Form Operators and Functions

Expressions in calculations, constraints, and relevants can contain operators and functions.

Operators

Math operators

	Explanation	Example
+	addition	${salary_income} + ${self_employed_income}
-	subtraction	${income} - ${expenses}
*	multiplication	${bill} * 1.18
div	division	${percent_int} div 100
mod	modulo (division remainder)	(${even_number} mod 2) = 0

Warning

Math operators only work with numbers.

• The addition operator cannot be used to concatenate strings. Use the concat() function instead.

• Empty values (that is, variables referencing unanswered questions) are actually empty strings, and will not be automatically converted to zero (0).

Comparison operators

Comparison operators are used to compare values. The result of a comparison is always True or False.

	Explanation	Example	Notes
=	equal to	${enrolled} = 'yes'	Can compare numbers or strings.
!=	not equal to	${enrolled} != 'yes'	Can compare numbers or strings.
>	greater than	${age} > 17
>=	greater than or equal to	${age} >= 18
<	less than	${age} < 65
<=	less than or equal to	${age} <= 64

Warning

• The relational operators (>, >=, <, <=) only work with numbers.

• Empty response values are not automatically converted to zero (0).

Boolean operators

Boolean operators combine two True or False values into a single True or False value.

	Explanation	Example
and	True if the expressions before and after are True	${age} > -1 and ${age} < 120
or	True if either of the expressions before or after are True	${age} < 19 or ${age} > 64

Path operators

	Explanation	Example	Notes
.	current question's value	. >= 18	Used in constraints.
..	current question's parent group	position(..)	Used with position() to get a parent repeat instance's index.

Note

Formally, these are not operators but rather XPath references to the current node (.) and the parent node (..). XPath paths can be used to reference nodes of a form.

Functions

See also

Functions in the ODK XForm Specification

Control flow

if(expression, then, else)

Returns then if expression evaluates to True. Otherwise, returns else.

position(xpath)

Returns an integer equal to the 1-indexed position of the current node within the node defined by xpath.

Most often this is used in the form position(..) to identify the current iteration index within a repeat group.

XLSForm

type	name	label	repeat_count	calculation
note	person_list_note	Please list the names of the people in your household.
begin_repeat	person	Member of household
text	name	Name
end_repeat
begin_repeat	person_details	Details	count(${person})
calculate	current_name			indexed-repeat(${name}, ${person}, position(..))
date	member_bday	Birthday of ${current_name}
end_repeat

once(expression)

Returns the value expression if the question's value is empty. Otherwise, returns the current value of the question.

This can be used to ensure that a random number is only generated once, or to store the first value entered for a question in a way that is retrievable even if the response is changed later.

Warning

This function is often misunderstood. Read when expressions are evaluated to learn more.

Accessing response values

Note

The response from most question types can be accessed using variables. Functions are needed for accessing responses to multi select questions and questions inside repeat groups.

Select questions

selected(space_delimited_array, string)

Returns True if string is a member of space_delimited_array, otherwise returns False.

Commonly used to determined if a specific choice was selected in a select question. (This is possible because a reference to a select question returns a space-delimited array of choice names.)

XLSForm

survey

type	name	label	hint	relevant	constraint
select_multiple medical_issues	what_issues	Have you experienced any of the following?	Select all that apply.
select_multiple cancer_types	what_cancer	What type of cancer have you experienced?	Select all that apply.	selected(${what_issues}, 'cancer')
select_multiple diabetes_types	what_diabetes	What type of diabetes do you have?	Select all that apply.	selected(${what_issues}, 'diabetes')
begin_group	blood_pressure	Blood pressure reading	selected(${what_issues}, 'hypertension')
integer	systolic_bp	Systolic			. > 40 and . < 400
integer	diastolic_bp	Diastolic			. >= 20 and . <= 200
end_group
text	other_health	List other issues.		selected(${what_issues}, 'other')
note	after_health_note	This note is after all health questions.

