Converting and printing Decimal of a char in C

Question

After reading a single character as input, how do I determine what type of character it is? For example, how do I determine whether the character is a number, a letter or a dot?

Answer 1

You can use

• the function isdigit to determine whether a character is a digit,
• the function isalpha to determine whether a character is an alphabetic letter,
• the function ispunct to determine whether a character is a punctuation mark,
• the function isspace to determine whether a character is a whitespace character.

You can find a full list of all character classification functions of the C standard library here.

If you want to restrict the comparison to "a dot" (i.e. not any punctuation mark), then you can compare the value of the character directly with '.'.

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

int main( void )
{
    int c;

    printf( "Please enter a single character: " );
    c = getchar();

    if ( isdigit( c ) )
    {
        printf( "The input is a digit.\n" );
    }
    else if ( isalpha( c ) )
    {
        printf( "The input is an alphabetical letter.\n" );
    }
    else if ( c == '.' )
    {
        printf( "The input is a dot.\n" );
    }
    else if ( ispunct( c ) )
    {
        printf( "The input is a punctuation character.\n" );
    }
    else if ( isspace( c ) )
    {
        printf( "The input is a whitespace character.\n" );
    }
    else
    {
        printf( "The input is not a digit, letter, punctuation or whitespace character.\n" );
    }
}

This program has the following behavior:

Please enter a single character: j
The input is an alphabetical letter.

Please enter a single character: 7
The input is a digit.

Please enter a single character: .
The input is a dot.

Please enter a single character: ;
The input is a punctuation character.

Please enter a single character:  
The input is a whitespace character.