Write a C Program to Print Prime Numbers From 1 to N.

In this article, prints all the prime numbers from 1 to N without using any structure and function. The program checks each number in the range [1, N] to see if it is a prime number or not.

Here's a C program to Print Prime Numbers From 1 to N:

#include<stdio.h>

#include<conio.h>

void main()

{

    int N, i, j, isPrime;

    // Get the value of N from the user

    printf("Enter a number N: ");

    scanf("%d", &N);

    // Print prime numbers from 1 to N

    printf("Prime numbers from 1 to %d are:\n", N);

    for (i = 2; i <= N; i++) {

        isPrime = 1; // Assume the number is prime initially

        // Check if the number i is divisible by any number from 2 to i-1

        for (j = 2; j < i; j++) {

            if (i % j == 0) {

                isPrime = 0; // If i is divisible by j, then it's not a prime number

                break;

            }

        }

        // If isPrime is still 1, then the number i is prime

        if (isPrime == 1) {

            printf("%d ", i);

        }

    }

    printf("\n");

}

• We start by declaring the necessary variables, namely 'N' to store the upper limit, 'i' to iterate through the numbers from 2 to N, 'j' to check for divisors, and 'isPrime' to store whether the current number is prime or not.
• The user is prompted to enter a value for 'N' using scanf.
• We then use a for loop to iterate through each number from 2 to N. Prime numbers start from 2 since 1 is not considered a prime number.
• Inside the outer loop, we set 'isPrime' to 1, assuming the current number (i) is prime initially.
• We use another nested for loop to check if the number i is divisible by any number from 2 to i-1. If i is divisible by any of these numbers, it means i is not a prime number, and we set isPrime to 0 and break out of the inner loop.
• If isPrime is still 1 after the inner loop, then the number i is prime. We print it using printf.
• The process continues until the outer loop finishes checking all numbers from 2 to N.
• The program will then print the prime numbers from 1 to N.

Output:

Note:  This program works fine for smaller values of N, but it may not be efficient for very large N due to the nested loops, which leads to a time complexity of approximately O(N^2). For more efficient prime number generation, you can look into optimized algorithms like the Sieve of Eratosthenes.