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ANSI Common Lisp   17 Sequences   17.3 Dictionary of Sequences
17.3.15 position, position-if, position-if-not Function
Syntax:
position item sequence &key from-end test test-not start end key    position

position-if predicate sequence &key from-end start end key    position

position-if-not predicate sequence &key from-end start end key    position

Arguments and Values:
item - an object.

sequence - a proper sequence.

predicate - a designator for a function of one argument that returns a generalized boolean.

from-end - a generalized boolean. The default is false.

test - a designator for a function of two arguments that returns a generalized boolean.

test-not - a designator for a function of two arguments that returns a generalized boolean.

start, end - bounding index designators of sequence. The defaults for start and end are 0 and nil, respectively.

key - a designator for a function of one argument, or nil.

position - a bounding index of sequence, or nil.

Description:
position, position-if, and position-if-not each search sequence for an element that satisfies the test.

The position returned is the index within sequence of the rightmost (if from-end is true [see correction note below]) or of the leftmost (if from-end is false) element that satisfies the test; otherwise nil is returned. The index returned is relative to the left-hand end of the entire sequence, regardless of the value of start, end, or from-end.

Examples:
 (position #\a "baobab" :from-end t)  4
 (position-if #'oddp '((1) (2) (3) (4)) :start 1 :key #'car)  2
 (position 595 '())   NIL
 (position-if-not #'integerp '(1 2 3 4 5.0))  4 
Exceptional Situations:
Should be prepared to signal an error of type type-error if sequence is not a proper sequence.
See Also:
find, Section 3.6 Traversal Rules and Side Effects
Notes:
The :test-not argument is deprecated.

The function position-if-not is deprecated.

Correction (not part of ANS):
The text under Description "The position returned is the index within sequence of the rightmost (if from-end is true..." has been corrected. It originally read `leftmost...rightmost'.
Allegro CL Implementation Details:
None.

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