The Field Calculator button in the attribute table allows to perform calculations on basis of existing attribute values or defined functions, e.g to calculate length or area of geometry features. The results can be written to a new attribute column or it can be used to update values in an already existing column.
You have to bring the vector layer in editing mode, before you can click on the field calculator icon to open the dialog (see figure_attributes_3). In the dialog you first have to select whether you want to only update selected features, create a new attribute field where the results of the calculation will be added or update an existing field.
Figure Attributes 3:
If you choose to add a new field, you need to enter a field name, a field type (integer, real or string), the total field width, and the field precision (see figure_attributes_3). For example, if you choose a field width of 10 and a field precision of 3 it means you have 6 signs before the dot, then the dot and another 3 signs for the precision.
The Function List contains functions as well as fields and values. View the help function in the Selected Function Help. In Expression you see the calculation expressions you create with the Function List. The most commonly used operators, see Operators.
In the Function List, click on Fields and Values to view all attributes of the attribute table to be searched. To add an attribute to the Field calculator Expression field, double click its name in the Fields and Values list. Generally you can use the various fields, values and functions to construct the calculation expression or you can just type it into the box. To display the values of a field, you just right click on the appropriate field. You can choose between Load top 10 unique values and Load all unique values. On the right side opens the Field Values list with the unique values. To add a value to the Field calculator Expression box, double click its name in the Field Values list.
The Operators, Math, Conversions, String, Geometry and Record groups provides several functions. In Operators you find mathematical operators. Find Math for mathematical functions. The Conversions group contains functions that convert one data type to another. The String group provides functions for data strings. In the Geometry group you find functions for geometry objects. With Record group functions you can add a numeration to your data set. To add a function to the Field calculator Expression box, click on the > and then doubleclick the function.
A short example illustrates how the field calculator works. We want to calculate the length in km of the railroads layer from the QGIS sample dataset:
The available functions are listed below.
The field calculator Function list with the Selected Function Help , Operators and Expression menu are also available through the rule-based rendering in the Style menu of the Layer properties and the expression based labeling in the Labeling core application.
This group contains operators e.g + - *
a + b a plus b a - b a minus b a * b a multiplied by b a / b a divided by b a % b a modulo b for example 7 % 2 = 1 -> 2 fits into 7 three times rest is 1 a ^ b a power b for example 2^2=4 or 2^3=8 a = b a and b are equal a > b a is larger than b a < b a is smaller than b a <> b a and b are not equal a != b a and b are not equal a <= b a is less than or equal to b a >= b a is larger than or equal to b a ~ b a matches the regular expression b + a positive sign - a negative value of a || joins two values together into a string 'Hello' || ' world' LIKE returns 1 if the string matches the supplied pattern ILIKE returns 1 if the string matches case-insensitive the supplied pattern. ILIKE can be used instead of LIKE to make the match case-insensitive IS returns 1 if a is the same as b OR returns 1 when condition a or b is true AND returns 1 when condition a and b are true NOT returns 1 if a is not the same as b column name "column name" value of the field column name 'string' a string value 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
This group contains functions to handle conditional checks in expressions.
CASE evaluates multiple expressions and return a result CASE ELSE evaluates multiple expressions and return a result coalesce returns the first non-NULL value from the expression list regexp_match returns true if any part of a string matches the supplied regular expression
This group contains math functions e.g square root, sin and cos
sqrt(a) square root of a abs returns the absolute value of a number. sin(a) sinus of a cos(a) cosinus of a tan(a) tangens of a asin(a) arcussinus of a acos(a) arcuscosinus of a atan(a) arcustangens of a atan2(y,x) arcustangens of y/x using the signs of the two arguments to determine the quadrant of the result exp exponential of an value ln value of the natural logarithm of the passed expression log10 value of the base 10 logarithm of the passed expression log value of the logarithm of the passed value and base round number to number of decimal places rand random integer within the range specified by the minimum and maximum argument (inclusive) randf random float within the range specified by the minimum and maximum argument (inclusive) max largest value in a set of values min smallest value in a set of values clamp restricts an input value to a specified range scale_linear transforms a given value from an input domain to an output range using linear interpolation scale_exp transforms a given value from an input domain to an output range using an exponential curve floor rounds a number downwards ceil rounds a number upwards $pi pi as value for calculations
This group contains functions to convert on data type to another e.g string to integer, integer to string.