choices

list_name	name	label
medical_issues	cancer	Cancer
medical_issues	diabetes	Diabetes
medical_issues	hypertension	Hypertension
medical_issues	other	Other
cancer_types	lung	Lung cancer
cancer_types	skin	Skin cancer
cancer_types	prostate	Prostate cancer
cancer_types	breast	Breast cancer
cancer_types	other	Other
diabetes_types	type_1	Type 1 (Insulin dependent)
diabetes_types	type_2	Type 2 (Insulin resistant)

selected-at(space_delimited_array, n)

Returns the string at the n ^th position of the space_delimited_array. (The array is zero-indexed.) Returns an empty string if the index does not exist.

This can be used to get the name of a selected choice from a multi-select question. (This is possible because a reference to a select question returns a space-delimited array of choice names.)

Note

If used to get a choice name from a select question, this function returns the name, not the label, of the selected choice. To get the label in the current language, use jr:choice-name().

XLSForm

survey

type	name	label	hint	calculation
select_multiple colors	color_prefs	What colors do you like?	Select three.
calculate	color_0			selected-at(${color_prefs}, 0)
calculate	color_1			selected-at(${color_prefs}, 1)
calculate	color_2			selected-at(${color_prefs}, 2)
note	color_note	Selected colors:	${color_0} <br> ${color_1} <br> ${color_2}

choices

list_name	name	label
colors	red	Red
colors	blue	Blue
colors	yellow	Yellow
colors	green	Green
colors	orange	Orange
colors	purple	Purple

count-selected(multi_select_question)

Returns the number of choices selected in multi_select_question.

XLSForm

survey

type	name	label	hint	constraint	constraint_message
select_multiple colors	color_prefs	What colors do you like?	Select three.	count-selected(.)=3	Select exactly three.

choices

list_name	name	label
colors	red	Red
colors	blue	Blue
colors	yellow	Yellow
colors	green	Green
colors	orange	Orange
colors	purple	Purple

jr:choice-name(choice_name, 'select_question')

Returns the label value, in the active language, associated with the choice_name in the list of choices for the select_question.

Note

You have to wrap the select_question reference in quotes.

'${question_name}'

XLSForm

survey

type	name	label::English	label::Español	hint::English	hint:Español	calculation
select_multiple colors	color_prefs	What colors do you like?	¿Qué colores te gustan?	Select three.	Seleccione tres.
calculate	color_0					jr:choice-name( selected-at(${color_prefs}, 0), '${color_prefs}')
calculate	color_1					jr:choice-name( selected-at(${color_prefs}, 1), '${color_prefs}')
calculate	color_2					jr:choice-name( selected-at(${color_prefs}, 2), '${color_prefs}')
note	color_note	Selected colors:	Colores seleccionados:	${color_0} <br> ${color_1} <br> ${color_2}	${color_0} <br> ${color_1} <br> ${color_2}

choices

list_name	name	label::English	label::Español
colors	red	Red	Rojo
colors	blue	Blue	Azul
colors	yellow	Yellow	Amarillo
colors	green	Green	Verde
colors	orange	Orange	Anaranjado
colors	purple	Purple	Púrpura

Repeat groups

Helpful terms

nodeset

A collection of XML nodes. In XLSForms, this is typically a collection of response values.

Outside a repeat group, referring to a question by name will return a nodeset containing all the responses to that question.

Nodesets can also be created by joining two or more nodes with pipes: /data/age | /data/name.

indexed-repeat(name, group, i[, sub_grp, sub_i[, sub_sub_grp, sub_sub_i]])

Returns the response value of question name from the repeat-group group, in iteration i.

Nested repeat groups can be accessed using the sub and sub_sub parameters.

See also

Referencing repeated questions from inside the repeat

XLSForm

type	name	label	repeat_count	calculation
note	person_list_note	Please list the names of the people in your household.
begin_repeat	person	Member of household
text	name	Name
end_repeat
begin_repeat	person_details	Details	count(${person})
calculate	current_name			indexed-repeat(${name}, ${person}, position(..))
date	member_bday	Birthday of ${current_name}
end_repeat

count(nodeset)

Returns the number of items in nodeset. This can be used to count the number of repetitions in a repeat group.

