How do you write a C Program to Print all the Prime Numbers between 1 and 100.?

Answer 1

C does not prove a built-in function to determine if a number is prime or not, thus we must write one. The implementation is reasonably trivial:

bool is_prime (unsigned int value) {

if (value<2) return false;

if (!(value%2)) return value==2;

unsigned int max_factor, factor;

max_factor = sqrt (value) + 1;

for (factor=3; factor<max_factor; factor+=2)

if (!(value%factor)) return false;

return true;

}

That is; we return false when value is less than 2 (because 2 is the first prime).

If the value is even, we return true if the value is 2 (because 2 is the only even prime).

For all other values greater than 2, we divide the value by increasing odd factors (3, 5, 7, 9, etc) up to the square root of the value. If the value is evenly divisible by any of these (potential) factors, the number cannot be prime so we return false. If none of the potential factors evenly divides into the value, the value is prime so we return true.

Given this function, we can now create another function that will determine the next prime after any given value (the given value need not be prime).

unsigned int next_prime (unsigned int value) {

while (!is_prime (++value));

return value;

}

Here we increment the value until the is_prime() function returns true, at which point we return the value, which is now prime.

Now we can print all the prime numbers from 1 to 100:

int main (void) {

int n;

n=1;

while ((num=next_prime (num))<=100) {

printf ("%d\n", num);

}

return 0;

}