# Prime number generator in C

I have to print numbers between two limits n and m, t times.

I created t variable that stores a number of test cases.

Outer for loop iterates for every test cases.

Inner for loop prints primes from m to n.

# Code

#include <stdio.h>
#include <stdlib.h>

int is_prime(int);

int main(void) {
int t, m, n;
scanf("%d", &t);
for (int i = 0; i < t; i++) {
scanf("%d %d", &m, &n);
for (int j = m; j <= n; j++) {
if (is_prime(j)) {
printf("%d\n", j);
}
}
if (i < t - 1) printf("\n");
}

return 0;
}

int is_prime(int num) {
if (num <= 1) return 0;
for (int i = 2; i * i <= num; i++) {
if (num % i == 0) {
return 0;
}
}
return 1;
}


Can I have review?

The only even prime number is two, so with a little extra coding you can ignore even numbers in the for loop. For even more speed use a sieve, though maybe save that for when you get a 'TLE' failure.

A more formal code layout would be good as well, your if statements need expanding to include {...}. More vertical space (blank lines) a well as explanatory comments help. When you come back to your old code six months later you will thank yourself.

I prefer to set 0 and 1 as FALSE and TRUE to make code easier to read YMMV.

int is_prime(int num) {
if (num <= 1) {
return FALSE;
}

// Even numbers.
if (num % 2 == 0) {
return num == 2;  // 2 is the only even prime.
}

// Odd numbers.
// Start loop at 3 and step 2 to skip even divisors.
for (int i = 3; i * i <= num; i += 2) {
if (num % i == 0) {
return FALSE;
}
}
return TRUE;
}

• Just include <stdbool.h> to avoid having to define it yourself, and get the lowercase versions at the same time. Commented Jul 10, 2017 at 5:13
• @TamoghnaChowdhury: Thanks. My C is very rusty. Commented Jul 10, 2017 at 7:49

Good that OP's code handles values <= 0, yet could have used is_prime(unsigned num) instead. Further: this is a good place for the only even prime detect.

Corner concern: The i * i <= num test fails for large num, like num = INT_MAX as i*i is always <= than INT_MAX or it is int overflow - which is undefined behavior (UB).

Preference: Use bool for return values that are either 0 or 1.

Many modern compilers/processors calculate the remainder and quotient for little/no additional cost. Use that as an exit condition.

bool is_prime(uintmax_t num) {
if (num <= 3) {
return num >= 2;
}
uintmax_t q = num;
for (uintmax_t i = 3; i <= q; i += 2) {
if (num % i == 0) {
return false;
}
q = num / i;
}
return num%2;
}


The next step in prime detection is use of the Sieve of Eratosthenes

Note for future: Code does not check the return value of scanf(). This is OK for test code, but not for code under test.