C Program to find prime numbers in a given range

In this article, we will write a C Program to find prime numbers in a given range. For example, if user enters the range from 1 to 10, then program should print prime numbers between 1 to 10, which are 2, 3, 5 and 7.

C Program to print prime numbers in a range

In this program, user is asked to enter the start and end range. The program, then prints all the prime numbers in the entered range. The explanation of the program is at the end of the code.

#include <stdio.h>
#include <stdbool.h>

// Function to check if a number is prime
bool isPrime(int n) {
    if (n <= 1) return false;
    if (n <= 3) return true;
    if (n % 2 == 0 || n % 3 == 0) return false;
    for (int i = 5; i * i <= n; i += 6) {
        if (n % i == 0 || n % (i + 2) == 0) return false;
    }
    return true;
}

// Function to find and print prime numbers in a given range
void findPrimesInRange(int start, int end) {
    printf("Prime numbers between %d and %d are: ", start, end);
    for (int i = start; i <= end; i++) {
        if (isPrime(i)) {
            printf("%d ", i);
        }
    }
    printf("\n");
}

int main() {
    int start, end;
    printf("Enter the start of the range: ");
    scanf("%d", &start);
    printf("Enter the end of the range: ");
    scanf("%d", &end);

    findPrimesInRange(start, end);

    return 0;
}

Output:

Enter the start of the range: 1
Enter the end of the range: 50
Prime numbers between 1 and 50 are: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 

Explanation of the program:

1. Include Header files:

#include <stdio.h>
#include <stdbool.h>

• #include <stdio.h>: It is required for printf and scanf functions.
• #include <stdbool.h>: It is required for bool type, which we have used as a return type for isPrime() function.

2. Prime Number Check Function: It checks whether a given number is prime or not.

bool isPrime(int n) {
    // Numbers less than or equal to 1 are not prime
    if (n <= 1) return false; 

    // 2 and 3 are prime numbers 
    if (n <= 3) return true;   

    // Multiples of 2 and 3 are not prime
    if (n % 2 == 0 || n % 3 == 0) return false;  

    // Check for factors from 5 to sqrt(n) using 6k ± 1 rule
    for (int i = 5; i * i <= n; i += 6) {
        if (n % i == 0 || n % (i + 2) == 0) 
          return false;
    }
    // If no factors are found, the number is prime
    return true;  
}

• It returns false for numbers less than or equal to 1 as these are not prime numbers.
• It returns true for 2 and 3 as these are prime numbers.
• It checks whether the number is divisible by 2 or 3, if it is then it returns false as multiples of 2 and 3 are not prime.
• For remaining numbers, it checks using a loop that runs from 5 to square root of n, in a loop of i+=6.

3. Function to Find Primes in a Range: This function prints the prime numbers in the given range

void findPrimesInRange(int start, int end) {
    printf("Prime numbers between %d and %d are: ", start, end);
    for (int i = start; i <= end; i++) {
        if (isPrime(i)) {
            printf("%d ", i);  // Print the prime number
        }
    }
    printf("\n");
}

• The start and end variables represents the range.
• It runs a loop from start to end.
• It checks each number in the range by calling isPrime() function.
• If isPrime() function returns true for a number, it prints the number.

4. Main function:

int main() {
    int start, end;
    printf("Enter the start of the range: ");
    scanf("%d", &start);
    printf("Enter the end of the range: ");
    scanf("%d", &end);

    findPrimesInRange(start, end);

    return 0;
}

• It prompts the user to enter the start and end of the range using printf and read the user entered values using scanf.
• It calls findPrimesInRange with entered range values to print all prime numbers in entered range.
• Finally, it returns 0 to indicate that program executed successfully.

Related C Programs:

• C Program to print prime numbers from 1 to 100 (or 1 to N)
• C Program to check whether a number is prime or not
• Java Program to check prime number
• Python Program to check if a number is prime or not