Question

#include <iostream>

using namespace std;
int main(int argc, char *argv[]) 
{
   int i=-5;
   while(~(i))
   {
      cout<<i;
      ++i;
   }

 }

The output is -5,-4,-3,-2.
Shouldn’t it print values till -1?Why is it only till -2.
And please explain me the difference between ‘not’ and ‘negation’ operators.When ever I write a program they were the source for bugs.

while(i)

I know that the loop condition will be true for positive and negative i’s except 0.

while(!i) vs while(~i)

For what values of ‘i’ the above two loops get executed?

Answer 1

When i gets to -1, the value of ~i is ~-1, or 0, so the while loop stops executing. The ! operator works because it does something completely different; it results in 1 for 0 values and 0 for all other values. ~ is a bitwise negation.

A little more in detail:

• ~ takes each bit in a number and toggles it. So, for example, 10010₂ would become 01101₂
• -1 is all ones in binary when a two’s complement signed integer.
• ~0b…11111111 is 0.

However:

• !0 is 1, !anythingElse is 0
• -1 is not 0
• !-1 is still 0

And if you actually want to loop including i == -1, just use while (i) instead of while (~i).