The GNU C Programming Tutorial

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Character handling

Let's examine some simple library functions and see how they are used. Some of the functions that are available on GNU systems for handling individual characters are described below. They are all macros, so the usual caveats about macro parameters apply. (See Macro functions.) All of the functions below accept single variables of type char as parameters. To use any of them, you must include the system header file ctype.h; the library used is simply glibc, which is linked automatically.

isalnum: Returns true if and only if the parameter is alphanumeric: that is, an alphabetic character (see isalpha) or a digit (see isdigit).
isalpha: Returns true if and only if the parameter is alphabetic. An alphabetic character is any character from A through Z or a through z.
isascii: Returns true if and only if the parameter is a valid ASCII character: that is, it has an integer value in the range 0 through 127. (Remember, the char type in C is actually a kind of integer!)
iscntrl: Returns true if and only if the parameter is a control character. Control characters vary from system to system, but are usually defined as characters in the range 0 to 31.
isdigit: Returns true if and only if the parameter is a digit in the range 0 through 9.
isgraph: Returns true if and only if the parameter is graphic: that is, if the character is either alphanumeric (see isalnum) or punctuation (see ispunct). All graphical characters are valid ASCII characters, but ASCII also includes non-graphical characters such as control characters (see iscntrl) and whitespace (see isspace).
islower: Returns true if and only if the parameter is a lower-case alphabetic character (see isalpha).
isprint: Returns true if and only if the parameter is a printable character: that is, the character is either graphical (see isgraph) or a space character.
ispunct: Returns true if and only if the parameter is a punctuation character.
isspace: Returns true if and only if the parameter is a whitespace character. What is defined as whitespace varies from system to system, but it usually includes space characters and tab characters, and sometimes newline characters.
isupper: Returns true if and only if the parameter is an upper-case alphabetic character (see isalpha).
isxdigit: Returns true if and only if the parameter is a valid hexadecimal digit: that is, a decimal digit (see isdigit), or a letter from a through f or A through F.
toascii: Returns the parameter stripped of its eighth bit, so that it has an integer value from 0 through 127 and is therefore a valid ASCII character. (See isascii.)
tolower: Converts a character into its lower-case counterpart. Does not affect characters which are already in lower case.
toupper: Converts a character into its upper-case counterpart. Does not affect characters which are already in upper case.

/********************************************************/
/*                                                      */
/* Demonstration of character utility functions         */
/*                                                      */
/********************************************************/

#include <stdio.h>
#include <ctype.h>

#define  allchars    ch = 0; isascii(ch); ch++

int main ()           /* A criminally long main program! */

{
  char ch;

  printf ("\n\nVALID CHARACTERS FROM isgraph:\n\n");
  for (allchars)
    {
      if (isgraph(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\nVALID CHARACTERS FROM isalnum:\n\n");
  for (allchars)
    {
      if (isalnum(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\nVALID CHARACTERS FROM isalpha:\n\n");
  for (allchars)
    {
      if (isalpha(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\nVALID CHARACTERS FROM isupper:\n\n");
  for (allchars)
    {
      if (isupper(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\nVALID CHARACTERS FROM islower:\n\n");
  for (allchars)
    {
      if (islower(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\nVALID CHARACTERS FROM isdigit:\n\n");
  for (allchars)
    {
      if (isdigit(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\nVALID CHARACTERS FROM isxdigit:\n\n");
  for (allchars)
    {
      if (isxdigit(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\nVALID CHARACTERS FROM ispunct:\n\n");
  for (allchars)
    {
      if (ispunct(ch))
	{
	  printf ("%c ",ch);
	}
    }

  printf ("\n\n");

  return 0;
}

The output of the above code example is as follows:

VALID CHARACTERS FROM isgraph:

! " # $ % & ' ( ) * + , - . / 0 1 2 3 4 5 6 7 8 9 : ; < = > ? @ A B C D
E F G H I J K L M N O P Q R S T U V W X Y Z [ \ ] ^ _ ` a b c d e f g h
i j k l m n o p q r s t u v w x y z { | } ~

VALID CHARACTERS FROM isalnum:

0 1 2 3 4 5 6 7 8 9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
a b c d e f g h i j k l m n o p q r s t u v w x y z

VALID CHARACTERS FROM isalpha:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j
k l m n o p q r s t u v w x y z

VALID CHARACTERS FROM isupper:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

VALID CHARACTERS FROM islower:

a b c d e f g h i j k l m n o p q r s t u v w x y z

VALID CHARACTERS FROM isdigit:

0 1 2 3 4 5 6 7 8 9

VALID CHARACTERS FROM isxdigit:

0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f

VALID CHARACTERS FROM ispunct:

! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~