Thread: Bitmanipulation in C

The task is to return the number of bits set to true.

Here is my code:

Code:

int cardinalityBS(PBitSet _this) {
    int s, counter = 0;


    for(s = 0; s < 31; s++) {
        counter += _this->bits & 1;
        _this->bits <<= 1;
    }


    return counter;
}

The code is not working, since whenever I set _this->bits to a number, it returns me the wrong result. My question is: How could I fix this problem because the code looks perfect for me.

Shouldn't it be

Code:

coutner += _this->bits & (1 << s);

Thank you very much for your answer, can I ask just one more question?

Code:

void clearBS(PBitSet _this, int fromIndex, int toIndex) {
     int c;


     for(c = fromIndex; c < toIndex; c++) {
        _this->bits &= (0 << c);
     }
}

The goal is here to set the bits of an integer from "fromIndex" to "toIndex" to false but the code above does not work. (Whenever I print the integer number to test my code, if my fromIndex and toIndex are bigger than 0, my number will be 0 either.) What did I do wrong?

Hmm... Look at this.

Code:

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


void clear_bits(const int       begin,
                const int       end,
                      unsigned *x)
{
    if (begin < 0 || end < 0 || begin > 32 || end > 32)
        return;


    unsigned cpy = *x;


    for (int i = begin; i < end; ++i)
    {
        cpy &= ~(1 << i);
    }


    *x = cpy;
}


int main(void)
{
    // all bits are set because I couldn't
    // get UINT_MAX to work
    unsigned x = 0xffffffff;


    clear_bits(22, 32, &x);


    printf("%u\n", x);
}

You're problem was that you need to be using

Code:

i &= ~(1 << bit)

Thanks you, it's working now. But why is i &= ~(1 << bit) actually not the same as i &= (0 << bit) ??

This would be a good read for you : How do you set, clear and toggle a single bit in C/C++? - Stack Overflow

Hmm... phantom, is this what you had in mind? Or do you have a better implementation?

Code:

void set_bits(const int       begin,
              const int       end,
                    unsigned *x)
{
    const int n = end - begin;


    unsigned val = (1 << n) - 1;
    val <<= begin;


    unsigned tmp = *x;


    tmp |= val;


    *x = tmp;
}


int main(void)
{
    unsigned x = 0;


    set_bits(1, 5, &x);    


    printf("%u\n", x);


    return 0;
}