Question

There is a binary string 10001110 which I know is using 2’s complement. I know it will be 8 bits.

It is given to me in the form of an unsigned int which equals 142.

I then need to convert this back to 10001110

then invert all the bits and add one resulting in it equaling 01110010

and then convert this to a signed int equaling -114.

How can I do this? I’m relatively new to C and have spent ages trying to figure it out!

Essentially I want to write a function that takes the unsigned int 142 and returns the signed int -114

Answer 1

Here’s the solution I finally came up with.

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

int getBit(int no, int bit) // get bit at certain position
{
    bit--;
    return (no & (1 << bit)) >> bit;
}

void complementNBits(int *no, int count) // invert certain number of bits
{
    int i;
    for(i=0; i<count; i++)
    {
        *no ^= 1 << i;
    };
}

int main() 
{
    unsigned int unsignedNo=142;

    int negative = 0;   
    signed int signedNo=0;

    // check if MSB is set to determine if negative

    if (getBit(unsignedNo, 8)) // if MSB is set
    {
        negative = 1;
        // invert these 8 bits and add 1. the rest of the MSBs will already be 0
        complementNBits(&unsignedNo, 8);
        unsignedNo++;
    }

    signedNo = unsignedNo;
    if (negative)
    {
        signedNo = signedNo * -1;
    }

    printf("THE SIGNED INTEGER: %i\n", signedNo);

}

Thanks for the help everyone!