Question

C language

int is_prime(int n)
{
if (n <= 1) return 0;
if (n % 2 == 0 && n > 2) return 0;
for (int i = 3; i < n / 2; i += 2)
if (n % i == 0)
return 0;
return 1;
}

What do the three lines in this function mean?

Answer 1

Given function actually finds whether given integer is a positive prime number or not, if its apositive prime number it returns 1 and 0 otherwise.

first line [if (n <= 1)] checks whether given integer is less than 1 then it returns 0.

second line [if(n%2==0 && n>2)] checks for even number greater than 2 and returns 0 for those even numbers.

Now consider the bold 3 lines of code:

for loop starts with value of i=3, each iteration it increments i by 2 until n/2.

i.e. i values are 3,5,7,...n/2

In each iteration it checks whether given number divides i, so its not prime and returns 0.

Now after termination of for loop all cases to check nnot prime is over so the number is prime and returns 1.