XLSForm

type	name	label	repeat_count	calculation
note	person_list_note	Please list the names of the people in your household.
begin_repeat	person	Member of household
text	name	Name
end_repeat
begin_repeat	person_details	Details	count(${person})
calculate	current_name			indexed-repeat(${name}, ${person}, position(..))
date	member_bday	Birthday of ${current_name}
end_repeat

count-non-empty(nodeset)

Returns the number of non-empty members of nodeset.

sum(nodeset)

Returns the sum of the members of nodeset.

Can be used to tally responses to a repeated select question.

XLSForm

survey

type	name	label	calculation
begin_repeat	guest_details	Guest details
text	guest_name	Guest name
select_one meal_options	meal_preference	Meal preference
calculate	chkn		if(${meal_preference} = 'chicken', 1, 0 )
calculate	fsh		if(${meal_preference} = 'fish', 1, 0 )
calculate	veg		if(${meal_preference} = 'vegetarian', 1, 0 )
end_repeat
calculate	chkn_count		sum(${chkn})
calculate	fsh_count		sum(${fsh})
calculate	veg_count		sum(${veg})

choices

list_name	name	label
meal_options	chicken	Chicken
meal_options	fish	Fish
meal_options	vegetarian	Vegetarian

max(nodeset)

Returns the largest member of nodeset.

XLSForm

survey

type	name	label	calculation
begin_repeat	child_questions	Questions about child
text	child_name	Child's name
integer	child_age	Child's age
end_repeat
calculate	age_of_oldest_child		max(${child_age})

min(nodeset)

Returns the smallest member of nodeset.

XLSForm

survey

type	name	label	calculation
begin_repeat	child_questions	Questions about child
text	child_name	Child's name
integer	child_age	Child's age
end_repeat
calculate	age_of_youngest_child		min(${child_age})

Warning

The min() and max() functions only work sets of numbers. Empty values (that is, variables referencing unanswered questions) are actually empty strings, and will not be automatically converted to zero (0).

Strings

Searching and matching strings

regex(string, expression)

Returns True if string is an exact and complete match for expression.

See also

Using regular expressions

XLSForm

survey

type	name	label	constraint	constraint_message
text	middle_initial	What is your middle initial?	regex(., '\p{L}')	Just the first letter.

contains(string, substring)

Returns True if the string contains the substring.

starts-with(string, substring)

Returns True if string begins with substring.

ends-with(string, substring)

Returns True if the string ends with substring.

substr(string, start[, end])

Returns the substring of string beginning at the index start and extending to (but not including) index end (or to the termination of string, if end is not provided). Members of string are zero-indexed.

substring-before(string, target)

Returns the substring of string before the first occurrence of the target substring. If the target is not found, or string begins with the target substring, then this will return an empty string.

substring-after(string, target)

Returns the substring of string after the first occurrence of the target substring. If the target is not found this will return an empty string.

translate(string, fromchars, tochars)

Returns a copy of string, where every occurrence of a character in fromchars is replaced by the corresponding character in tochars. If fromchars is longer than tochars then every occurrence of a character in fromchars that does not have a corresponding character in tochars will be removed.

string-length(string)

Returns the number of characters in string. If no value is passed in, returns the number of characters in the value of the question that this function call is tied to which can be useful in a constraint expression.

normalize-space(string)

Returns a string with normalized whitespace by stripping leading and trailing whitespace of string and replacing sequences of whitespace characters with a single space. If no value is passed in, normalizes whitespace of the value of the question that this function call is tied to.

Combining strings

concat(arg [, arg [, arg [, arg [...]]]])

Concatenates one or more arguments into a single string. If any arg is a nodeset, the values within the set are concatenated into a string.

join(separator, nodeset)

Joins the members of nodeset, using the string separator.

