CODEWITHSUNDEEP
cbinary
étale-cohomology
étale-cohomology

Reputation: 1861

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);

}

Upvotes: 2

Views: 100

Answers (1)

jxh
jxh

Reputation: 70392

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;
}

Upvotes: 3

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