logical AND and OR in c

Question

In a && b , this returns true if both a and b are equal to 1. If a=-1 and b=-1 then also the expression returns true.Similar is the case with a||b,where a=-1 and b=0,it returns true. Can anybody please explain the reason.

Answer 1

a && b returns 1 when both a and b are nonzero, not just when they're equal to 1. Otherwise it returns 0.

a || b returns 1 when at least one of a or b is nonzero, not just when one of them is equal to 1. Otherwise it returns 0.

To give some examples:

   0 &&  0 -> 0
   1 &&  0 -> 0
   1 &&  1 -> 1
   2 &&  1 -> 1
  -1 && -1 -> 1
-100 &&  0 -> 0

   0 ||  0 -> 0
   1 ||  0 -> 1
   0 ||  1 -> 1
  -1 ||  0 -> 1
-100 || 20 -> 1

Answer 2

C11 (n1570) §6.5.13 al 3 p 99 say :

The && operator shall yield 1 if both of its operands compare unequal to 0; otherwise, it yields 0.

-1 is a nonzero value, so -1 && -1 is 1.

Answer 3

On the MSDN page for &&:

The logical-AND operator produces the value 1 if both operands have nonzero values.

Clearly both operands are -1 in your example so they will produce 1.

On the same page, listed for ||

If either operand has a nonzero value, the result is 1.

In your case one operand is -1, therefore the result is 1

Answer 4

AND returns 1 if both operands have non-zero values. OR returns 1 if either operand has a non-zero value.