“I remember in one interview I was asked to write a function that returns true iff x is a power of two, so I wrote return 1 == __builtin_popcount(x). They liked that.”
I’m no longer a programmer, but I wondered why it wasn’t “return (__builtin_popcount(x) == 1)” - just out of interest.
If you accidentally write "return x = 1" when x is a variable, you always return true. If you return "1 = x", you cause a syntax error. So some people have gotten into the habit of writing constants on the left, even if the return value of __builtin_popcount is not assignable.
> I’m no longer a programmer, but I wondered why it wasn’t “return (__builtin_popcount(x) == 1)” - just out of interest.
== is a boolian comparison operator. Therefore, we are returning true or false depending on how the expression evaluates. __builtin_popcount(x) will return the number of set '1' bits in the binary int x. Since powers of 2 in binary are always a single 1 followed by 0's, this expression is checking whether the number of 1's in the binary representation of x is equal to 1, and returning true if this is the case. Otherwise, if there are more than 1 '1's, this indicates that x is not a multiple of 2, and should return false.
Example: 15 in binary is 01111. There are 4 '1's, so the 1 == 4 comparison returns false. Increment to 16, or 10000, and there is exactly 1 '1', and so the comparison 1 == 1 returns true.
