14.2. Expressions¶

Based on layer data and prebuilt or user defined functions, Expressions offer a powerful way to manipulate attribute value, geometry and variables in order to dynamically change the geometry style, the content or position of the label, the value for diagram, the height of a layout item, select some features, create virtual field …

14.2.1. The Expression string builder ¶

Main dialog to build expressions, the Expression string builder is available from many parts in QGIS and, can particularly be accessed when:

clicking the button;
selecting features with the ^{Select By Expression…} tool;
editing attributes with e.g. the ^{Field calculator} tool;
manipulating symbology, label or layout item parameters with the ^{Data defined override} tool (see Data defined override setup);
building a geometry generator symbol layer;
doing some geoprocessing.

The Expression builder dialog offers access to the:

Expression tab which, thanks to a list of predefined functions, helps to write and check the expression to use;
Function Editor tab which helps to extend the list of functions by creating custom ones.

Some use cases of expressions:

From Field Calculator, calculate a «pop_density» field using existing «total_pop» and «area_km2» fields:
```
"total_pop" / "area_km2"
```

Update the field «density_level» with categories according to the «pop_density» values:

CASE WHEN "pop_density" < 50 THEN 'Low population density'
     WHEN "pop_density" >= 50 and "pop_density" < 150 THEN 'Medium population density'
     WHEN "pop_density" >= 150 THEN 'High population density'
END

Update a region layer field with the names (comma separated) of contained airports:

aggregate('airport_layer', 'concatenate', "name", within($geometry, geometry(@parent)), ', ')

Apply a categorized style to all the features according to whether their average house price is smaller or higher than 10000€ per square metre:
```
"price_m2" > 10000
```
Using the «Select By Expression…» tool, select all the features representing areas of “High population density” and whose average house price is higher than 10000€ per square metre:
```
"density_level" = 'High population density' and "price_m2" > 10000
```
Likewise, the previous expression could also be used to define which features should be labeled or shown in the map.

Using expressions offers you a lot of possibilities.

Совет

Use named parameters to improve the expression reading

Some functions require many parameters to be set. The expression engine supports the use of named parameters. This means that instead of writing the cryptic expression clamp( 1, 2, 9), you can use clamp( min:=1, value:=2, max:=9). This also allows arguments to be switched, e.g. clamp( value:=2, max:=9, min:=1). Using named parameters helps clarify what the arguments for an expression function refer to, which is helpful when you are trying to interpret an expression at a later date!

14.2.2. List of functions ¶

The Expression tab provides the main interface to write expressions using functions, layer’s fields and values. It contains following widgets:

An expression editor area to type or paste expressions. Autocompletion is available to speed expression writing:
- Corresponding variables, function names and field names to the input text are shown below: use the Up and Down arrows to browse the items and press Tab to insert in the expression or simply click on the wished item.
- Function parameters are shown while filling them.
QGIS also checks the expression rightness and highlights all the errors using:
- Underline: for unknown functions, wrong or invalid arguments;
- Marker: for every other error (eg, missing parenthesis, unexpected character) at a single location.
Совет

Document your expression with comments

When using complex expression, it is good practice to add text either as a multiline comment or inline comments to help you remember.
```
/*
Labels each region with its highest (in altitude) airport(s)
and altitude, eg 'AMBLER : 264m' for the 'Northwest Artic' region
*/
with_variable(
  'airport_alti', -- stores the highest altitude of the region
  aggregate(
    'airports',
    'max',
    "ELEV", -- the field containing the altitude
    -- and limit the airports to the region they are within
    filter := within( $geometry, geometry( @parent ) )
  ),
    aggregate( -- finds airports at the same altitude in the region
      'airports',
      'concatenate',
      "NAME",
      filter := within( $geometry, geometry( @parent ) )
        and "ELEV" = @airport_alti
    )
    || ' : ' || @airport_alti || 'm'
    -- using || allows regions without airports to be skipped
)
```
Under the expression editor, an Output preview displays the result of the expression evaluated on the first feature of the layer. In case of error, it indicates it and you can access details with the provided hyperlink.
A function selector displays the list of functions, variables, fields… organized in groups. A search box is available to filter the list and quickly find a particular function or field. Double-clicking an item adds it to the expression editor.
A help panel displays help for each selected item in the function selector.

Совет

Press Ctrl+Click when hovering a function name in an expression to automatically display its help in the dialog.

A field’s values widget shown when a field is selected in the function selector helps to fetch features attributes. Double-clicking a value adds it to the expression editor.

Совет

The right panel, showing functions help or field values, can be collapsed (invisible) in the dialog. Press the Show Values or Show Help button to get it back.

Рис. 14.65 The Expression tab¶

14.2.2.1. Aggregates Functions ¶

This group contains functions which aggregate values over layers and fields.

Function	Описание
aggregate	Returns an aggregate value calculated using features from another layer
array_agg	Returns an array of aggregated values from a field or expression
collect	Returns the multipart geometry of aggregated geometries from an expression
concatenate	Returns all aggregated strings from a field or expression joined by a delimiter
concatenate_unique	Returns all unique aggregated strings from a field or expression joined by a delimiter
count	Returns the count of matching features
count_distinct	Returns the count of distinct values
count_missing	Returns the count of missing (null) values
iqr	Returns the calculated inter quartile range from a field or expression
majority	Returns the aggregate majority of values (most commonly occurring value) from a field or expression
max_length	Returns the maximum length of strings from a field or expression
maximum	Returns the aggregate maximum value from a field or expression
mean	Returns the aggregate mean value from a field or expression
median	Returns the aggregate median value from a field or expression
min_length	Returns the minimum length of strings from a field or expression
minimum	Returns the aggregate minimum value from a field or expression
minority	Returns the aggregate minority of values (least commonly occurring value) from a field or expression
q1	Returns the calculated first quartile from a field or expression
q3	Returns the calculated third quartile from a field or expression
range	Returns the aggregate range of values (maximum - minimum) from a field or expression
relation_aggregate	Returns an aggregate value calculated using all matching child features from a layer relation
stdev	Returns the aggregate standard deviation value from a field or expression
sum	Returns the aggregate summed value from a field or expression

