CODEWITHSUNDEEP
cif-statementreturnreturn-value
Lester John
Lester John

Reputation: 23

Function confusion in return value, what is true and false explicitly?

I am new to C, and in StackExchange here and all other sources, 0 is success, else is false. In this function to print prime numbers, why does it only print prime numbers if return value is 1?

like if I go (is_prime(num) == 0), then it will not print prime number, but if just said is_prime(num) it automatically assumes (is_prime(num) == 1) ?

This has confused me, please clarify because the value will switch between 0 and 1, but why the bias automatically?

int is_prime(int num){
   int isPrime = 1;
   int i;

   for(i = 2; i <= sqrt(num); i++){
      if(num % i == 0){
         isPrime =  0;
      }
   }

   return isPrime;
}

Upvotes: 2

Views: 1905

Answers (2)

John Kugelman
John Kugelman

Reputation: 361585

I am new to C, and in StackExchange here and all other sources, 0 is success, else is false.

That's backwards. In C, 0 is false and non-zero values are true.

if (number)    <==>   if (number != 0)
if (!number)   <==>   if (number == 0)

This is true most of the time and is the most common integer → boolean relationship. There are a few exceptions where the mapping is reversed:

  • The return value of main() is 0 for success, non-zero for failure. The reason for this is if a program fails you may want to distinguish between errors by returning different exit codes. For instance, 1 for "bad command line arguments", 2 for "file not found", 3 for "could not connect to server", etc. The meaning of these exit codes is application-dependent.

  • Many POSIX system calls such as close() and connect() use the same idea, returning 0 for success and -1 for errors. For these functions you must write if (connect(...) != 0) rather than if (connect(...)).

Note that these exceptions are not part of the C language itself, but rather with commonly-used C functions.

Upvotes: 3

qaphla
qaphla

Reputation: 4733

In an if statement, or any case where a boolean value (True or false) is being tested, in C, at least, 0 represents false, and any nonzero integer represents true.

If, for some reason, your isPrime function returned -17 given some input x, isPrime(x) would still be considered true if taken as a boolean, and thus if you had some code inside an if block whose condition was isPrime(x), that code would be run, because -17 is nonzero.

Upvotes: 1

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