C Program to Check Whether a Number is Prime or not
Given a positive integer N. The task is to write a C program to check if the number is prime or not.
Definition:
A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. The first few prime numbers are {2, 3, 5, 7, 11, ….}
The idea to solve this problem is to iterate through all the numbers starting from 2 to sqrt(N) using a for loop and for every number check if it divides N. If we find any number that divides, we return false. If we did not find any number between 2 and sqrt(N) which divides N then it means that N is prime and we will return True.
Why did we choose sqrt(N)?
The reason is that the smallest and greater than one factor of a number cannot be more than the sqrt of N. And we stop as soon as we find a factor. For example, if N is 49, the smallest factor is 7. For 15, smallest factor is 3.
Below is the C program to check if a number is prime:
C++
// C++ program to check if a// number is prime#include <iostream>#include <math.h>using namespace std;int main(){ int n, i, flag = 1; // Ask user for input cout <<"Enter a number: \n"; // Store input number in a variable cin >> n ; // Iterate from 2 to sqrt(n) for (i = 2; i <= sqrt(n); i++) { // If n is divisible by any number between // 2 and n/2, it is not prime if (n % i == 0) { flag = 0; break; } } if (n <= 1) flag = 0; if (flag == 1) { cout << n <<" is a prime number"; } else { cout << n <<" is not a prime number"; } return 0;}// This code is contributed by shivanisinghss2110. |
C
// C program to check if a// number is prime#include <math.h>#include <stdio.h>int main(){ int n, i, flag = 1; // Ask user for input printf("Enter a number: \n"); // Store input number in a variable scanf("%d", &n); // Iterate from 2 to sqrt(n) for (i = 2; i <= sqrt(n); i++) { // If n is divisible by any number between // 2 and n/2, it is not prime if (n % i == 0) { flag = 0; break; } } if (n <= 1) flag = 0; if (flag == 1) { printf("%d is a prime number", n); } else { printf("%d is not a prime number", n); } return 0;} |
Output:
Enter a number: 11 11 is a prime number
Time Complexity: O(n1/2)
Auxiliary Space: O(1)
.png)
