Iterators

| OPENMP API Specification: Version 5.0 November 2018

2.1.6 Iterators

Iterators are identifiers that expand to multiple values in the clause on which they appear.

The syntax of the iterator modifier is as follows:

 
iterator(iterators-definition)

where iterators-definition is one of the following:

 
iterator-specifier [, iterators-definition ]

where iterator-specifier is one of the following:

 
[ iterator-type ]  identifier = range-specification

where:

identifier is a base language identifier.

∙ iterator-type is a type name.

∙ iterator-type is a type specifier.
range-specification is of the form begin:end[:step], where begin and end are expressions for which their types can be converted to iterator-type and step is an integral expression.

In an iterator-specifier, if the iterator-type is not specified then the type of that iterator is of int type.

In an iterator-specifier, if the iterator-type is not specified then the type of that iterator is default integer.

In a range-specification, if the step is not specified its value is implicitly defined to be 1.

An iterator only exists in the context of the clause in which it appears. An iterator also hides all accessible symbols with the same name in the context of the clause.

The use of a variable in an expression that appears in the range-specification causes an implicit reference to the variable in all enclosing constructs.

The values of the iterator are the set of values i₀, …, i_N-1 where:

i₀ = (iterator-type) begin,
i_j = (iterator-type) (i_j-1 + step), and
if step > 0,
- i₀ < (iterator-type) end,
- i_N-1 < (iterator-type) end, and
- (iterator-type) (i_N-1 + step) ≥ (iterator-type) end;
if step < 0,
- i₀ > (iterator-type) end,
- i_N-1 > (iterator-type) end, and
- (iterator-type) (i_N-1 + step) ≤ (iterator-type) end.

The values of the iterator are the set of values i₁, …, i_N where:

i₁ = begin,
i_j = i_j-1 + step, and
if step > 0,
- i₁ ≤ end,
- i_N ≤ end, and
- i_N + step > end;
if step < 0,
- i₁ ≥ end,
- i_N ≥ end, and
- i_N + step < end.

The set of values will be empty if no possible value complies with the conditions above.

For those clauses that contain expressions that contain iterator identifiers, the effect is as if the list item is instantiated within the clause for each value of the iterator in the set defined above, substituting each occurrence of the iterator identifier in the expression with the iterator value. If the set of values of the iterator is empty then the effect is as if the clause was not specified.

The behavior is unspecified if i_j + step cannot be represented in iterator-type in any of the i_j + step computations for any 0 ≤ j < N in C/C++ or 0 < j ≤ N in Fortran.

Restrictions

An expression that contains an iterator identifier can only appear in clauses that explicitly allow expressions that contain iterators.
The iterator-type must not declare a new type.

∙ The iterator-type must be an integral or pointer type. ∙ The iterator-type must not be const qualified.

∙ The iterator-type must be an integer type.
If the step expression of a range-specification equals zero, the behavior is unspecified.
Each iterator identifier can only be defined once in an iterators-definition.
Iterators cannot appear in the range-specification.

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