.

The ^{Field Calculator} button in the attribute
table allows you to perform calculations on the basis of existing attribute values or
defined functions, for instance, to calculate length or area of geometry features. The
results can be written to a new attribute column, or they can be used to update
values in an existing column.

You will need to bring the vector layer into editing mode, before you can click on the field calculator icon to open the dialog (see figure_attributes_3). In the dialog, you first must 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 digits before the dot, then the dot and another 3 digits 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**. For 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, the **Field Values**
list opens 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 provide
several functions. In *Operators*, you find mathematical operators.
Look in *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 double click 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:

- Load the shapefile
`railroads.shp`in QGIS and press^{Open Attribute Table}. - Click on
^{Toggle editing mode}and open the^{Field Calculator}dialog. - Select the
*Create a new field*checkbox to save the calculations into a new field. - Add
`length`as Output field name and`real`as Output field type, and define Output field width to be 10 and Precision, 3. - Now double click on function
`$length`in the*Geometry*group to add it into the Field calculator expression box. - Complete the expression by typing ‘’/ 1000’’ in the Field calculator expression box and click
**[Ok]**. - You can now find a new column
`length`in the attribute table.

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.

**Operators**

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, or 2 fits into 7 three times with remainder 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
```

**Conditionals**

This group contains functions to handle conditional checks in expressions.

```
CASE evaluates multiple expressions and returns a result
CASE ELSE evaluates multiple expressions and returns 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
```

**Mathematical Functions**

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) sine of a
cos(a) cosine of a
tan(a) tangent of a
asin(a) arcsin of a
acos(a) arccos of a
atan(a) arctan of a
atan2(y,x) arctan of y/x using the signs of the two arguments
to determine the quadrant of the result
exp exponential of a 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 round 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
```

**Conversions**

This group contains functions to convert one 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 converts number to string
todatetime converts a string into Qt data time type
todate converts a string into Qt data type
totime converts a string into Qt time type
tointerval converts a string to an interval type (can be used to take days,
hours, months, 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 an interval
month extract the month part from a date, or the number of months from an interval
week extract the week number from a date, or the number of weeks from an interval
day extract the day from a date, or the number of days from an interval
hour extract the hour from a datetime or time, or the number
of hours from an interval
minute extract the minute from a datetime or time, or the number
of minutes from an interval
second extract the second from a datetime or time, or the number
of minutes from an interval
```

**String Functions**

This group contains functions that operate on strings (e.g., that 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 white space (spaces, tabs, etc.) from a string
length length of string a
replace returns a string with 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
```

**Color Functions**

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
```

**Geometry Functions**

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

```
xat retrieves an 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 length 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 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 (this 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 b
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
```

**Record Functions**

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 a context menu with options to load sample values from the selected field.