Miguel Pinto - 3 months ago 27

C Question

So I'm currently trying to finish some university homework that deals with bitwise manipulations, and one of the exercises is giving me the worst kind of trouble, the one where you dont know where you went wrong. The exercise is as follows:

Implement the function int activate_bits(int a, int left, int right ) that should ’activate’

all the bits to the left of left and to the right of right on the number a (excluding the bits

left and right).

My code regarding the activate_bits function is the following

`#include <stdio.h>`

unsigned int activate_bits(unsigned a, int left, int right){

int mask1 = 1;

int mask2 = 1;

int mask3 = 0;

int i;

/* assuming an int as only 8 bits for convenience, what I want to do here is

shift the least significant bit to the left, then add 1,as many times as

necessary(according to the parameter right), so that the number 00000001

becomes 00000011 and so forth */

for (i= (right -1); i<right ; i++){

mask1 << 1;

mask1 = mask1 +1 ;

}

/* doing the same as above here, checking how many activated bits the second

mask should actually have by doing (32 - left ) */

for (i = (32 - left); i < 0; i--){

mask2 << 1;

mask2 = mask2 +1 ;

}

/* now I'm shifting the second mask as many times as needed so it is placed

after the bit position indicated by the left parameter */

mask2 << left;

mask3 = mask1 + mask2;

return a | mask3;

}

Can anyone help me as to why this is giving me a wrong result ?

Thanks in advance

Answer

`mask1 << 1;`

doesn't do what you think it does. It's not like `mask1++`

which increments `mask1`

; it's the same as having the line `mask1 + 1`

- the result is evaluated but not stored anywhere.

Try doing this:

`mask1 = mask1 << 1`

Or, for brevity's sake:

`mask1 <<= 1`