Question

C program question:

If i have a program that asks the user for a positive integer an i save that in the variable n. And then i have a while statement like so :
while (n) {
invert (P)
n = n & n-1
}

what is that while loop saying? Is it saying "while true"? isn't it always true?

Answer 1

C defines false as 0. C defines true as not false. Any non-zero, non-null value is true.

Answer 2

So if i had the fallowing code, since n does not equal zero it will always be true so then it would always loop to the while after n = n & (n-1)?

int main ()
{
int n, p;
p=0;
printf("Enter a positive integer:");
scanf("%d",&n);

while (n) {
invert(p);
n = n & (n-1);
}
printf("Odd parity is %d\n", p);
invert(p);
printf("Even parity is %d\n", p);
return 0;
}

Answer 3

If n is positive, then the loop will execute at least 1 time. Note the n = n & (n-1) which will mostlikely decrease the value of n, if not set it to 0. So The while(n) look will execute 1 or more times in this cases. If n is negative on the other hand, the loop might keep on going for a while, I think it will actually be infinite if n is < 0. So it would be wise to check for n to be > 0 before going into the while loop.

Answer 4

What does n = n & (n-1) do?

Answer 5

n = n & (n-1) ;
lets say that n =3
then n = 3 &2 ??
converting 3 and 2 to binary notation
(11) & 10 = 10 = 2
so no the loop will eventually terminate when n&(n-1) is zero

Answer 6

A single ampersand is a bitwise and. A double ampersand is used in complex (more than one) conditionals to require that the conditionals on each side of it are both true:

if( ( 2 == 2 ) && ( 3 == 3 ) )
puts( "Yes" );

will print.

Answer 7

More precisely && is a logic AND operation, while & is a bitwise AND operation. So && will return true when both operand are logically TRUE.