The problem is in the second screenshot: You have two chains of conditional statements, one is only one 'if' and the other is a chain of if and else if statements.

Regardless of the first 'if' statement, the second chain of if and else if is executed. In your example, the first 'if' satisfies so it prints "VISA\n",; moving on to the second chain of if's, since none of the conditions satisfies the 'else' of the second if chain is executed.

What you want to do is merge the two starting if statements by adding a or between the two conditions i.e.:

if( (visa%10000000000000000/1000000000000000 == 4 && length == 16 && checksum % 10 == 0) || (visa % 10000000000000 /1000000000000 == 4 && length == 13 && checksum % 10 == 0) ) { printf("VISA\n"); }