3

I'm not sure how the while loop works in the following simple piece of code

short CountBits(unsigned int x){
    short num_bits = 0;
    while (x){
        num_bits += x & 1;
        x >>= 1;
    }
    return num_bits;
}

How does an unsigned integer evaluate to True or False?

Sentinel
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4 Answers4

5

In the given context, x must be converted to true or false.

From the C++11 Standard (4.12/1):

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

Think of 

while (x){ ... }

as

while (x != 0){ ... }
R Sahu
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2

True is any integer which is not equal to 0. Thus if x evaluates to 0 then the loop breaks.

Archmede
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2

"How does an unsigned integer evaluate to True or False"? The same way any numeric value evaluates to true or false: 0 is false, any other value is true. Some people would write the test as while (x != 0); that's exactly the same thing.

Pete Becker
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1

For any integral number in C++, on most machines, 0 will evaluate to false. As such, when X becomes 0, the loop will terminate. The loop will continue while x is a non-zero value.

Rachel Casey
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    Tell us more about those machines where the loop doesn't terminate when `x` becomes `0` – M.M May 01 '17 at 05:18
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    C++ has a standard, it specifies that x being 0 does make the loop terminate – M.M May 01 '17 at 21:11