Examples:

Return the maximum of the «passengers» field from features in the layer grouped by «station_class» field:
```
maximum("passengers", group_by:="station_class")
```
Calculate the total number of passengers for the stations inside the current atlas feature:
```
aggregate('rail_stations','sum',"passengers",
  intersects(@atlas_geometry, $geometry))
```

Return the mean of the «field_from_related_table» field for all matching child features using the „my_relation“ relation from the layer:

relation_aggregate('my_relation', 'mean', "field_from_related_table")

or:

relation_aggregate(relation:='my_relation', aggregate := 'mean',
  expression := "field_from_related_table")

14.2.2.2. Array Functions ¶

This group contains functions to create and manipulate arrays (also known as list data structures). The order of values within the array matters, unlike the „map“ data structure, where the order of key-value pairs is irrelevant and values are identified by their keys.

Function	Описание
array	Returns an array containing all the values passed as parameter
array_all	Returns true if an array contains all the values of a given array
array_append	Returns an array with the given value added at the end
array_cat	Returns an array containing all the given arrays concatenated
array_contains	Returns true if an array contains the given value
array_distinct	Returns an array containing distinct values of the given array
array_filter	Returns an array with only the items for which an expression evaluates to true
array_find	Returns the index (0 for the first one) of a value within an array. Returns -1 if the value is not found.
array_first	Returns the first value of an array
array_foreach	Returns an array with the given expression evaluated on each item
array_get	Returns the Nth value (0 for the first one) of an array
array_insert	Returns an array with the given value added at the given position
array_intersect	Returns true if any element of array_1 exists in array_2
array_last	Returns the last element of an array
array_length	Returns the number of elements of an array
array_prepend	Returns an array with the given value added at the beginning
array_remove_all	Returns an array with all the entries of the given value removed
array_remove_at	Returns an array with the given index removed
array_reverse	Returns the given array with array values in reversed order
array_slice	Returns the values of the array from the start_pos argument up to and including the end_pos argument
array_sort	Returns the provided array with its elements sorted
array_to_string	Concatenates array elements into a string separated by a delimiter and using optional string for empty values
generate_series	Creates an array containing a sequence of numbers
regexp_matches	Returns an array of all strings captured by capturing groups, in the order the groups themselves appear in the supplied regular expression against a string
string_to_array	Splits string into an array using supplied delimiter and optional string for empty values

14.2.2.3. Color Functions ¶

This group contains functions for manipulating colors.

Function	Описание
color_cmyk	Returns a string representation of a color based on its cyan, magenta, yellow and black components
color_cmyka	Returns a string representation of a color based on its cyan, magenta, yellow, black and alpha (transparency) components
color_grayscale_average	Applies a grayscale filter and returns a string representation from a provided color
color_hsl	Returns a string representation of a color based on its hue, saturation, and lightness attributes
color_hsla	Returns a string representation of a color based on its hue, saturation, lightness and alpha (transparency) attributes
color_hsv	Returns a string representation of a color based on its hue, saturation, and value attributes
color_hsva	Returns a string representation of a color based on its hue, saturation, value and alpha (transparency) attributes
color_mix_rgb	Returns a string representing a color mixing the red, green, blue, and alpha values of two provided colors based on a given ratio
color_part	Returns a specific component from a color string, eg the red component or alpha component
color_rgb	Returns a string representation of a color based on its red, green, and blue components
color_rgba	Returns a string representation of a color based on its red, green, blue, and alpha (transparency) components
create_ramp	Returns a gradient ramp from a map of color strings and steps
darker	Returns a darker (or lighter) color string
lighter	Returns a lighter (or darker) color string
project_color	Returns a color from the project’s color scheme
ramp_color	Returns a string representing a color from a color ramp
set_color_part	Sets a specific color component for a color string, eg the red component or alpha component

14.2.2.4. Conditional Functions ¶

This group contains functions to handle conditional checks in expressions.

Function	Описание
CASE WHEN … THEN … END	Evaluates an expression and returns a result if true. You can test multiple conditions
CASE WHEN … THEN … ELSE … END	Evaluates an expression and returns a different result whether it’s true or false. You can test multiple conditions
coalesce	Returns the first non-NULL value from the expression list
if	Tests a condition and returns a different result depending on the conditional check
nullif(value1, value2)	Returns a null value if value1 equals value2 otherwise it returns value1. This can be used to conditionally substitute values with NULL.
try	Tries an expression and returns its value if error-free, an alternative value (if provided) or Null if an error occurs

Some example:

Send back a value if the first condition is true, else another value:

CASE WHEN "software" LIKE '%QGIS%' THEN 'QGIS' ELSE 'Other' END

14.2.2.5. Conversions Functions ¶

This group contains functions to convert one data type to another (e.g., string to integer, integer to string).

Function	Описание
to_date	Converts a string into a date object
to_datetime	Converts a string into a datetime object
to_dm	Converts a coordinate to degree, minute
to_dms	Converts coordinate to degree, minute, second
to_int	Converts a string to integer number
to_interval	Converts a string to an interval type (can be used to take days, hours, months, etc. of a date)
to_real	Converts a string to a real number
to_string	Converts number to string
to_time	Converts a string into a time object

14.2.2.6. Custom Functions ¶

This group contains functions created by the user. See Function Editor for more details.