Converting to and from strings

boolean-from-string(string)

Returns True if string is "true" or "1". Otherwise, False.

string(arg)

Converts arg to a string.

Encoding and decoding strings

digest(data, algorithm, encoding method (optional))

Computes and returns the hash value of the data string using the indicated hash algorithm string, and encoding this hash value using the optional encoding string.

Options for the algorithm are MD5, SHA-1, SHA-256, SHA-384, SHA-512.

If the third parameter is not specified, the encoding is base64. Valid options for the encoding are base64 and hex.

This function can be useful if, for example, someone wants to build a unique identifier from sensitive data like a national ID number without compromising that data.

base64-decode(string)

Decodes all bytes from the input using the Base64 encoding scheme, assuming that the encoded bytes represent UTF-8 characters. Returns a UTF-8 character string.

extract-signed(string, public-key)

Given a base64-encoded, signed string and public key as inputs, verifies that the first 64 bytes are a valid Ed25519 signature. If the signature is valid, returns the message (non-signature) portion of the contents as a UTF-8 string. If the signature is not valid, returns an empty string.

Math

Warning

Math functions (except number()) only work with number values.

You can use number() to convert a string of digits to a number, but it is usually better to get a number value directly.

Empty values (that is, variables referencing unanswered questions) are actually empty strings, and will not be automatically converted to zero (0).

Number handling

round(number, places)

Rounds a decimal number to some number of decimal places.

int(number)

Truncates the fractional portion of a decimal number to return an integer.

number(arg)

Converts arg to number value.

If arg is a string of digits, returns the number value.

If arg is True, returns 1. If arg is False, returns 0.

If arg cannot be converted, returns NaN (not a number).

Calculation

pow(number, power)

Raises a number to a power.

log(number)

Returns the natural log of number.

log10(number)

Returns the base-10 log of number.

abs(number)

Returns the absolute value of number.

sin(number)

Returns the sine of number.

cos(number)

Returns the cosine of number.

tan(number)

Returns the tangent of number.

asin(number)

Returns the arc sine of number.

acos(number)

Returns the arc cosine of number.

atan(number)

Returns the arctan of number.

atan2(y, x)

Returns the multi-valued inverse tangent of y, x.

sqrt(number)

Returns the square root of number.

exp(x)

Returns e^x.

exp10(x)

Returns 10^x.

pi()

Returns an approximation of the mathematical constant π.

Date and time

today()

Returns the current date without a time component.

now()

Returns the current datetime in ISO 8601 format, including the timezone.

Warning

This function is often misused. Read when expressions are evaluated to learn more.

Converting dates and time

decimal-date-time(dateTime)

Converts dateTime value to the number of days since January 1, 1970 (the Unix Epoch).

This is the inverse of date().

date(days)

Converts an integer representing a number of days from January 1, 1970 (the Unix Epoch) to a standard date value.

This is the inverse of decimal-date-time().

decimal-time(time)

Converts time to a number representing a fractional day. For example, noon is 0.5 and 6:00 PM is 0.75.

Formatting dates and times for display

format-date(date, format)

Returns date as a string formatted as defined by format.

The following identifiers are used in the format string:

%Y	4-digit year
%y	2-digit year
%m	0-padded month
%n	numeric month
%b	short text month (Jan, Feb, Mar…)
%d	0-padded day of month
%e	day of month
%a	short text day (Sun, Mon, Tue…).

Note

Month and day abbreviations are language and locale specific. If form locale can be determined, that locale will be used. Otherwise, the device locale will be used.

format-date-time(dateTime, format)

Returns dateTime as a string formatted as defined by format.

The identifiers list in format-date() are available, plus the following:

%H	0-padded hour (24-hr time)
%h	hour (24-hr time)
%M	0-padded minute
%S	0-padded second
%3	0-padded millisecond ticks.

Geography

area(nodeset | geoshape)

Returns the area, in square meters, of either a nodeset of geopoints or a geoshape value.

