Sean Callahan Sean Callahan - 2 months ago 36
C Question

Bitwise operations and shifts

Im having some trouble understanding how and why this code works the way it does. My partner in this assignment finished this part and I cant get ahold of him to find out how and why this works. I've tried a few different things to understand it, but any help would be much appreciated. This code is using 2's complement and a 32-bit representation.

/*
* fitsBits - return 1 if x can be represented as an
* n-bit, two's complement integer.
* 1 <= n <= 32
* Examples: fitsBits(5,3) = 0, fitsBits(-4,3) = 1
* Legal ops: ! ~ & ^ | + << >>
* Max ops: 15
* Rating: 2
*/
int fitsBits(int x, int n) {
int r, c;
c = 33 + ~n;
r = !(((x << c)>>c)^x);
return r;
}

Answer
c = 33 + ~n;

This calculates how many high order bits are remaining after using n low order bits.

((x << c)>>c

This fills the high order bits with the same value as the sign bit of x.

!(blah ^ x)

This is equivalent to

blah == x
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