14.2.2.7. Date and Time Functions ¶

This group contains functions for handling date and time data.

Function	Описание
age	Returns as an interval the difference between two dates or datetimes
day	Extracts the day from a date or datetime, or the number of days from an interval
day_of_week	Returns a number corresponding to the day of the week for a specified date or datetime
epoch	Returns the interval in milliseconds between the unix epoch and a given date value
hour	Extracts the hour from a datetime or time, or the number of hours from an interval
minute	Extracts the minute from a datetime or time, or the number of minutes from an interval
month	Extracts the month part from a date or datetime, or the number of months from an interval
now	Returns current date and time
second	Extracts the second from a datetime or time, or the number of seconds from an interval
week	Extracts the week number from a date or datetime, or the number of weeks from an interval
year	Extracts the year part from a date or datetime, or the number of years from an interval

This group also shares several functions with the Conversions Functions ( to_date, to_time, to_datetime, to_interval) and String Functions (format_date) groups.

Some examples:

Get today’s month and year in the «month_number/year» format:
```
format_date(now(),'MM/yyyy')
-- Returns '03/2017'
```

Besides these functions, subtracting dates, datetimes or times using the - (minus) operator will return an interval.

Adding or subtracting an interval to dates, datetimes or times, using the + (plus) and - (minus) operators, will return a datetime.

Get the number of days until QGIS 3.0 release:

to_date('2017-09-29') - to_date(now())
-- Returns <interval: 203 days>

The same with time:

to_datetime('2017-09-29 12:00:00') - to_datetime(now())
-- Returns <interval: 202.49 days>

Get the datetime of 100 days from now:

now() + to_interval('100 days')
-- Returns <datetime: 2017-06-18 01:00:00>

Примечание

Storing date and datetime and intervals on fields

The ability to store date, time and datetime values directly on fields may depend on the data source’s provider (e.g., Shapefile accepts date format, but not datetime or time format). The following are some suggestions to overcome this limitation:

date, Datetime and time can be stored in text type fields after using the to_format() function.
Intervals can be stored in integer or decimal type fields after using one of the date extraction functions (e.g., day() to get the interval expressed in days)

14.2.2.8. Fields and Values ¶

Contains a list of fields from the layer.

Double-click a field name to have it added to your expression. You can also type the field name (preferably inside double quotes) or its alias.

To retrieve fields values to use in an expression, select the appropriate field and, in the shown widget, choose between 10 Samples and All Unique. Requested values are then displayed and you can use the Search box at the top of the list to filter the result. Sample values can also be accessed via right-clicking on a field.

To add a value to the expression you are writing, double-click on it in the list. If the value is of a string type, it should be simple quoted, otherwise no quote is needed.

14.2.2.9. Files and Paths Functions ¶

This group contains functions which manipulate file and path names.

Function	Описание
base_file_name	Returns the base name of the file without the directory or file suffix.
file_exists	Returns true if a file path exists.
file_name	Returns the name of a file (including the file extension), excluding the directory.
file_path	Returns the directory component of a file path, without the file name
file_size	Returns the size (in bytes) of a file.
file_suffix	Returns the file extension from a file path.
is_directory	Returns true if a path corresponds to a directory.
is_file	Returns true if a path corresponds to a file.

14.2.2.10. Fuzzy Matching Functions ¶

This group contains functions for fuzzy comparisons between values.

Function	Описание
hamming_distance	Returns the number of characters at corresponding positions within the input strings where the characters are different
levensheim	Returns the minimum number of character edits (insertions, deletions or substitutions) required to change one string to another. Measure the similarity between two strings
longest_common_substring	Returns the longest common substring between two strings
soundex	Returns the Soundex representation of a string

14.2.2.11. General Functions ¶

This group contains general assorted functions.

Function	Описание
env	Gets an environment variable and returns its content as a string. If the variable is not found, `NULL` will be returned.
eval	Evaluates an expression which is passed in a string. Useful to expand dynamic parameters passed as context variables or fields.
is_layer_visible	Returns true if a specified layer is visible
layer_property	Returns a property of a layer or a value of its metadata. It can be layer name, crs, geometry type, feature count…
var	Returns the value stored within a specified variable. See Variables below
with_variable	Creates and sets a variable for any expression code that will be provided as a third argument. Useful to avoid repetition in expressions where the same value needs to be used more than once.

14.2.2.12. Geometry Functions ¶

This group contains functions that operate on geometry objects (e.g. buffer, transform, $area).

