Generating random number between [-1, 1] in C?

Question

I have seen many questions on SO about this particular subject but none of them has any answer for me, so I thought of asking this question.

I wanted to generate a r

Answer 1

I had a similar question a while back and thought that it might be more efficient to just generate the fractional part directly. I did some searching and came across an interesting fast floating point rand that doesn't use floating point division or multiplication or a int->float cast can be done with some intimate knowledge of the internal representation of a float:

float sfrand( void )
{
    unsigned int a=(rand()<<16)|rand();  //we use the bottom 23 bits of the int, so one
                                         //16 bit rand() won't cut it.
    a=(a&0x007fffff) | 0x40000000;  

    return( *((float*)&a) - 3.0f );
}

The first part generates a random float from [2^1,2^2), subtract 3 and you have [-1, 1). This of course may be too intimate for some applications/developers but it was just what I was looking for. This mechanism works well for any range that is a power of 2 wide.