Common Lisp the Language, 2nd Edition

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# 14.1. Simple Sequence Functions

Most of the following functions perform simple operations on a single sequence; make-sequence constructs a new sequence.

[Function]
elt sequence index

This returns the element of sequence specified by index, which must be a non-negative integer less than the length of the sequence as returned by length. The first element of a sequence has index 0.

(Note that elt observes the fill pointer in those vectors that have fill pointers. The array-specific function aref may be used to access vector elements that are beyond the vector's fill pointer.)

setf may be used with elt to destructively replace a sequence element with a new value.

[Function]
subseq sequence start &optional end

This returns the subsequence of sequence specified by start and end. subseq always allocates a new sequence for a result; it never shares storage with an old sequence. The result subsequence is always of the same type as the argument sequence.

setf may be used with subseq to destructively replace a subsequence with a sequence of new values; see also replace.

[Function]
copy-seq sequence

A copy is made of the argument sequence; the result is equalp to the argument but not eq to it.

```(copy-seq x) == (subseq x 0)
```

but the name copy-seq is more perspicuous when applicable.

[Function]
length sequence

The number of elements in sequence is returned as a non-negative integer. If the sequence is a vector with a fill pointer, the ``active length'' as specified by the fill pointer is returned (see section 17.5).

[Function]
reverse sequence

The result is a new sequence of the same kind as sequence, containing the same elements but in reverse order. The argument is not modified.

[Function]
nreverse sequence

The result is a sequence containing the same elements as sequence but in reverse order. The argument may be destroyed and re-used to produce the result. The result may or may not be eq to the argument, so it is usually wise to say something like (setq x (nreverse x)), because simply (nreverse x) is not guaranteed to leave a reversed value in x.

X3J13 voted in March 1989 (REMF-DESTRUCTION-UNSPECIFIED)   to clarify the permissible side effects of certain operations. When the sequence is a list, nreverse is permitted to perform a setf on any part, car or cdr, of the top-level list structure of that list. When the sequence is an array, nreverse is permitted to re-order the elements of the given array in order to produce the resulting array.

[Function]
make-sequence type size &key :initial-element

This returns a sequence of type type and of length size, each of whose elements has been initialized to the :initial-element argument. If specified, the :initial-element argument must be an object that can be an element of a sequence of type type. For example:

```(make-sequence '(vector double-float)
100
:initial-element 1d0)
```

If an :initial-element argument is not specified, then the sequence will be initialized in an implementation-dependent way.

X3J13 voted in January 1989 (ARGUMENTS-UNDERSPECIFIED)   to clarify that the type argument must be a type specifier, and the size argument must be a non-negative integer less than the value of array-dimension-limit.

X3J13 voted in June 1989 (SEQUENCE-TYPE-LENGTH)   to specify that make-sequence should signal an error if the sequence type specifies the number of elements and the size argument is different.

X3J13 voted in March 1989 (CHARACTER-PROPOSAL)   to specify that if type is string, the result is the same as if make-string had been called with the same size and :initial-element arguments.

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