Function	Описание
$area	Returns the area size of the current feature
$geometry	Returns the geometry of the current feature (can be used for processing with other functions)
$length	Returns the length of the current line feature
$perimeter	Returns the perimeter of the current polygon feature
$x	Returns the X coordinate of the current feature
$x_at(n)	Returns the X coordinate of the nth node of the current feature’s geometry
$y	Returns the Y coordinate of the current feature
$y_at(n)	Returns the Y coordinate of the nth node of the current feature’s geometry
angle_at_vertex	Returns the bisector angle (average angle) to the geometry for a specified vertex on a linestring geometry. Angles are in degrees clockwise from north
area	Returns the area of a geometry polygon feature. Calculations are in the Spatial Reference System of this geometry
azimuth	Returns the north-based azimuth as the angle in radians measured clockwise from the vertical on point_a to point_b
boundary	Returns the closure of the combinatorial boundary of the geometry (ie the topological boundary of the geometry - see also Boundary).
bounds	Returns a geometry which represents the bounding box of an input geometry. Calculations are in the Spatial Reference System of this geometry (see also Bounding boxes)
bounds_height	Returns the height of the bounding box of a geometry. Calculations are in the Spatial Reference System of this geometry
bounds_width	Returns the width of the bounding box of a geometry. Calculations are in the Spatial Reference System of this geometry
buffer	Returns a geometry that represents all points whose distance from this geometry is less than or equal to distance. Calculations are in the Spatial Reference System of this geometry (see also Buffer)
buffer_by_m	Creates a buffer along a line geometry where the buffer diameter varies according to the M values at the line vertices (see also Variable width buffer (by M value))
centroid	Returns the geometric center of a geometry (see also Centroids)
closest_point	Returns the point on a geometry that is closest to a second geometry
collect_geometries	Collects a set of geometries into a multi-part geometry object (see also Collect geometries)
combine	Returns the combination of two geometries
contains(a,b)	Returns 1 (true) if and only if no points of b lie in the exterior of a, and at least one point of the interior of b lies in the interior of a
convex_hull	Returns the convex hull of a geometry (this represents the minimum convex geometry that encloses all geometries within the set) (see also Convex hull)
crosses	Returns 1 (true) if the supplied geometries have some, but not all, interior points in common
difference(a,b)	Returns a geometry that represents that part of geometry a that does not intersect with geometry b (see also Difference)
disjoint	Returns 1 (true) if the geometries do not share any space together
distance	Returns the minimum distance (based on Spatial Reference System) between two geometries in projected units
distance_to_vertex	Returns the distance along the geometry to a specified vertex
end_point	Returns the last node from a geometry (see also Extract specific vertices)
extend	Extends the start and end of a linestring geometry by a specified amount (see also Extend lines)
exterior_ring	Returns a line string representing the exterior ring of a polygon geometry, or null if the geometry is not a polygon
extrude(geom,x,y)	Returns an extruded version of the input (Multi-) Curve or (Multi-)Linestring geometry with an extension specified by X and Y
flip_coordinates	Returns a copy of the geometry with the X and Y coordinates swapped (see also Swap X and Y coordinates)
force_rhr	Forces a geometry to respect the Right-Hand-Rule (see also Force right-hand-rule)
geom_from_gml	Returns a geometry created from a GML representation of geometry
geom_from_wkt	Returns a geometry created from a well-known text (WKT) representation
geom_to_wkt	Returns the well-known text (WKT) representation of the geometry without SRID metadata
geometry	Returns a feature’s geometry
geometry_n	Returns the nth geometry from a geometry collection, or null if the input geometry is not a collection
hausdorff_distance	Returns basically a measure of how similar or dissimilar two geometries are, with a lower distance indicating more similar geometries
inclination	Returns the inclination measured from the zenith (0) to the nadir (180) on point_a to point_b
interior_ring_n	Returns the geometry of the nth interior ring from a polygon geometry, or null if the geometry is not a polygon
intersection	Returns a geometry that represents the shared portion of two geometries (see also Intersection)
intersects	Tests whether a geometry intersects another. Returns 1 (true) if the geometries spatially intersect (share any portion of space) and 0 if they don’t
intersects_bbox	Tests whether a geometry’s bounding box overlaps another geometry’s bounding box. Returns 1 (true) if the geometries spatially intersect (share any portion of space) their bounding box, or 0 if they don’t
is_closed	Returns true if a line string is closed (start and end points are coincident), false if a line string is not closed, or null if the geometry is not a line string
length	Returns length of a line geometry feature (or length of a string)
line_interpolate_angle	Returns the angle parallel to the geometry at a specified distance along a linestring geometry. Angles are in degrees clockwise from north.
line_interpolate_point	Returns the point interpolated by a specified distance along a linestring geometry. (see also Interpolate point on line)
line_locate_point	Returns the distance along a linestring corresponding to the closest position the linestring comes to a specified point geometry.
line_merge	Returns a (Multi-)LineString geometry, where any connected LineStrings from the input geometry have been merged into a single linestring.
line_substring	Returns the portion of a line or curve geometry falling betweeen specified start and end distances (measured from the beginning of the line) (see also Line substring)
m	Returns the M value of a point geometry
make_circle	Creates a circular geometry based on center point and radius
make_ellipse	Creates an elliptical geometry based on center point, axes and azimuth
make_line	Creates a line geometry from a series or an array of point geometries
make_point(x,y,z,m)	Returns a point geometry from X and Y (and optional Z or M) values
make_point_m(x,y,m)	Returns a point geometry from X and Y coordinates and M values
make_polygon	Creates a polygon geometry from an outer ring and optional series of inner ring geometries
make_rectangle_3points	Creates a rectangle from 3 points
make_regular_polygon	Creates a regular polygon
make_square	Creates a square from a diagonal
make_triangle	Creates a triangle polygon
minimal_circle	Returns the minimal enclosing circle of an input geometry (see also Minimum enclosing circles)
nodes_to_points	Returns a multipoint geometry consisting of every node in the input geometry (see also Extract vertices)
num_geometries	Returns the number of geometries in a geometry collection, or null if the input geometry is not a collection
num_interior_rings	Returns the number of interior rings in a polygon or geometry collection, or null if the input geometry is not a polygon or collection
num_points	Returns the number of vertices in a geometry
num_rings	Returns the number of rings (including exterior rings) in a polygon or geometry collection, or null if the input geometry is not a polygon or collection
offset_curve	Returns a geometry formed by offsetting a linestring geometry to the side. Distances are in the Spatial Reference System of this geometry. (see also Offset lines)
order_parts	Orders the parts of a MultiGeometry by a given criteria
oriented_bbox	Returns a geometry representing the minimal oriented bounding box of an input geometry (see also Oriented minimum bounding box)
overlaps	Tests whether a geometry overlaps another. Returns 1 (true) if the geometries share space, are of the same dimension, but are not completely contained by each other
perimeter	Returns the perimeter of a geometry polygon feature. Calculations are in the Spatial Reference System of this geometry
point_n	Returns a specific node from a geometry (see also Extract specific vertices)
point_on_surface	Returns a point guaranteed to lie on the surface of a geometry (see also Point on Surface)
pole_of_inaccessibility	Calculates the approximate pole of inaccessibility for a surface, which is the most distant internal point from the boundary of the surface (see also Pole of inaccessibility)
project	Returns a point projected from a start point using a distance, a bearing (azimuth) and an elevation in radians (see also Project points (Cartesian))
relate	Tests or returns the Dimensional Extended 9 Intersection Model (DE-9IM) representation of the relationship between two geometries
reverse	Reverses the direction of a line string by reversing the order of its vertices (see also Reverse line direction)
segments_to_lines	Returns a multi line geometry consisting of a line for every segment in the input geometry (see also Explode lines)
shortest_line	Returns the shortest line joining two geometries. The resultant line will start at geometry 1 and end at geometry 2
simplify	Simplifies a geometry by removing nodes using a distance based threshold (see also Simplify)
simplify_vw	Simplifies a geometry by removing nodes using an area based threshold (see also Simplify)
single_sided_buffer	Returns a geometry formed by buffering out just one side of a linestring geometry. Distances are in the Spatial Reference System of this geometry (see also Single sided buffer)
smooth	Smooths a geometry by adding extra nodes which round off corners in the geometry (see also Smooth)
start_point	Returns the first node from a geometry (see also Extract specific vertices)
sym_difference	Returns a geometry that represents the portions of two geometries that do not intersect (see also Symmetrical difference)
tapered_buffer	Creates a buffer along a line geometry where the buffer diameter varies evenly over the length of the line (see also Tapered buffers)
touches	Tests whether a geometry touches another. Returns 1 (true) if the geometries have at least one point in common, but their interiors do not intersect
transform	Returns the geometry transformed from the source CRS to the destination CRS (see also Reproject layer)
translate	Returns a translated version of a geometry. Calculations are in the Spatial Reference System of the geometry (see also Translate)
union	Returns a geometry that represents the point set union of the geometries
wedge_buffer	Returns a wedge shaped buffer originating from a point geometry given an angle and radii (see also Create wedge buffers)
within (a,b)	Tests whether a geometry is within another. Returns 1 (true) if geometry a is completely inside geometry b
x	Returns the X coordinate of a point geometry, or the X coordinate of the centroid for a non-point geometry
x_max	Returns the maximum X coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
x_min	Returns the minimum X coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
y	Returns the Y coordinate of a point geometry, or the Y coordinate of the centroid for a non-point geometry
y_max	Returns the maximum Y coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
y_min	Returns the minimum Y coordinate of a geometry. Calculations are in the Spatial Reference System of this geometry
z	Returns the Z coordinate of a point geometry

