Explanation of Q4

#1

Please explain question 4. I could not understand

#2

In this question you have to tell what this function does

the answer is " true if X is a power 2 "
Here is what the expression (x && !(x & (x - 1))); means:

  • The initial x part means x != 0
  • The (x & (x-1)) part means " x is not a power of two" (i.e. is zero or has more than one bit set)
  • The !(x & (x-­1)) expression means " x is a power of two"

Therefore, overall expression means " x is a positive power of two".

To see why x & (x-1) helps you find powers of two, consider what happens to numbers that have a single bit set vs. the numbers that have two or more bits set. A number with a single bit set, when decremented, would produce a sequence that looks like this:

000100000..000 <== x   // when x is power of two
000011111..111 <== x-1
#3

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