What happens when a variable is assigned to zero in an `if` condition?

Question

It would be helpful if anybody can explain this.

int main()
{
 int a=0;
 if(a=0)
       printf("a is zero\t");
 else
       printf("a is not zero\t");
 printf("Value of a is %d\n",a);
 return 0;
}

output of this is

a is not zero   Value of a is 0 

Answer 1

The result of the assignment is the value of the expression.

Therefore:

if (a = 0)

is the same as:

if (0)

which is the same as:

if (false)

which will force the else path.

Answer 2

if(a=0)
       printf("a is zero\t");
 else
       printf("a is not zero\t");

These messages are precisely backwards. The statement after the if is executed if the condition isn't 0, and the statement after the else is executed if the condition is 0, so this should be

if(a=0)
       printf("a is not zero\t");
 else
       printf("a is zero\t");

Or, equivalently but more clearly,

a = 0;
if(a)
       printf("a is not zero\t");
 else
       printf("a is zero\t");

Which, together with

printf("Value of a is %d\n",a);

would print

a is zero   Value of a is 0 

as expected.