Some examples:

Return the X coordinate of the current feature’s centroid:
```
x( $geometry )
```

Send back a value according to feature’s area:

CASE WHEN $area > 10 000 THEN 'Larger' ELSE 'Smaller' END

You can manipulate the current geometry using the variable $geometry to create a buffer or get a point on the geometry’s surface:
```
buffer( $geometry, 10 )
point_on_surface( $geometry )
```

Given a point feature, generate a closed line (using make_line) around the point’s geometry:

make_line(
  -- using an array of points placed around the original
  array_foreach(
    -- list of angles for placing the projected points (every 90°)
    array:=generate_series( 0, 360, 90 ),
    -- translate the point 20 units in the given direction (angle)
    expression:=project( $geometry, distance:=20, azimuth:=radians( @element ) )
  )
)

14.2.2.13. Layout Functions ¶

This group contains functions to manipulate print layout items properties.

Function	Описание
item_variables	Returns a map of variables from a layout item inside this print layout

An example:

Get the scale of the „Map 0“ in the current print layout:
```
map_get( item_variables('Map 0'), 'map_scale')
```

14.2.2.14. Map Layers ¶

This group contains a list of the available layers in the current project. This offers a convenient way to write expressions referring to multiple layers, such as when performing aggregates, attribute or spatial queries.

It also provides some convenient functions to manipulate layers.

Function	Описание
decode_uri	Takes a layer and decodes the uri of the underlying data provider. Available information depends on the data provider type.

14.2.2.15. Maps Functions ¶

This group contains functions to create or manipulate keys and values of map data structures (also known as dictionary objects, key-value pairs, or associative arrays). Unlike the list data structure where values order matters, the order of the key-value pairs in the map object is not relevant and values are identified by their keys.

Function	Описание
from_json	Loads a json-formatted string
hstore_to_map	Creates a map from a hstore-formatted string
json_to_map	Creates a map from a json-formatted string
map	Returns a map containing all the keys and values passed as pair of parameters
map_akeys	Returns all the keys of a map as an array
map_avals	Returns all the values of a map as an array
map_concat	Returns a map containing all the entries of the given maps. If two maps contain the same key, the value of the second map is taken.
map_delete	Returns a map with the given key and its corresponding value deleted
map_exist	Returns true if the given key exists in the map
map_get	Returns the value of a map, given it’s key
map_insert	Returns a map with an added key/value
map_to_hstore	Merges map elements into a hstore-formatted string
map_to_json	Merges map elements into a json-formatted string
to_json	Creates a json-formatted string from a map, an array or other value

14.2.2.16. Mathematical Functions ¶

This group contains math functions (e.g., square root, sin and cos).

