Prime Number in C

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A prime number in C is a Positive Number that can be divided exactly only by itself.

What is a prime number?

A prime number is a positive integer greater than 1 that has no positive integer divisors other than 1 and itself. In other words, a prime number is a number that can only be divided evenly by 1 and itself.

For example, the first six prime numbers are 2, 3, 5, 7, 11, and 13. These numbers can only be divided evenly by 1 and themselves. The number 4, for example, is not a prime number because it can be divided evenly by 2 and itself.

Prime numbers play an important role in number theory and have many applications in computer science and cryptography.

Approach to check prime number in C language

Here is one approach to check whether a given number is prime in C:

First, check for the base cases. If the number is less than or equal to 1, it is not prime and the function can return 0. If the number is 2 or 3, it is prime and the function can return 1.
Next, check for divisibility by 2 and 3. If the number is divisible by 2 or 3, it is not prime and the function can return 0.
To check for divisibility by other integers, use a for loop to iterate through all integers in the range [5, sqrt(n)]. For each integer, check if the number is divisible by it or by (it+2). If it is divisible by any of these integers, it is not prime and the function can return 0.
If the number has passed all the checks above, it is considered prime and the function can return 1.

Pseudo code to check prime number in C language

Here is some sample pseudocode for checking whether a given number is prime in C:

int is_prime(int n) {
    if (n <= 1) {
        return 0;
    }
    if (n <= 3) {
        return 1;
    }
    if (n % 2 == 0 || n % 3 == 0) {
        return 0;
    }
    for (int i = 5; i * i <= n; i = i + 6) {
        if (n % i == 0 || n % (i + 2) == 0) {
            return 0;
        }
    }
    return 1;
}

This code uses a simple algorithm to check whether the input number is prime. It first checks for the base cases (n <= 1 and n <= 3), then checks if the number is divisible by 2 or 3. If the number passes those checks, it uses a for loop to check for divisibility by all integers in the range [5, sqrt(n)]. If the number is not divisible by any of those integers, it is considered prime and the function returns 1. Otherwise, it returns 0.

C program to check prime numbers.

Here is a sample C program that uses the approach I described earlier to check whether a given number is prime

#include <stdio.h>
#include <math.h>

int is_prime(int n) {
    if (n <= 1) {
        return 0;
    }
    if (n <= 3) {
        return 1;
    }
    if (n % 2 == 0 || n % 3 == 0) {
        return 0;
    }
    for (int i = 5; i * i <= n; i = i + 6) {
        if (n % i == 0 || n % (i + 2) == 0) {
            return 0;
        }
    }
    return 1;
}

int main() {
    int num;
    printf("Enter a positive integer: ");
    scanf("%d", &num);
    if (is_prime(num)) {
        printf("%d is a prime number\n", num);
    } else {
        printf("%d is not a prime number\n", num);
    }
    return 0;
}

This program uses the is_prime function to check whether a given number is prime. The user is prompted to enter a positive integer, and the function is called with this number as an argument. The function returns 1 if the number is prime and 0 if it is not. The program then prints the appropriate message to the console.

Other Programs :

Fibonacci Series in C