It takes into account the circumference of the Earth around the Equator but does not take altitude into account.

distance(nodeset | geoshape | geotrace | geopoint, geopoint [, geopoint [...]])

Returns the distance, in meters, of either:

• a nodeset of geopoints

survey

type	name	label	calculation
begin_repeat	points
geopoint	point	Record a Geopoint
end_repeat
calculate	calculated_distance		distance(${point})
note	display_distance	Calculated distance: ${calculated_distance}

• the perimeter of a geoshape

survey

type	name	label	calculation
geoshape	shape	Record a Geoshape
calculate	calculated_distance		distance(${shape})
note	display_distance	Calculated distance: ${calculated_distance}

• the length of a geotrace value

survey

type	name	label	calculation
geotrace	trace	Record a Geotrace
calculate	calculated_distance		distance(${trace})
note	display_distance	Calculated distance: ${calculated_distance}

• a list of geopoints either specified as strings or references to other fields

survey

type	name	label	calculation
geopoint	point1	Record a Geopoint
geopoint	point2	Record a Geopoint
calculate	dist		distance(${point1}, ${point2})
note	dist_note	Calculated distance: ${dist}

The distance function takes into account the circumference of the Earth around the equator but does not take altitude into account. The longer the line segments are, the less accurate the computed distance will be. Additionally, distance calculations closer to the equator are more accurate than ones close to the poles.

You can use the distance function for things like basic reverse geocoding and basic geofencing. See this sample form for multiple examples.

geofence(geopoint, geoshape)

Returns True if the specified point is inside the specified geoshape, False otherwise.

survey

type	name	label	relevant	appearance
geoshape	shape	Specify a shape to use as a fence
geopoint	point	Select a point to see whether it is in the fence		placement-map
note	in_note	Point is inside the fence	geofence(${point}, ${shape})
note	out_note	Point is outside the fence	${point} != '' and not(geofence(${point}, ${shape}))

You can also find this example in Google Sheets

The geofence function is helpful for things like validating that a data collector is the expected location when filling out a form. If you don't need to define precise boundaries, you can instead use the distance() function to validate that a data collector is within a certain distance of the center of the target location.

If you need to validate that data collectors are at an indoor location, keep in mind that location capture is generally inaccurate indoors unless there are cellular and WiFi signals available. You can address this by asking data collector to capture the location of the building front door before entering or by defining a fence that is, for example, 5 to 10 meters outside the real boundaries of the building.

Utility

random()

Returns a random number between 0.0 (inclusive) and 1.0 (exclusive).

Warning

This function is often misused. Read when expressions are evaluated to learn more.

randomize(nodeset[, seed])

Returns a shuffled nodeset.

A shuffle with a numeric seed is deterministic and reproducible.

The primary use for this function is to randomize the order of choices for a select question. The documentation on select widgets describes how this is done in XLSForm.

randomize() can only be used in a context where a nodeset is accepted. Note that questions of type calculate cannot reference a nodeset.

uuid([length])

Without argument, returns a random RFC 4122 version 4 compliant UUID.

With an argument it returns a random GUID of specified length.

boolean(arg)

Returns True if arg is:

• a number other than zero

• a non-empty string

• a non-empty collection

• a comparison or expressions that evaluates to True.

Returns False if arg is:

• the number 0

• an empty string

• an empty collection

• a comparison or expression that evaluates to False.

not(arg)

Returns the opposite of boolean(arg).

coalesce(arg1, arg2)

Returns first non-empty value of the two arg s. Returns an empty string if both are empty or non-existent.

checklist(min, max, response[, response[, response[, ...]]])

Returns True if the number of response s that are exactly the string "yes" is between min and max, inclusive.

Set min or max to -1 to make the argument not applicable.

weighted-checklist(min, max, reponse, weight[, response, weight[, response, weight[, response, weight[, ... ]]])

Returns True if the sum of the weight s of each response that is exactly the string "yes" is between min and max, inclusive.

Set min or max to -1 to make the argument not

true()

Evaluates to True.

false()

Evaluates to False.

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