Diego Alonso Cortez Diego Alonso Cortez - 3 months ago 8
C Question

Binary integer to fraction between 0 and 1 in base 10, in C

Eg. how can I transform the integer

7
to the float
0.111
?

The naive way would be to convert
7
to the string
111
, convert that to the integer
111
and then divide by 1000 to get
0.111
. Is there a better/faster way, though?

Here's a working example.

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>

// Convert 32-bit uint to string
// http://stackoverflow.com/questions/699968/display-the-binary-representation-of-a-number-in-c
const char* bin(uint32_t n){
uint N = 32;
static unsigned char ucharBuffer[32+1];
char *p_buffer = ucharBuffer;

if(!n)
return "0";

// Work from the end of the buffer back
p_buffer += N;
*p_buffer-- = '\0';

//For each bit (going backwards) store character
while(n){
if (N-- == 0)
return NULL;
*p_buffer-- = ((n & 1) == 1) ? '1' : '0';
n >>= 1;}

return p_buffer+1;}


int main(){

uint INPUT = 7;
const char *p = bin(INPUT);
char *end;

printf("%d -> ", INPUT);
printf("'%.*s' -> ", (uint)(end-p), p);
printf("%.3f\n", (double) strtol(p, &end, 10)/1000);

}

jxh jxh
Answer

You don't need to convert into a string. C has binary operators that work just fine for this purpose.

double binary_inverse (unsigned input) {
    double x = 0;
    while (input) {
        x += input & 0x1;
        x /= 10;
        input >>= 1;
    }
    return x;
}