prefixp

Arguments: prefix sequence &optional prefix-length

The default methods for all types of sequences do the following: prefix and sequence must be sequences. prefix-length must be nil or a fixnum less than or equal to the length or prefix. In what follows, prefix-used is prefix if prefix-length is nil and (subseq prefix 0 prefix-length) if prefix-length is a fixnum.

This function returns a non-nil value if sequence starts with prefix-used and returns nil otherwise. nil is always returned if prefix-used is longer than sequence.

The value returned when sequence does start with prefix-used is the length of prefix-used. If prefix-used is shorter than sequence, this value is also the index of the element just after the prefix-used portion of sequence.

Examples

(prefixp "foo" "foobar") => 3 ;; The length of "foo" and the
                                 ;; index of #\b in "foobar"
(prefixp "foo" "foo") => 3 ;; the length of "foo"
(prefixp "foo1" "foo2") => nil
(prefixp "foo1" "foo") => nil
(prefixp '(1 2 3) '(1 2 3 4 5 6 7)) => 3
(prefixp (list #\f) "foo")  => 1
(prefixp "f" (vector \#f #\o)) => 1

;; Some arguments always return the same value.
;; So if PREFIX-USED has length 0, 0 is returned:
(prefixp "" [any-sequence]) -> 0
(prefixp nil [any-sequence]) -> 0
(prefixp [any-sequence] [any-sequence] 0) -> 0

;; Examples with prefix-length non-nil:
(prefixp "foo" "foo2" 3) -> 3
(prefixp "foo1" "foo2" 3) -> 3
(prefixp "foo12345" "foo2" 3) -> 3
(prefixp "foo12345" "foo2" 4) -> nil

;; USE CASE: Suppose the value of a variable is a string made up 
;; of an OS name followed by a version number. Then if PREFIX
;; is a particular OS name, we can test for whether the OS 
;; named in the variable is an instance of that OS:

(defvar *os-version* "CENTOS-4.5.21")
(defun centos-p (str) (prefixp "CENTOS" str))
(centos-p *os-version*) -> 6 ;; that is true
(setq *os-version* "REDHAT-4.5.21")
(centos-p *os-version*) -> nil

Copyright (c) 1998-2019, Franz Inc. Oakland, CA., USA. All rights reserved.
This page was not revised from the 10.0 page.
Created 2019.8.20.