Sean Callahan - 3 months ago 54

C Question

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
```