CLHS: Function REVERSE, NREVERSE

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Function REVERSE, NREVERSE

Syntax:

reverse sequence => reversed-sequence

nreverse sequence => reversed-sequence

Arguments and Values:

sequence---a proper sequence.

reversed-sequence---a sequence.

Description:

reverse and nreverse return a new sequence of the same kind as sequence, containing the same elements, but in reverse order.

reverse and nreverse differ in that reverse always creates and returns a new sequence, whereas nreverse might modify and return the given sequence. reverse never modifies the given sequence.

For reverse, if sequence is a vector, the result is a fresh simple array of rank one that has the same actual array element type as sequence. If sequence is a list, the result is a fresh list.

For nreverse, if sequence is a vector, the result is a vector that has the same actual array element type as sequence. If sequence is a list, the result is a list.

For nreverse, sequence might be destroyed and re-used to produce the result. The result might or might not be identical to sequence. Specifically, when sequence is a list, nreverse is permitted to setf any part, car or cdr, of any cons that is part of the list structure of sequence. When sequence is a vector, nreverse is permitted to re-order the elements of sequence in order to produce the resulting vector.

Examples:

     (setq str "abc") =>  "abc"
     (reverse str) =>  "cba"
     str =>  "abc"
     (setq str (copy-seq str)) =>  "abc"
     (nreverse str) =>  "cba"
     str =>  implementation-dependent
     (setq l (list 1 2 3)) =>  (1 2 3)
     (nreverse l) =>  (3 2 1)
     l =>  implementation-dependent

Side Effects:

nreverse might either create a new sequence, modify the argument sequence, or both. (reverse does not modify sequence.)

Affected By: None.

Exceptional Situations:

Should be prepared to signal an error of type type-error if sequence is not a proper sequence.

See Also: None.

Notes: None.

The following X3J13 cleanup issue, not part of the specification, applies to this section:

- REMF-DESTRUCTION-UNSPECIFIED:X3J13-MAR-89

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