Extracting bits using bit manipulation

Question

I have a 32-bit unsigned int and I need to extract bits at given positions and make a new number out of those bits. For example, if I have a 0xFFFFFFFF and want bits 0,10,11 my

Answer 1

Beware, untested code:

for(i = 0; i < count; i++) 
{
    bitmask = 1 << positions[i];
    bit = (bytes & bitmask)!=0;
    result = (result << 1)|bit;
}

Answer 2

Since you are picking off individual bits, there's no reason to make the bit mask a variable; just shift the desired bit into the units bit, and use a mask of 1. E.g.:

...
result = (2*result) | ((bytes >> positions[i]) & 1);
...

Many compilers generate the same code for 2*result and result<<1, so use whichever you like.

Note, if you are designing the interface and don't have good reasons for using short integers for positions[] and count as you do, then don't. Be consistent and specify all the integers the same way.