toint converts a string to integer number toreal converts a string to real number tostring convert number to string todatetime convert a string into Qt data time type todate convert a string into Qt data type totime convert a string into Qt time type tointerval converts a string to a interval type. Can be used to take days, hours, month, etc off a date
Date and Time Functions
This group contains functions for handling date and time data.
$now current date and time age difference between two dates year extract the year part from a date, or the number of years from a Interval month extract the month part from a date, or the number of months from a Interval week extract the week number from a date, or the number of weeks from a Interval day extract the day from a date, or the number of days from a Interval hour extract the hour from a datetime or time, or the number of hours from a Interval minute extract the minute from a datetime or time, or the number of minutes from a Interval second extract the second from a datetime or time, or the number of minutes from a Interval
This group contains functions that operate on strings e.g replace, convert to upper case.
lower convert string a to lower case upper convert string a to upper case title converts all words of a string to title case (all words lower case with leading capital letter) trim removes all leading and trailing whitespace (spaces, tabs, etc) from a string length length of string a replace returns a string with the the supplied string replaced regexp_replace(a,this,that) returns a string with the supplied regular expression replaced regexp_substr returns the portion of a string which matches a supplied regular expression substr(*a*,from,len) returns a part of a string concat concatenates several strings to one strpos returns the index of a regular expression in a string left returns a substring that contains the n leftmost characters of the string right returns a substring that contains the n rightmost characters of the string rpad returns a string with supplied width padded using the fill character lpad returns a string with supplied width padded using the fill character format formats a string using supplied arguments format_number returns a number formatted with the locale separator for thousands. Also truncates the number to the number of supplied places format_date formats a date type or string into a custom string format
This group contains functions for manipulating colors.
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 ramp_color returns a string representing a color from a color ramp 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_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
This group contains functions that operate on geometry objects e.g length, area.
xat retrieves a x coordinate of the current feature yat retrieves a y coordinate of the current feature $area returns the area size of the current feature $length returns the area size of the current feature $perimeter returns the perimeter length of the current feature $x returns the x coordinate of the current feature $y returns the y coordinate of the current feature $geometry returns the geometry of the current feature. Can be used for processing with other functions. geomFromWKT returns a geometry created from a Well-Known Text (WKT) representation. geomFromGML returns a geometry from a GML representation of geometry bbox disjoint returns 1 if the Geometries do not share any space together intersects returns 1 if the geometries spatially intersect (share any portion of space) and 0 if they don't touches returns 1 if the geometries have at least one point in common, but their interiors do not intersect crosses returns 1 if the supplied geometries have some, but not all, interior points in common. contains returns 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 overlaps returns 1 if the Geometries share space, are of the same dimension, but are not completely contained by each other. within returns 1 if the geometry a is completely inside geometry b buffer returns a geometry that represents all points whose distance from this geometry is less than or equal to distance centroid returns the geometric center of a geometry convexHull returns the convex hull of a geometry. It represents the minimum convex geometry that encloses all geometries within the set difference returns a geometry that represents that part of geometry a that does not intersect with geometry a distance returns the minimum distance (based on spatial ref) between two geometries in projected units intersection returns a geometry that represents the shared portion of geometry a and geometry b symDifference returns a geometry that represents the portions of a and b that do not intersect combine returns the combination of geometry a and geometry b union returns a geometry that represents the point set union of the geometries geomToWKT returns the Well-Known Text (WKT) representation of the geometry without SRID metadata
This group contains functions that operate on record identifiers.
$rownum returns the number of the current row $id returns the feature id of the current row $scale returns the current scale of the map canvas
Fields and Values
Contains a list of fields from the layer. Sample values can also be accessed via right-click.
Select the field name from the list then right-click to access context menu with options to load sample values from the selected field.