You use at in OCL when you need the item at a specified one-based position in a collection.

Syntax

collection->at(index : Integer) : T

collection is the collection to read from. index is an integer position, and T is the type of the returned element.

The first position is 1, not 0.

Get an item by position

1. Start with the collection whose item you want to retrieve.
2. Choose its one-based position: use 1 for the first item, 2 for the second item, and so on.
3. Call at with that position.

For example, this expression retrieves the first Department instance:

Department.allInstances->at(1)

The result is one Department: the element at position 1 in the collection returned by Department.allInstances.

One-based indexing

Expression	Position requested
Department.allInstances->at(1)	First element
Department.allInstances->at(2)	Second element

Do not use at(0) when you mean the first element with this operator. If your expression uses zero-based indexing, use at0 instead:

Department.allInstances->at0(0)

This retrieves the first element using the zero-based at0 convention.

Choose between at and at0

Operator	First element	Second element
at	at(1)	at(2)
at0	at0(0)	at0(1)

Keep the indexing convention consistent within an expression. For example, if you select the first item with at(1), use at(2) to select the next item; do not switch to at0(1) unless you intend to use zero-based indexing.

See also

• Documentation:Learn OCL
• Documentation:OCLOperators at0
• Documentation:OCLOperators atTime