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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html> <head> <meta name="generator" content="HTML Tidy for Linux/x86 (vers 1st October 2002), see www.w3.org"> <title>Standard C String & Character</title> </head> <body bgcolor="#ffffff"> <table width="100%" bgcolor="#eeeeff"> <tr> <td><a href="index.html">cppreference.com</a> -> <a href="stdstring.html">Standard C String & Character</a> -> Details</td> </tr> </table> <h1>Standard C String & Character</h1> <hr> <h2><a name="atof">atof</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <stdlib.h> double atof( const char *str );</pre> </td> </tr> </table> <p>The function atof() converts <i>str</i> into a double, then returns that value. <i>str</i> must start with a valid number, but can be terminated with any non-numerical character, other than "E" or "e". For example,</p><pre> x = atof( "42.0is_the_answer" );</pre> <p>results in x being set to 42.0.</p> <i>Related topics:</i><br> <strong><a href="#atoi">atoi()</a> and <a href="#atol">atol()</a>.</strong> <hr> <h2><a name="atoi">atoi</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <stdlib.h> int atoi( const char *str );</pre> </td> </tr> </table> <p>The atoi() function converts <i>str</i> into an integer, and returns that integer. <i>str</i> should start with some sort of number, and atoi() will stop reading from <i>str</i> as soon as a non-numerical character has been read. For example,</p><pre> i = atoi( "512.035" );</pre> <p>would result in i being set to 512.</p> <i>Related topics:</i><br> <strong><a href="#atof">atof()</a> and <a href="#atol">atol()</a>.</strong> <hr> <h2><a name="atol">atol</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <stdlib.h> long atol( const char *str );</pre> </td> </tr> </table> <p>The function atol() converts <i>str</i> into a long, then returns that value. atol() will read from <i>str</i> until it finds any character that should not be in a long. The resulting truncated value is then converted and returned. For example,</p><pre> x = atol( "1024.0001" );</pre> <p>results in x being set to 1024L.</p> <i>Related topics:</i><br> <strong><a href="#atof">atof()</a> and <a href="#atoi">atoi()</a>.</strong> <hr> <h2><a name="isalnum">isalnum</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isalnum( int ch );</pre> </td> </tr> </table> <p>The function isalnum() returns non-zero if its argument is a numeric digit or a letter of the alphabet. Otherwise, zero is returned.</p><pre> char c; scanf( "%c", &c ); if( isalnum(c) ) printf( "You entered the alphanumeric character %c\n", c );</pre> <i>Related topics:</i><br> <strong><a href="#isalpha">isalpha()</a>, <a href="#iscntrl">iscntrl()</a>, <a href= "#isdigit">isdigit()</a>, <a href="#isgraph">isgraph()</a>, <a href="#isprint">isprint()</a>, <a href= "#ispunct">ispunct()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="isalpha">isalpha</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isalpha( int ch );</pre> </td> </tr> </table> <p>The function isalpha() returns non-zero if its argument is a letter of the alphabet. Otherwise, zero is returned.</p><pre> char c; scanf( "%c", &c ); if( isalpha(c) ) printf( "You entered a letter of the alphabet\n" );</pre> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#iscntrl">iscntrl()</a>, <a href= "#isdigit">isdigit()</a>, <a href="#isgraph">isgraph()</a>, <a href="#isprint">isprint()</a>, <a href= "#ispunct">ispunct()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="iscntrl">iscntrl</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int iscntrl( int ch );</pre> </td> </tr> </table> <p>The iscntrl() function returns non-zero if its argument is a control character (between 0 and 0x1F or equal to 0x7F). Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#isalpha">isalpha()</a>, <a href= "#isdigit">isdigit()</a>, <a href="#isgraph">isgraph()</a>, <a href="#isprint">isprint()</a>, <a href= "#ispunct">ispunct()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="isdigit">isdigit</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isdigit( int ch );</pre> </td> </tr> </table> <p>The function isdigit() returns non-zero if its argument is a digit between 0 and 9. Otherwise, zero is returned.</p><pre> char c; scanf( "%c", &c ); if( isdigit(c) ) printf( "You entered the digit %c\n", c );</pre> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#isalpha">isalpha()</a>, <a href= "#iscntrl">iscntrl()</a>, <a href="#isgraph">isgraph()</a>, <a href="#isprint">isprint()</a>, <a href= "#ispunct">ispunct()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="isgraph">isgraph</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isgraph( int ch );</pre> </td> </tr> </table> <p>The function isgraph() returns non-zero if its argument is any printable character other than a space (if you can see the character, then isgraph() will return a non-zero value). Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#isalpha">isalpha()</a>, <a href= "#iscntrl">iscntrl()</a>, <a href="#isdigit">isdigit()</a>, <a href="#isprint">isprint()</a>, <a href= "#ispunct">ispunct()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="islower">islower</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int islower( int ch );</pre> </td> </tr> </table> <p>The islower() function returns non-zero if its argument is a lowercase letter. Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#isupper">isupper()</a></strong> <hr> <h2><a name="isprint">isprint</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isprint( int ch );</pre> </td> </tr> </table> <p>The function isprint() returns non-zero if its argument is a printable character (including a space). Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#isalpha">isalpha()</a>, <a href= "#iscntrl">iscntrl()</a>, <a href="#isdigit">isdigit()</a>, <a href="#isgraph">isgraph()</a>, <a href= "#ispunct">ispunct()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="ispunct">ispunct</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int ispunct( int ch );</pre> </td> </tr> </table> <p>The ispunct() function returns non-zero if its argument is a printing character but neither alphanumeric nor a space. Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#isalpha">isalpha()</a>, <a href= "#iscntrl">iscntrl()</a>, <a href="#isdigit">isdigit()</a>, <a href="#isgraph">isgraph()</a>, <a href= "#isprint">isprint()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="isspace">isspace</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isspace( int ch );</pre> </td> </tr> </table> <p>The isspace() function returns non-zero if its argument is some sort of space (i.e. single space, tab, vertical tab, form feed, carriage return, or newline). Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#isalpha">isalpha()</a>, <a href= "#iscntrl">iscntrl()</a>, <a href="#isdigit">isdigit()</a>, <a href="#isgraph">isgraph()</a>, and <a href="#ispunct">ispunct()</a>.</strong> <hr> <h2><a name="isupper">isupper</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isupper( int ch );</pre> </td> </tr> </table> <p>The isupper() function returns non-zero if its argument is an uppercase letter. Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#tolower">tolower()</a></strong> <hr> <h2><a name="isxdigit">isxdigit</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <ctype.h> int isxdigit( int ch );</pre> </td> </tr> </table> <p>The function isxdigit() returns non-zero if its argument is a hexidecimal digit (i.e. A-F, a-f, or 0-9). Otherwise, zero is returned.</p> <i>Related topics:</i><br> <strong><a href="#isalnum">isalnum()</a>, <a href="#isalpha">isalpha()</a>, <a href= "#iscntrl">iscntrl()</a>, <a href="#isdigit">isdigit()</a>, <a href="#isgraph">isgraph()</a>, <a href= "#ispunct">ispunct()</a>, and <a href="#isspace">isspace()</a>.</strong> <hr> <h2><a name="memchr">memchr</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <string.h> void *memchr( const void *buffer, int ch, size_t count );</pre> </td> </tr> </table> <p>The memchr() function looks for the first occurrence of <i>ch</i> within <i>count</i> characters in the array pointed to by <i>buffer</i>. The return value points to the location of the first occurrence of <i>ch</i>, or NULL if <i>ch</i> isn't found. For example:</p><pre> char names[] = "Alan Bob Chris X Dave"; if( memchr(names,'X',strlen(names)) == NULL ) printf( "Didn't find an X\n" ); else printf( "Found an X\n" );</pre> <i>Related topics:</i><br> <strong><a href="#memcpy">memcpy()</a> and <a href="#strstr">strstr()</a>.</strong> <hr> <h2><a name="memcmp">memcmp</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <string.h> int memcmp( const void *buffer1, const void *buffer2, size_t count );</pre> </td> </tr> </table> <p>The function memcmp() compares the first <i>count</i> characters of <i>buffer1</i> and <i>buffer2</i>. The return values are as follows:</p> <table bgcolor="#eeeeff"> <tr> <td><strong>Value</strong></td> <td><strong>Explanation</strong></td> </tr> <tr> <td>less than 0</td> <td>buffer1 is less than buffer2</td> </tr> <tr> <td>equal to 0</td> <td>buffer1 is equal to buffer2</td> </tr> <tr> <td>greater than 0</td> <td>buffer1 is greater than buffer2</td> </tr> </table> <p><i>Related topics:</i><br> <strong><a href="#memchr">memchr()</a>, <a href="#memcpy">memcpy()</a>, and <a href= "#strcmp">strcmp()</a>.</strong></p> <hr> <h2><a name="memcpy">memcpy</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <string.h> void *memcpy( void *to, const void *from, size_t count );</pre> </td> </tr> </table> <p>The function memcpy() copies <i>count</i> characters from the array <i>from</i> to the array <i>to</i>. memcpy() returns <i>to</i>. The behavior of memcpy() is undefined if <i>to</i> and <i>from</i> overlap.</p> <i>Related topics:</i><br> <strong><a href="#memmove">memmove()</a>.</strong> <hr> <h2><a name="memmove">memmove</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <string.h> void *memmove( void *to, const void *from, size_t count );</pre> </td> </tr> </table> <p>The memmove() function is identical to <a href="#memcpy">memcpy()</a>, except that it works even if <i>to</i> and <i>from</i> overlap.</p> <i>Related topics:</i><br> <strong><a href="#memcpy">memcpy()</a>.</strong> <hr> <h2><a name="memset">memset</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <string.h> void *memset( void *buffer, int ch, size_t count );</pre> </td> </tr> </table> <p>The function memset() copies <i>ch</i> into the first <i>count</i> characters of <i>buffer</i>, and returns <i>buffer</i>. memset() is useful for intializing a section of memory to some value. For example, this command:</p><pre> memset( the_array, '\0', sizeof(the_array) );</pre> <p>is a very efficient way to set all values of <i>the_array</i> to zero.</p> <i>Related topics:</i><br> <strong><a href="#memcmp">memcmp()</a>, <a href="#memcpy">memcpy()</a>, and <a href= "#memmove">memmove()</a>.</strong> <hr> <h2><a name="strcat">strcat</a></h2> <i>Syntax:</i> <table bgcolor="#ccccff"> <tr> <td><pre> #include <string.h> char *strcat( char *str1, const char *str2 );</pre> </td> </tr> </table> <p>The strcat() function concatenates <i>str2</i> onto the end of <i>str1</i>, and returns <i>str1</i>. For example:</p><pre> printf( "Enter your name: " ); scanf( "%s", name ); title = strcat( name, " the Great" );⌨️ 快捷键说明
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