Reference
C Library
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C Library
cassert (assert.h)
cctype (ctype.h)
cerrno (errno.h)
cfloat (float.h)
ciso646 (iso646.h)
climits (limits.h)
clocale (locale.h)
cmath (math.h)
csetjmp (setjmp.h)
csignal (signal.h)
cstdarg (stdarg.h)
cstddef (stddef.h)
cstdio (stdio.h)
cstdlib (stdlib.h)
cstring (string.h)
ctime (time.h)
cctype (ctype.h)
isalnum
isalpha
iscntrl
isdigit
isgraph
islower
isprint
ispunct
isspace
isupper
isxdigit
tolower
toupper


isalnum

function
<cctype> 
int isalnum ( int c );

Check if character is alphanumeric

Checks if parameter c is either a decimal digit or an uppercase or lowercase letter.

The result is true if either isalpha or isdigit would also return true.

Notice that what is considered a letter may depend on the locale being used; In the default C locale, what constitutes a letter is what returns true by either isupper or islower.

For a detailed chart on what the different ctype functions return for each character of the standard ANSII character set, see the reference for the <cctype> header.

In C++, a locale-specific template version of this function (isalnum) exists in header <locale>.

Parameters

c
Character to be checked, casted as an int, or EOF.

Return Value

A value different from zero (i.e., true) if indeed c is either a digit or a letter. Zero (i.e., false) otherwise.

Example

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/* isalnum example */
#include <stdio.h>
#include <ctype.h>
int main ()
{
  int i;
  char str[]="c3po...";
  i=0;
  while (isalnum(str[i])) i++;
  printf ("The first %d characters are alphanumeric.\n",i);
  return 0;
}


Output:
The first 4 characters are alphanumeric.

See also

isalpha Check if character is alphabetic (function)
isdigit Check if character is decimal digit (function)