Character Processing

Character Processing (<ctype.h>)

The <ctype.h> header file declares several functions for testing and converting characters.

• For each function, the argument is an int whose value must be EOF or representable as an unsigned char.

• The return value is an integer (non-zero = true, 0 = false).

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Character Testing Functions

1. Alphanumeric Check

int isalnum(int c);

Returns a nonzero integer if c is an alphanumeric ASCII character.
Otherwise returns 0.

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2. Alphabet Check

int isalpha(int c);

Returns nonzero if c is an alphabetic ASCII character, else 0.

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3. Control Character Check

int iscntrl(int c);

Returns nonzero if c is:

• ASCII DEL character (177 octal / 0x7F hex), OR

• Any non-printing ASCII character (< 40 octal / 0x20 hex)

Else returns 0.

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4. Digit Check

int isdigit(int c);

Returns nonzero if c is a decimal digit (0–9), else 0.

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5. Graphic Character Check

int isgraph(int c);

Returns nonzero if c is a printing character except space, else 0.

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6. Lowercase Check

int islower(int c);

Returns nonzero if c is a lowercase alphabet, else 0.

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7. Printable Character Check

int isprint(int c);

Returns nonzero if c is a printable ASCII character including space, else 0.

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8. Punctuation Check

int ispunct(int c);

Returns nonzero if c is an ASCII punctuation character
(any printable, non-alphanumeric character > 40 octal / 0x20 hex).

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9. White Space Check

int isspace(int c);

Returns nonzero if c is a whitespace character.

Standard whitespace characters:

• Space ' '

• Form feed '\f'

• New line '\n'

• Carriage return '\r'

• Horizontal tab '\t'

• Vertical tab '\v'

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10. Uppercase Check

int isupper(int c);

Returns nonzero if c is an uppercase alphabet, else 0.

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11. Hexadecimal Digit Check

int isxdigit(int c);

Returns nonzero if c is a hex digit (0–9, A–F, a–f).

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Character Conversion Functions

Convert to Lowercase

int tolower(int c);

Converts uppercase letter to lowercase.
If not uppercase, value remains unchanged.

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Convert to Uppercase

int toupper(int c);

Converts lowercase letter to uppercase.
If not lowercase, value remains unchanged.

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Example 1: Count Alphabets, Digits, and Spaces

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

int main(void)
{
   char str[100];
   int i, dc=0, al=0, sp=0;

   printf("Enter a string \n");
   fgets(str,100,stdin);

   for(i=0;i<strlen(str);i++)
   {
      if(isalpha(str[i])) al++;
      if(isdigit(str[i])) dc++;
      if(isspace(str[i])) sp++;
   }

   printf("Number of digits=%d\n",dc);
   printf("Number of alphabets=%d\n",al);
   printf("Number of spaces=%d\n",sp);
}

Output

Enter a string
this is test123
Number of digits=3
Number of alphabets=10
Number of spaces=3

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Example 2: Check Uppercase or Lowercase Alphabet

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

int main()
{
    char ch;
    printf("Enter a character\n");
    ch = getchar();

    if(isupper(ch))
        printf("uppercase\n");
    else
        printf("lower case\n");
}

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Example 3: Count Uppercase and Lowercase Characters (University Question)

#include <stdio.h>

int main() {
    char str[100];
    int uppercase_count = 0, lowercase_count = 0;
    int i = 0;

    printf("Enter a string: ");
    fgets(str, sizeof(str), stdin);

    while (str[i] != '\0') {
        if (str[i] >= 'A' && str[i] <= 'Z') {
            uppercase_count++;
        }
        else if (str[i] >= 'a' && str[i] <= 'z') {
            lowercase_count++;
        }
        i++;
    }

    printf("Number of uppercase characters: %d\n", uppercase_count);
    printf("Number of lowercase characters: %d\n", lowercase_count);

    return 0;
}

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Note

We can also use built-in <ctype.h> functions such as:

• isupper()

• islower()

to simplify such programs.