Function	Описание
abs	Returns the absolute value of a number
acos	Returns the inverse cosine of a value in radians
asin	Returns the inverse sine of a value in radians
atan	Returns the inverse tangent of a value in radians
atan2(y,x)	Returns the inverse tangent of Y/X by using the signs of the two arguments to determine the quadrant of the result
azimuth(a,b)	Returns the north-based azimuth as the angle in radians measured clockwise from the vertical on point a to point b
ceil	Rounds a number upwards
clamp	Restricts an input value to a specified range
cos	Returns the cosine of an angle in radians
degrees	Converts from radians to degrees
exp	Returns exponential of a value
floor	Rounds a number downwards
inclination	Returns the inclination measured from the zenith (0) to the nadir (180) on point_a to point_b.
ln	Returns the natural logarithm of the passed expression
log	Returns the value of the logarithm of the passed value and base
log10	Returns the value of the base 10 logarithm of the passed expression
max	Returns the largest not null value in a set of values
min	Returns the smallest not null value in a set of values
pi	Returns the value of pi for calculations
radians	Converts from degrees to radians
rand	Returns the random integer within the range specified by the minimum and maximum argument (inclusive)
randf	Returns the random float within the range specified by the minimum and maximum argument (inclusive)
round	Rounds to number of decimal places
scale_exp	Transforms a given value from an input domain to an output range using an exponential curve
scale_linear	Transforms a given value from an input domain to an output range using linear interpolation
sin	Returns the sine of an angle in radians
sqrt	Returns the square root of a value
tan	Returns the tangent of an angle in radians

14.2.2.17. Operators ¶

This group contains operators (e.g., +, -, *). Note that for most of the mathematical functions below, if one of the inputs is NULL then the result is NULL.

Function	Описание
a + b	Addition of two values (a plus b)
a - b	Subtraction of two values (a minus b).
a * b	Multiplication of two values (a multiplied by b)
a / b	Division of two values (a divided by b)
a % b	Remainder of division of a by b (eg, 7 % 2 = 1, or 2 fits into 7 three times with remainder 1)
a ^ b	Power of two values (for example, 2^2=4 or 2^3=8)
a < b	Compares two values and evaluates to 1 if the left value is less than the right value (a is smaller than b)
a <= b	Compares two values and evaluates to 1 if the left value is less than or equal to the right value
a <> b	Compares two values and evaluates to 1 if they are not equal
a = b	Compares two values and evaluates to 1 if they are equal
a != b	a and b are not equal
a > b	Compares two values and evaluates to 1 if the left value is greater than the right value (a is larger than b)
a >= b	Compares two values and evaluates to 1 if the left value is greater than or equal to the right value
a ~ b	a matches the regular expression b
\|\|	Joins two values together into a string. If one of the values is NULL the result will be NULL
„\n“	Inserts a new line in a string
LIKE	Returns 1 if the first parameter matches the supplied pattern
ILIKE	Returns 1 if the first parameter matches case-insensitive the supplied pattern (ILIKE can be used instead of LIKE to make the match case-insensitive)
a IS b	Tests whether two values are identical. Returns 1 if a is the same as b
a OR b	Returns 1 when condition a or condition b is true
a AND b	Returns 1 when conditions a and b are true
NOT	Negates a condition
«Column_name»	Value of the field Column_name, take care to not be confused with simple quote, see below
„string“	a string value, take care to not be confused with double quote, see above
NULL	null value
a IS NULL	a has no value
a IS NOT NULL	a has a value
a IN (value[,value])	a is below the values listed
a NOT IN (value[,value])	a is not below the values listed

Some examples:

Joins a string and a value from a column name:
```
'My feature''s id is: ' || "gid"
```
Test if the «description» attribute field starts with the „Hello“ string in the value (note the position of the % character):
```
"description" LIKE 'Hello%'
```

14.2.2.18. Processing Functions ¶

This group contains functions that operate on processing algorithms.

Function	Описание
parameter	Returns the value of a processing algorithm input parameter

14.2.2.19. Rasters Functions ¶

This group contains functions to operate on raster layer.

Function	Описание
raster_statistic	Returns statistics from a raster layer
raster_value	Returns the raster band value at the provided point

14.2.2.20. Record and Attributes Functions ¶

This group contains functions that operate on record identifiers.

Function	Описание
$currentfeature	Returns the current feature being evaluated. This can be used with the „attribute“ function to evaluate attribute values from the current feature.
$id	Returns the feature id of the current row
attribute	Returns the value of a specified attribute from a feature
attributes	Returns a map of all attributes from a feature, with field names as map keys
get_feature	Returns the first feature of a layer matching a given attribute value
get_feature_by_id	Returns the feature of a layer matching the given feature ID
is_selected	Returns if a feature is selected
num_selected	Returns the number of selected features on a given layer
represent_value	Returns the configured representation value for a field value (convenient with some widget types)
sql_fetch_and_increment	Manage autoincrementing values in SQLite databases
uuid	Generates a Universally Unique Identifier (UUID) for each row. Each UUID is 38 characters long.

Some examples:

Return the first feature in layer «LayerA» whose field «id» has the same value as the field «name» of the current feature (a kind of jointure):
```
get_feature( 'layerA', 'id', attribute( $currentfeature, 'name') )
```

Calculate the area of the joined feature from the previous example:

area( geometry( get_feature( 'layerA', 'id', attribute( $currentfeature, 'name') ) ) )

14.2.2.21. String Functions ¶

This group contains functions that operate on strings (e.g., that replace, convert to upper case).

