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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<TITLE>18.3. String Construction and Manipulation</TITLE>
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<meta name="description" value=" String Construction and Manipulation">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3608 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3606 HREF="node164.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3602 HREF="node166.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3610 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3611 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3609 HREF="node168.html"> Structures</A>
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<B>Up:</B> <A NAME=tex2html3607 HREF="node164.html"> Strings</A>
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<B> Previous:</B> <A NAME=tex2html3603 HREF="node166.html"> String Comparison</A>
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<HR> <P>
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<H1><A NAME=SECTION002230000000000000000>18.3. String Construction and Manipulation</A></H1>
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<P>
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Most of the interesting operations on strings may be performed
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with the generic sequence functions described in chapter <A HREF="node141.html#KSEQUE">14</A>.
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The following functions perform additional operations that are specific
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to strings.
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<p>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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Note that this remark, predating the design of the Common Lisp Object System,
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uses the term ``generic'' in a generic sense and not necessarily
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in the technical sense used by CLOS
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(see chapter <A HREF="node15.html#DTYPES">2</A>).
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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<BR><b>[Function]</b><BR>
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<tt>make-string <i>size</i> &key :initial-element</tt><P>This returns a string (in fact a simple string)
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of length <i>size</i>, each of whose characters
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has been initialized to the <tt>:initial-element</tt> argument.
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If an <tt>:initial-element</tt> argument is not specified, then the string will
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be initialized in an implementation-dependent way.
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<P>
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<hr>
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<b>Implementation note:</b> It may be convenient to initialize the string
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to null characters, or to spaces, or to garbage (``whatever was there'').
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<hr>
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<P>
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A string is really just a one-dimensional array of ``string
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characters'' (that is, those characters that are members of type
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<tt>string-char</tt>). More complex character arrays may be constructed using the
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function <tt>make-array</tt>.
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in March 1989 (CHARACTER-PROPOSAL) <A NAME=18277> </A>
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to eliminate the type <tt>string-char</tt> and to add a keyword
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argument <tt>:element-type</tt> to <tt>make-string</tt>. The new function
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description is as follows.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>make-string <i>size</i> &key :initial-element :element-type</tt><P>This returns a simple string
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of length <i>size</i>, each of whose characters
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has been initialized to the <tt>:initial-element</tt> argument.
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If an <tt>:initial-element</tt> argument is not specified, then the string will
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be initialized in an implementation-dependent way.
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<P>
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The <tt>:element-type</tt> argument names the type of the elements
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of the string; a string is constructed of the most specialized type
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that can accommodate elements of the given type. If <tt>:element-type</tt>
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is omitted, the type <tt>character</tt> is the default.
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<P>
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X3J13 voted in January 1989
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(ARGUMENTS-UNDERSPECIFIED) <A NAME=18288> </A>
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to clarify that the <i>size</i> argument
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must be a non-negative integer less than the value of
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<tt>array-dimension-limit</tt>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Function]</b><BR>
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<tt>string-trim <i>character-bag</i> <i>string</i> <BR></tt><tt>string-left-trim <i>character-bag</i> <i>string</i> <BR></tt><tt>string-right-trim <i>character-bag</i> <i>string</i></tt><P><tt>string-trim</tt> returns a substring of <i>string</i>, with all characters in
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<i>character-bag</i> stripped off the beginning and end.
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The function <tt>string-left-trim</tt> is similar but strips characters
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off only the beginning; <tt>string-right-trim</tt> strips off only the end.
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The argument <i>character-bag</i> may be any sequence containing
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characters.
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For example:
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<P><pre>
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(string-trim '(#\Space #\Tab #\Newline) " garbanzo beans
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") => "garbanzo beans"
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(string-trim " (*)" " ( *three (silly) words* ) ")
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=> "three (silly) words"
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(string-left-trim " (*)" " ( *three (silly) words* ) ")
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=> "three (silly) words* ) "
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(string-right-trim " (*)" " ( *three (silly) words* ) ")
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=> " ( *three (silly) words"
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</pre><P>
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If no characters need to be trimmed from the <i>string</i>,
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then either the argument <i>string</i> itself or a copy of it may
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be returned, at the discretion of the implementation.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1989 (STRING-COERCION) <A NAME=18308> </A>
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to clarify string coercion (see <tt>string</tt>).
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Function]</b><BR>
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<tt>string-upcase <i>string</i> &key :start :end <BR></tt><tt>string-downcase <i>string</i> &key :start :end <BR></tt><tt>string-capitalize <i>string</i> &key :start :end</tt><P><tt>string-upcase</tt> returns a string just like <i>string</i> with all lowercase
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characters replaced by the corresponding uppercase characters. More
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precisely, each character of the result string is produced by applying
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the function <tt>char-upcase</tt> to the corresponding character of
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<i>string</i>.
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<P>
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<tt>string-downcase</tt> is similar, except that uppercase characters are
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converted to lowercase characters (using <tt>char-downcase</tt>).
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<P>
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The keyword arguments <tt>:start</tt> and <tt>:end</tt> delimit the portion
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of the string to be affected. The result is always of the same length
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as <i>string</i>, however.
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<P>
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The argument is not destroyed. However, if no characters in the argument
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require conversion, the result may be either the argument or a copy of it,
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at the implementation's discretion.
