hex string to integer...

**xion** · 07-31-2003

this is one of the k&r exercises that ive been stuck on. i was trying to do this manually and not using a stdio.h function. i wasnt aware this function existed. and may i add k&r doesnt assign exercises very well. i guess they expect you to already know whats out there. the use of pointers wasnt discussed and yet i had to figure out the basics of them and use it to do an exercise.

Code:

#include<stdio.h>

unsigned int htoi(char *s);

int main()
{
	char	s[10];
	int	i;
	int	c;

	for (i = 0; i <= (10 -1) && (c = getchar()) != EOF  && c != '\n'; i++)
		s[i] = c;

//do i need to add a '\n' and a '\0' here?  or will this suffice and put the '\0' at the end? 
						
	printf("%d\n",htoi(s));

	return 0;
}

unsigned int htoi(char *s)
{
	unsigned int n;
	int r;

	r = sscanf(s, "%x", &n);
	return n;
}

**Prelude** · 07-31-2003

and may i add k&r doesnt assign exercises very well. i guess they expect you to already know whats out there. the use of pointers wasnt discussed and yet i had to figure out the basics of them and use it to do an exercise.

You really aren't supposed to use the standard library for this exercise, that defeats the purpose. The exercise assumes that you only know what has been shown up to that point, which is certainly possible:

Code:

/* Assume ASCII */
int htoi(char s[])
{
    int i = 0; /* Iterate over s */
    int n = 0; /* Built up number */

    /* Remove "0x" or "0X" */
    if ( s[0] == '0' && s[1] == 'x' || s[1] == 'X' )
        i = 2;

    while ( s[i] != '\0' )
    {
        int t;

        if ( s[i] >= 'A' && s[i] <= 'F' )
            t = s[i] - 'A' + 10;
        else if ( s[i] >= 'a' && s[i] <= 'f' )
            t = s[i] - 'a' + 10;
        else if ( s[i] >= '0' && s[i] <= '9' )
            t = s[i] - '0';
        else
            return n;

        n = 16 * n + t;
        ++i;
    }

    return n;
}

**Dave_Sinkula** · 07-31-2003

>this is one of the k&r exercises that ive been stuck on.

The following links may be of interest.

**xion** · 07-31-2003

thanks for the response guys/girls.
prelude thanks for the informative post. i had trouble getting the logic down on how to calc from hex to int. by seeing your code i understand all but one thing.

Code:

n = 16 * n + t;

doesnt 16 have to be squared the 3rd time this loop goes around? then cubed the 4th time around? just to make sure.

man i hope this logic gets easier as i go along.

**Prelude** · 07-31-2003

>doesnt 16 have to be squared the 3rd time this loop goes around? then cubed the 4th time around?
No, that's not necessary. My htoi is basically the atoi function from pp. 43. The only real difference is working with base 16 and handling A-F as well as 0-9.

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