Shortest way to convert hex char to int in C?

Question

I'm looking for the least amount of code in C, in order to convert a char to int, where it flags -1 (or any error flag) if the char is not a valid hex digit.

here's what I came up with, is there a shorter way?

// input example
char input = 'f';   

// conversion segment
int x = input - '0';    
if (!((x >= 49 && x <= 54) || (x >= 0 &&  x <= 9))) x = -1;
if (x > 9) x -= 39;

// test print   
printf("%d", x);

I'd like 'a' to be converted to 10, 'f' to be 15, '0' to '9' to be their integer equivalents, and any other char not in those ranges to return -1. Trying to see the least amount of lines this can be done in. — Austin, Mar 30 '16 at 03:35
There isn't a standard function that does that directly. You could use `if (sscanf(&c, "%1x", &x) != 1) x = -1;` at a pinch. Alternatively, you can play is `isdigit()` and `isxdigit()` (and maybe `isupper()` or `islower()`) from `` to work out what to do. Or create a lookup table for possible (unsigned) `char` values initialized with `-1` except for the valid hex digits. — Jonathan Leffler, Mar 30 '16 at 03:36
I only ask because in the proposed option `'a' - '0'` gives `49`. So it's definetely not the shortest way as it is not a solution. — kaylum, Mar 30 '16 at 03:38
don't use hardcode values like 48, 57, 97 or 102. Use 'a', 'z', 'A', 'Z'... instead — phuclv, Mar 30 '16 at 03:41
sorry kaylum, I corrected the ascii values. Why did someone vote to close? I don't see another question that addresses this and I'm sure someone looking to shorten up their code may find it useful. — Austin, Mar 30 '16 at 03:44
"shorten up the code" is something you do for Obfuscated C contests. Any actual programmer would just write normal code using obvious logic. — M.M, Mar 30 '16 at 03:47
I'm voting to close this question as off-topic because writing shortest code may be fun, but is useless. — ugoren, Mar 30 '16 at 13:42

score 1 · Answer 1 · edited May 23 '17 at 12:16

This code assumes ASCII and converts all 256 characters codes into 256 different codes, partially '0'-'9' 'A'-'F' map to 0,1,...15.

For additional tricks and simplification see the post

unsigned char ch = GetData(); // Fetch 1 byte of incoming data;
if (!(--ch & 64)) {           // decrement, then if in the '0' to '9' area ...
  ch = (ch + 7) & (~64);      // move 0-9 next to A-Z codes
}
ch -= 54;                     // -= 'A' - 10 - 1
if (ch > 15) { 
  ; // handle error
}

score 0 · Accepted Answer · answered Mar 30 '16 at 03:58

0

Try this function:

isxdigit(c);

answered Mar 30 '16 at 03:58

eyalm

3,366
19
21

I knew there was a way! By the way for anyone else it requires `` – Austin Mar 30 '16 at 04:01
Actually this would have be more nice if it returned the int equivalent value rather than a 1, but still shortens things up a bit. – Austin Mar 30 '16 at 04:03
This does not convert per the post's title "convert hex char to int", it simply tests (true/false) if a character is a hexadecimal character - and takes a 256 bit/byte table to do it. – chux - Reinstate Monica Mar 30 '16 at 04:03
I agree. It only tests. But, as i understood it, the questions was also about getting an error. – eyalm Mar 30 '16 at 05:16

Shortest way to convert hex char to int in C?

2 Answers2