Function	Описание
char	Returns the character associated with a unicode code
concat	Concatenates several strings to one
format	Formats a string using supplied arguments
format_date	Formats a date type or string into a custom string format
format_number	Returns a number formatted with the locale separator for thousands (also truncates the number to the number of supplied places)
left(string, n)	Returns a substring that contains the n leftmost characters of the string
length	Returns length of a string (or length of a line geometry feature)
lower	converts a string to lower case
lpad	Returns a string padded on the left to the specified width, using the fill character
regexp_match	Returns the first matching position matching a regular expression within a string, or 0 if the substring is not found
regexp_replace	Returns a string with the supplied regular expression replaced
regexp_substr	Returns the portion of a string which matches a supplied regular expression
replace	Returns a string with the supplied string, array, or map of strings replaced by a string, an array of strings or paired values
right(string, n)	Returns a substring that contains the n rightmost characters of the string
rpad	Returns a string padded on the right to the specified width, using the fill character
strpos	Returns the first matching position of a substring within another string, or 0 if the substring is not found
substr	Returns a part of a string
title	Converts all words of a string to title case (all words lower case with leading capital letter)
trim	Removes all leading and trailing white space (spaces, tabs, etc.) from a string
upper	Converts string a to upper case
wordwrap	Returns a string wrapped to a maximum/minimum number of characters

About fields concatenation

You can concatenate strings or field values using either || or + operators or the concat function, with some special characteristics:

The + operator also means sum up expression, so if you have an integer (field or numeric value) operand, this can be error prone and you better use the others:
```
'My feature''s id is: ' + "gid" => triggers an error as gid is an integer
```

When any of the arguments is a NULL value, either || or + will return a NULL value. To return the other arguments regardless the NULL value, you may want to use the concat function:

"country_name" || NULL => NULL
concat('My feature''s id is: ', NULL) => My feature's id is
concat("firstname", "nickname", "lastname") => Chuck Norris (if empty nickname)
"firstname" + "nickname" + "lastname" => NULL (if one field is empty)

For other cases, do at your convenience:

'My country is ' + "country_name" + ' (' + "country_code" + ')'
'My country is ' || "country_name" || ' (' || "country_code" || ')'
concat('My country is ', "country_name", ' (', "country_code", ')')
# All the above return: My country is France (FR)

14.2.2.22. Variables ¶

This group contains dynamic variables related to the application, the project file and other settings. The availability of variables depends on the context:

from the ^{Select by expression} dialog
from the ^{Field calculator} dialog
from the layer properties dialog
from the print layout

To use these variables in an expression, they should be preceded by the @ character (e.g, @row_number).

Function	Описание
algorithm_id	The unique ID of an algorithm
atlas_feature	The current atlas feature (as feature object)
atlas_featureid	The current atlas feature ID
atlas_featurenumber	The current atlas feature number in the layout
atlas_filename	The current atlas file name
atlas_geometry	The current atlas feature geometry
atlas_layerid	The current atlas coverage layer ID
atlas_layername	The current atlas coverage layer name
atlas_pagename	The current atlas page name
atlas_totalfeatures	The total number of features in atlas
canvas_cursor_point	The last cursor position on the canvas in the project’s geographical coordinates
cluster_color	The color of symbols within a cluster, or NULL if symbols have mixed colors
cluster_size	The number of symbols contained within a cluster
current_feature	The feature currently being edited in the attribute form or table row
current_geometry	The geometry of the feature currently being edited in the form or the table row
fullextent_maxx	Maximum x value from full canvas extent (including all layers)
fullextent_maxy	Maximum y value from full canvas extent (including all layers)
fullextent_minx	Minimum x value from full canvas extent (including all layers)
fullextent_miny	Minimum y value from full canvas extent (including all layers)
geometry_part_count	The number of parts in rendered feature’s geometry
geometry_part_num	The current geometry part number for feature being rendered
geometry_point_count	The number of points in the rendered geometry’s part
geometry_point_num	The current point number in the rendered geometry’s part
grid_axis	The current grid annotation axis (eg, „x“ for longitude, „y“ for latitude)
grid_number	The current grid annotation value
item_id	The layout item user ID (not necessarily unique)
item_uuid	The layout item unique ID
layer	The current layer
layer_id	The ID of current layer
layer_name	The name of current layer
layout_dpi	The composition resolution (DPI)
layout_name	The layout name
layout_numpages	The number of pages in the layout
layout_page	The page number of the current item in the layout
layout_pageheight	The active page height in the layout (in mm)
layout_pagewidth	The active page width in the layout (in mm)
legend_column_count	The number of columns in the legend
legend_filter_by_map	Indicates if the content of the legend is filtered by the map
legend_filter_out_atlas	Indicates if the atlas is filtered out of the legend
legend_split_layers	Indicates if layers can be split in the legend
legend_title	The title of the legend
legend_wrap_string	The character(s) used to wrap the legend text
map_crs	The Coordinate reference system of the current map
map_crs_acronym	The acronym of the Coordinate reference system of the current map
map_crs_definition	The full definition of the Coordinate reference system of the current map
map_crs_description	The name of the Coordinate reference system of the current map
map_crs_ellipsoid	The acronym of the ellipsoid of the Coordinate reference system of the current map
map_crs_proj4	The Proj4 definition of the Coordinate reference system of the current map
map_crs_wkt	The WKT definition of the Coordinate reference system of the current map
map_extent	The geometry representing the current extent of the map
map_extent_center	The point feature at the center of the map
map_extent_height	The current height of the map
map_extent_width	The current width of the map
map_id	The ID of current map destination. This will be „canvas“ for canvas renders, and the item ID for layout map renders
map_layer_ids	The list of map layer IDs visible in the map
map_layers	The list of map layers visible in the map
map_rotation	The current rotation of the map
map_scale	The current scale of the map
map_units	The units of map measurements
notification_message	Content of the notification message sent by the provider (available only for actions triggered by provider notifications).
parent	Refers to the current feature in the parent layer, providing access to its attributes and geometry when filtering an aggregate function
project_abstract	The project abstract, taken from project metadata
project_area_units	The area unit for the current project, used when calculating areas of geometries
project_author	The project author, taken from project metadata
project_basename	The basename of current project’s filename (without path and extension)
project_creation_date	The project creation date, taken from project metadata
project_crs	The Coordinate reference system of the project
project_crs_arconym	The acronym of the Coordinate reference system of the project
project_crs_definition	The full definition of the Coordinate reference system of the project
project_crs_description	The description of the Coordinate reference system of the project
project_crs_ellipsoid	The ellipsoid of the Coordinate reference system of the project
project_crs_proj4	The Proj4 representation of the Coordinate reference system of the project
project_crs_wkt	The WKT (well known text) representation of the coordinate reference system of the project
project_distance_units	The distance unit for the current project, used when calculating lengths of geometries and distances
project_ellipsoid	The name of the ellipsoid of the current project, used when calculating geodetic areas or lengths of geometries
project_filename	The filename of the current project
project_folder	The folder of the current project
project_home	The home path of the current project
project_identifier	The project identifier, taken from the project’s metadata
project_keywords	The project keywords, taken from the project’s metadata
project_last_saved	Date/time when project was last saved.
project_path	The full path (including file name) of the current project
project_title	The title of current project
project_units	The units of the project’s CRS
qgis_locale	The current language of QGIS
qgis_os_name	The current Operating system name, eg „windows“, „linux“ or „osx“
qgis_platform	The QGIS platform, eg „desktop“ or „server“
qgis_release_name	The current QGIS release name
qgis_short_version	The current QGIS version short string
qgis_version	The current QGIS version string
qgis_version_no	The current QGIS version number
row_number	Stores the number of the current row
snapping_results	Gives access to snapping results while digitizing a feature (only available in add feature)
scale_value	The current scale bar distance value
symbol_angle	The angle of the symbol used to render the feature (valid for marker symbols only)
symbol_color	The color of the symbol used to render the feature
symbol_count	The number of features represented by the symbol (in the layout legend)
symbol_id	The Internal ID of the symbol (in the layout legend)
symbol_label	The label for the symbol (either a user defined label or the default autogenerated label - in the layout legend)
user_account_name	The current user’s operating system account name
user_full_name	The current user’s operating system user name
value	The current value
with_variable	Allows setting a variable for usage within an expression and avoid recalculating the same value repeatedly

