Two's Complement Representation of int16_t

Question

I am attempting to display a string representation of an int16_t's Two's Complement. I find the two's complement by (uint16_t)~value + 1;

How would I add the 16 bits to a string?

char* getBits(const int16_t value) {

    uint16_t complement = (uint16_t)~value + 1;
    char bits = malloc(16*sizeof(char*))

    for (int i = 0; i < 16; i++) {
        bits[i] = // something
    }
    return bits;
}

Add `'\0'` to the end of `bits` and use `printf("%s", bits)`. — Wolfram, Sep 30 '20 at 23:14
Please do _not_ edit your original code based on a response here. It makes the answers posted nonsensical (e.g. Barmar's change from `char` to `char *`). You may edit your question and append additional code to the bottom. Or, you may show your final/updated in an answer of your own [if you wish]. — Craig Estey, Sep 30 '20 at 23:36

Barmar · Accepted Answer · 2020-09-30T23:54:56.483

1

Shift complement right i bits, and test whether the low-order bit is 0 or 1 and put the corresponding character in bits.

bits[i] = (complement >> i) & 1 ? '1' : '0';

Also, you need to allocate an extra character in your string for the null terminator. And bits needs to be a pointer, while the element size is sizeof(char), not sizeof(char*).

char *bits = malloc(17*sizeof(char));
bits[16] = 0;

There's no need to use the formula (uint16_t)~value + 1. Converting a signed int to unsigned int automatically returns its twos complement value. So you can simply do:

uint16_t complement = (uint16_t)value;

edited Sep 30 '20 at 23:54

answered Sep 30 '20 at 23:23

Barmar

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Why not malloc((16+1)*sizeof(char)) – Marichyasana Sep 30 '20 at 23:50
why sizeof(char)? – 0___________ Oct 01 '20 at 00:17
I tend not to remove that when the OP uses it. It's redundant but meaningful. – Barmar Oct 01 '20 at 00:19

0___________ · Answer 2 · 2020-09-30T23:44:49.840

Your code has many issues and it wont even compile. You need to learn what is pointer and what is not. How to use malloc and where to sstore its result. The best way is to read a good C book.

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

char* getBits(const int16_t value) 
{
    uint16_t complement = value;
    char *bits = malloc(sizeof(value) * CHAR_BIT + 1);
    char *wrk = bits;

    if(bits)
    {
        for (unsigned i = 1 << 15; i; i >>= 1) 
        {
            *wrk++ = (complement & i) ? '1' : '0';
        }
        *wrk++ = 0;
    }
    return bits;
}



int main(void)
{
    for(int i = 0; i > -15; i --)
    {
        char *bits;
        printf("%d - %s\n", i, (bits = getBits(i)));
        free(bits);
    }
}

Two's Complement Representation of int16_t

2 Answers2