filter() Function

Calls a predicate for each item in a list and returns any items for which the returned value is true.

Parameters

filter( predicate, list, context )

Returns

Any Type Array

Using the function

The following sections cover important details about the filter() function.

Using the predicate parameter

In the predicate parameter of filter(), enter fn!functionName or a!functionName to reference an expression function, or rule!ruleName to reference a rule.

Using rules or functions with multiple arguments

To use rules or functions that take more than two arguments, use the merge() function. For example, given a rule g(x, y, z), reduce(rule!g, i, merge({a, b, c}, {d, e, f})) returns g(g(g(i, a, d), b, e), c, f).

Unsupported functions

The filter() function doesn't support a!forEach() in rules that you use as the predicate.

Examples

The following examples demonstrate the filter() function.

To experiment with examples, copy and paste the expression into an expression rule and click TEST RULE to see the output.

Filtering a list

This example filters out null and empty values from a list using a!isNotNullOrEmpty as the predicate.

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filter(
  a!isNotNullOrEmpty,
  {1, 2, null, 3, null, 4}
)

Returns {1, 2, 3, 4}.

Filtering with context

To help demonstrate how the context parameter works in looping expressions, let's look at an example.

Imagine you're building an expression that uses looping to analyze group membership in your application. You have four Group constants configured in the following ways:

The following expressions reference these constants to show how context works when checking items in a list.

Filtering a list with context

This example uses the context parameter to pass fixed values into each evaluation of the predicate.

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filter(
  a!isUserMemberOfGroup,
  {"user1", "user2", "user3"},
  cons!csaAdmins
)

Returns {"user1", "user3"} since user1 and user3 are members of the group referenced by cons!csaAdmins.

Filtering a list with context for more than 2 parameters

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filter(
  a!isUserMemberOfGroup,
  {"user1", "user2", "user3"},
  {cons!allGroups, true}
)

Returns {"user1"} when only user1 is a member of all the groups referenced in cons!allGroups. a!isUserMemberOfGroup has 3 parameters: username, groups, and matchAllGroups. The filter() list corresponds to username, and the list of values provided in the context parameter are used as the next two parameters, groups & matchAllGroups.

Filtering a list with context for more than two parameters which accept lists

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filter(
  a!isUserMemberOfGroup,
  {"user1", "user2", "user3"},
  merge({cons!csaAdmins, cons!csaProfs}, {true, true})
)

Returns {"user1"} when only user1 is a member of the groups referenced in cons!csaAdmins and cons!csaProfs. The merge() function maps the list values to the correct parameters in this example.

Filtering a list with a custom expression rule

This example filters dates using a helper rule, rule!isDateLater(value, referenceDate), which is defined as:

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ri!value > ri!referenceDate

Example filter:

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filter(
  rule!isDateLater,
  {date(1950, 2, 3), date(1950, 6, 23), date(1950, 5, 2)},
  date(1950, 4, 7)
)

Returns {6/23/1950, 5/2/1950}

Feature compatibility

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