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For example:
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<P><pre>
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(string-upcase "Dr. Livingstone, I presume?")
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=> "DR. LIVINGSTONE, I PRESUME?"
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(string-downcase "Dr. Livingstone, I presume?")
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=> "dr. livingstone, i presume?"
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(string-upcase "Dr. Livingstone, I presume?" <tt>:start</tt> 6 <tt>:end</tt> 10)
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=> "Dr. LiVINGstone, I presume?"
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</pre><P>
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<P>
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<tt>string-capitalize</tt> produces a copy of <i>string</i> such that,
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for every word in the copy, the first character of the word,
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if case-modifiable, is uppercase and
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any other case-modifiable characters in the word are lowercase.
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For the purposes of <tt>string-capitalize</tt>,
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a word is defined to be a
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consecutive subsequence consisting of alphanumeric characters or digits,
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delimited at each end either by a non-alphanumeric character
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or by an end of the string.
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For example:
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<P><pre>
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(string-capitalize " hello ") => " Hello "
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(string-capitalize
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¯"occlUDeD cASEmenTs FOreSTAll iNADVertent DEFenestraTION")
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=>\>"Occluded Casements Forestall Inadvertent Defenestration"
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(string-capitalize 'kludgy-hash-search) => "Kludgy-Hash-Search"
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(string-capitalize "DON'T!") => "Don'T!" ;<i>not</i> "Don't!"
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(string-capitalize "pipe 13a, foo16c") => "Pipe 13a, Foo16c"
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</pre><P>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1989 (STRING-COERCION) <A NAME=18333> </A>
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to clarify string coercion (see <tt>string</tt>).
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<hr>
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<b>Compatibility note:</b> Some very approximate Interlisp equivalents to
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<tt>string-upcase</tt>, <tt>string-downcase</tt>, and <tt>string-capitalize</tt>
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are <tt>u-case</tt>, <tt>l-case</tt> with second argument <tt>nil</tt>,
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and <tt>l-case</tt> with second argument <tt>t</tt>.
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<hr>
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<P>
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<BR><b>[Function]</b><BR>
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<tt>nstring-upcase <i>string</i> &key :start :end <BR></tt><tt>nstring-downcase <i>string</i> &key :start :end <BR></tt><tt>nstring-capitalize <i>string</i> &key :start :end</tt><P>These three functions are just like <tt>string-upcase</tt>,
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<tt>string-downcase</tt>, and <tt>string-capitalize</tt>
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but destructively modify the argument <i>string</i> by altering
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case-modifiable characters as necessary.
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<P>
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The keyword arguments <tt>:start</tt> and <tt>:end</tt> delimit the portion
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of the string to be affected. The argument <i>string</i> is returned as
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the result.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>string <i>x</i></tt><P>Most of the string
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functions effectively apply <tt>string</tt>
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to such of their arguments as are supposed to be
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strings.
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If <i>x</i> is a string, it is returned.
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If <i>x</i> is a symbol, its print name is returned.
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<p>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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If <i>x</i> is a string character (a character of type <tt>string-char</tt>),
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then a string containing that one character is returned.
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in March 1989 (CHARACTER-PROPOSAL) <A NAME=18365> </A>
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to eliminate the type <tt>string-char</tt> and to redefine the type
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<tt>string</tt> to be the union of one or more specialized vector
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types, the types of whose elements are subtypes of the type <tt>character</tt>.
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Presumably converting a character to a string always works according
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to this vote.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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In any other situation, an error is signaled.
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<P>
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To convert a sequence of characters to a string, use <tt>coerce</tt>.
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(Note that <tt>(coerce x 'string)</tt> will not succeed if <tt>x</tt> is a symbol.
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Conversely, <tt>string</tt> will not convert a list or other sequence
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to be a string.)
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<P>
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To get the string representation of a number or any other Lisp
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object, use <tt>prin1-to-string</tt>, <tt>princ-to-string</tt>,
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or <tt>format</tt>.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1989 (STRING-COERCION) <A NAME=18378> </A>
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to specify that the following functions perform coercion
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on their <i>string</i> arguments identical to that performed
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by the function <tt>string</tt>.
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<P>
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<pre>
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string= string-equal string-trim
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string< string-lessp string-left-trim
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string> string-greaterp string-right-trim
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string<= string-not-greaterp string-upcase
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string>= string-not-lessp string-downcase
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string/= string-not-equal string-capitalize
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</pre>
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<P>
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Note that <tt>nstring-upcase</tt>, <tt>nstring-downcase</tt>, and
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<tt>nstring-capitalize</tt> are absent from this list; because they modify destructively,
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the argument must be a string.
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<P>
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As part of the same vote X3J13 specified that <tt>string</tt>
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may perform additional implementation-dependent coercions
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but the returned value must be of type <tt>string</tt>.
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Only when no coercion is defined, whether standard or implementation-dependent,
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is <tt>string</tt> required to signal an error, in which case the error condition
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must be of type <tt>type-error</tt>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<P>
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<BR> <HR><A NAME=tex2html3608 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3606 HREF="node164.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3602 HREF="node166.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3610 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3611 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3609 HREF="node168.html"> Structures</A>
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<B>Up:</B> <A NAME=tex2html3607 HREF="node164.html"> Strings</A>
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<B> Previous:</B> <A NAME=tex2html3603 HREF="node166.html"> String Comparison</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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