# C Program to Check if a given Number is Prime number

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This is a C program to check whether a given number is prime or not.

Problem Description

This program takes a number and checks whether a given number is prime or not.

Problem Solution

1. Take a number as input.
2. Check if the number is divisible by any of the natural numbers starting from 2.
3. If it is, then it is not a prime number. Otherwise it is a prime number.
4. Exit.

Program/Source Code

Here is source code of the C program to check if a given number is prime. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

1. `/*`
2. ` * C program to check whether a given number is prime or not`
3. ` * and output the given number with suitable message.`
4. ` */`
5. `#include <stdio.h>`
6. `#include <stdlib.h>`
7. ` `
8. `void main()`
9. `{`
10. `    int num, j, flag;`
11. ` `
12. `    printf("Enter a number \n");`
13. `    scanf("%d", &num);`
14. ` `
15. `    if (num <= 1)`
16. `    {`
17. `        printf("%d is not a prime numbers \n", num);`
18. `        exit(1);`
19. `    }`
20. `    flag = 0;`
21. `    for (j = 2; j <= num / 2; j++)`
22. `    {`
23. `        if ((num % j) == 0)`
24. `        {`
25. `            flag = 1;`
26. `            break;`
27. `        }`
28. `    }`
29. `    if (flag == 0)`
30. `        printf("%d is a prime number \n", num);`
31. `     else`
32. `        printf("%d is not a prime number \n", num);`
33. `}`
Program Explanation

1. Take a number as input and store it in the variable num.
2. If the number is lesser than or equal to 1, then print the output as ” It is not a prime number”.
3. Initialize the variable flag to zero.
4. Using for loop, check if the input number is divisible by any of the natural numbers starting from 2.
5. If it is, then assign the variable flag with 1.
6. Print the output as “It is a prime number”, if the variable flag ==0.
7. Otherwise print the output as “It is not a prime number” and exit.

Runtime Test Cases
```
Enter a number
23
23 is a prime number

Enter a number
15
15 is not a prime number```

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