Char Comparison in C

A char variable is actually an 8-bit integral value. It will have values from 0 to 255. These are almost always ASCII codes, but other encodings are allowed. 0 stands for the C-null character, and 255 stands for an empty symbol.

So, when you write the following assignment:

char a="a"; 

It is the same thing as this on an ASCII system.

char a = 97;

So, you can compare two char variables using the >, <, ==, <=, >= operators:

char a="a";
char b = 'b';

if( a < b ) printf("%c is smaller than %c", a, b);
if( a > b ) printf("%c is smaller than %c", a, b);
if( a == b ) printf("%c is equal to %c", a, b);

Note that even if ASCII is not required, this function will work because C requires that the digits are in consecutive order:

int isdigit(char c) {
    if(c >= '0' && c <= '9') 
        return 1;
    return 0;
}