Why isn't my solution to K&R exercise 2-3 working?

I'm trying to solve exercise 2-3 in the book The C Programming Language, but my solution isn't working and I can't find out what I did wrong. The exercise says to write a function that takes a string containing a hexadecimal number and returns that number as an int.

int

Here is my code:

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

int htoi(char *s);

int main() {
char s[5];
scanf("%s", s);
printf("%d", htoi(s));
}

int htoi(char *s) {
int length, num = 0, i;
char c = 'A';
for (length = 0; c >= '0' && c <= '9' || c >= 'A' && c <= 'F'; length++)
c = toupper(s[length]);
for (i = 0; i <= length; i++) {
c = toupper(s[i]);
if (c >= '0' && c <= '9')
num += (c - '0') * pow(16, length - i);
else if (c >= 'A' && c <= 'F')
num += (c - 'A' + 10) * pow(16, length - i);
}
return num;
}


Why isn't it working?

isxdigit()

pow()

c >= 'A' && c <= 'F'

'0'

'9'

pow

val = (val << 4) + hexVal(ch)

int val

hexVal

ch - '0'

'a'

'f'

switch

case 'a': case 'A': return 10;

2 Answers
2

You need to change pow(16, length - i) to pow(16, length - i - 2).

pow(16, length - i)

pow(16, length - i - 2)

"Why 2?" you may ask.

Well, you need to learn how to debug programs. First of, your length does not seem to be properly calculated. Print that value and you'll see that it is actually one more than expected. Usually, C-programmers do not count the '' terminator when talking about the length of a string, and the standard function strlen does not either.

length

''

strlen

So you should remove 1 from length after it is calculated. And then it will make sense. For instance, 4=4*10⁰ and 40=4*10¹. The digit on position n should be raised to the power n-1. After that you use pow(16, length - i - 1). Notice it's 1 instead of 2 when we have correct length.

pow(16, length - i - 1)

Other comments on your code

Unless you have a specific reason to do so, I would not write a custom function to calculate the length. Instead, use strlen and check for correctness later. In this program it seems that you can assume that the string always will be correct. So I would have written it like this:

strlen

int htoi(char * s) {
int length = strlen(s), num = 0, i;

for (i = 0; i < length; i++) {
int factor = pow(16, length - i - 1), digit;
int c = toupper(s[i]);
if (c >= '0' && c <= '9')
digit = c - '0';
else if (c >= 'A' && c <= 'F')
digit = c - 'A' + 10;
num += digit * factor;
}
return num;
}


I also refactored it a little bit.

NULL

NUL

''

NUL

There are multiple problems in your code:

scanf()

fgets()

scanf("%4s", s)

scanf()

htoi

A

F

length

char

é

num

16

pow(16, length - i -

if

Here is a corrected version:

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

int htoi(const char *s);

int main() {
char s[100];
if (fgets(s, sizeof s, stdin))
printf("%dn", htoi(s));
return 0;
}

int htoi(const char *s) {
unsigned int num = 0;
int i;
for (i = 0; isxdigit((unsigned char)s[i]); i++) {
if (c >= '0' && c <= '9')
num = num * 16 + c - '0';
else
num = num * 16 + 10 + toupper(c) - 'A';
}
return num;
}


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