🚀⚡Beginner

Print Prime Numbers From 1 to N

Display all primes up to N

This program prints all prime numbers from 1 to N, where N is entered by the user. A prime number is a number greater than 1 that has no divisors other than 1 and itself.

📝 Example

Input: N = 20

Output:

2 3 5 7 11 13 17 19

📋 Algorithm

Take N as input from the user
Loop through numbers from 2 to N
For each number, check if it's prime
If prime, print the number

C Program to Print Prime Numbers from 1 to N

prime_1_to_n.c

1#include <stdio.h>
2#include <stdbool.h>
3
4// Function to check if a number is prime
5bool isPrime(int num) {
6    if (num <= 1) return false;
7    if (num == 2) return true;
8    if (num % 2 == 0) return false;
9    
10    // Check odd numbers up to sqrt(num)
11    for (int i = 3; i * i <= num; i += 2) {
12        if (num % i == 0)
13            return false;
14    }
15    return true;
16}
17
18int main() {
19    int n;
20    
21    printf("Enter N: ");
22    scanf("%d", &n);
23    
24    printf("Prime numbers from 1 to %d:\n", n);
25    
26    for (int i = 2; i <= n; i++) {
27        if (isPrime(i)) {
28            printf("%d ", i);
29        }
30    }
31    printf("\n");
32    
33    return 0;
34}

$ ./prime_1_to_n

Enter N: 50\nPrime numbers from 1 to 50:\n2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

📖 Code Explanation

Code	Explanation
bool isPrime(int num)	Function that returns true if num is prime
if (num <= 1)	Numbers ≤ 1 are not prime
if (num % 2 == 0)	Even numbers (except 2) are not prime
i * i <= num	Only check up to √num for efficiency
i += 2	Skip even numbers (check only odd)

Simple Version (Without Function)

prime_simple.c

1#include <stdio.h>
2
3int main() {
4    int n, i, j, isPrime;
5    
6    printf("Enter N: ");
7    scanf("%d", &n);
8    
9    printf("Prime numbers from 1 to %d:\n", n);
10    
11    for (i = 2; i <= n; i++) {
12        isPrime = 1;  // Assume prime
13        
14        for (j = 2; j * j <= i; j++) {
15            if (i % j == 0) {
16                isPrime = 0;  // Not prime
17                break;
18            }
19        }
20        
21        if (isPrime)
22            printf("%d ", i);
23    }
24    
25    return 0;
26}