Some examples:

Return the X coordinate of a map item center in layout:

x( map_get( item_variables( 'map1'), 'map_extent_center' ) )

Return, for each feature in the current layer, the number of overlapping airport features:

aggregate( layer:='airport', aggregate:='count', expression:="code",
               filter:=intersects( $geometry, geometry( @parent ) ) )

Get the object_id of the first snapped point of a line:

with_variable(
  'first_snapped_point',
  array_first( @snapping_results ),
  attribute(
    get_feature_by_id(
      map_get( @first_snapped_point, 'layer' ),
      map_get( @first_snapped_point, 'feature_id' )
    ),
    'object_id'
  )
)

14.2.2.23. Recent Functions ¶

This group contains recently used functions. Depending on the context of its usage (feature selection, field calculator, generic), recently applied expressions are added to the corresponding list (up to ten expressions), sorted from more to less recent. This makes it easy to quickly retrieve and reapply previously used expressions.

14.2.3. Function Editor ¶

With the Function Editor tab, you are able to write your own functions in Python language. This provides a handy and comfortable way to address particular needs that would not be covered by the predefined functions.

Рис. 14.66 The Function Editor tab¶

To create a new function:

Press the ^{New File} button.
Enter a name to use in the form that pops up and press OK.

A new item of the name you provide is added in the left panel of the Function Editor tab; this is a Python .py file based on QGIS template file and stored in the /python/expressions folder under the active user profile directory.
The right panel displays the content of the file: a python script template. Update the code and its help according to your needs.
Press the Save and Load Functions button. The function you wrote is added to the functions tree in the Expression tab, by default under the Custom group.
Enjoy your new function.
If the function requires improvements, enable the Function Editor tab, do the changes and press again the Save and Load Functions button to make them available in the file, hence in any expression tab.

Custom Python functions are stored under the user profile directory, meaning that at each QGIS startup, it will auto load all the functions defined with the current user profile. Be aware that new functions are only saved in the /python/expressions folder and not in the project file. If you share a project that uses one of your custom functions you will need to also share the .py file in the /python/expressions folder.

Here’s a short example on how to create your own functions:

from qgis.core import *
from qgis.gui import *

@qgsfunction(args='auto', group='Custom')
def my_sum(value1, value2, feature, parent):
    """
    Calculates the sum of the two parameters value1 and value2.
    <h2>Example usage:</h2>
    <ul>
      <li>my_sum(5, 8) -> 13</li>
      <li>my_sum("field1", "field2") -> 42</li>
    </ul>
    """
    return value1 + value2

The short example creates a function my_sum that will give you a function with two values. When using the args='auto' function argument the number of function arguments required will be calculated by the number of arguments the function has been defined with in Python (minus 2 - feature, and parent).

This function can then be used in expressions:

Рис. 14.67 Custom Function added to the Expression tab¶

Further information about creating Python code can be found in the Поваренная книга разработчика PyQGIS.