Converting an ASCII number to a Hex number in C without using library functions

Question

I want to convert an 32-bit ASCII number (e.g. "FE257469") to the equivalent 32-bit hex number which will be stored on a 32-bit variable. Most importnatly, I want to do that without using any library function like sscanf(), atoi(), etc.

Any ideas on that?

Thank in advance.

Is this homework or why do you want to do it without using a library function? — x4u
– x4u, Commented Jun 3, 2011 at 21:13
How is FE257469 a 32-bit number? If each character is ASCII, then each character would be 7/8 bits.. — Colin
– Colin, Commented Jun 3, 2011 at 21:15
:) No guys, it's not. I know that it sounds fairly simple but I just wondered if there is an easy way of doing this (I can't really think clearly at the moment after so many hours of studying!) — limp
– limp, Commented Jun 3, 2011 at 21:23
@Colin strictly speaking is not a 32-bit number but it represents a 32-bit number, that's what I meant — limp
– limp, Commented Jun 3, 2011 at 21:24

x4u · Accepted Answer · 2011-06-03 21:40:29Z

1

Here is an implementation of such a function based on a switch:

unsigned int parseHex( char *str )
{
    unsigned int value = 0;

    for(;; ++str ) switch( *str )
    {
        case '0': case '1': case '2': case '3': case '4':
        case '5': case '6': case '7': case '8': case '9':
            value = value << 4 | *str & 0xf;
            break;
        case 'A': case 'B': case 'C': case 'D': case 'E': case 'F':
        case 'a': case 'b': case 'c': case 'd': case 'e': case 'f':
            value = value << 4 | 9 + *str & 0xf;
            break;
        default:
            return value;
    }
}

answered Jun 3, 2011 at 21:40

x4u

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Comments

Jerry Coffin · Accepted Answer · 2011-06-03 21:23:10Z

1

The usual way is something like:

initialize result to 0

convert one digit of input to decimal to get current digit
multiply result by 16
add current digit to result
repeat steps 1-3 for remaining digits

answered Jun 3, 2011 at 21:23

Jerry Coffin

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