Merge pull request #69 from sccalabr/issue-67

C Program to Check Prime Numbers

Author: devmalik7

C/prime_checker/README.md

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+# Prime Number Checker
+
+A C program to check if a number is prime or not.
+
+## What it does
+
+Takes a number as input and tells you whether it's prime or not. Pretty straightforward.
+
+## How to compile
+
+```bash
+gcc prime_checker.c -o prime_checker -lm
+```
+
+The `-lm` flag is needed for the math library.
+
+## Usage
+
+```bash
+./prime_checker
+```
+
+Then just enter a number when it asks.
+
+## Examples
+
+```
+Prime Number Checker
+Enter a number: 17
+17 is Prime
+```
+
+```
+Prime Number Checker
+Enter a number: 24
+24 is Not Prime
+```
+
+## How it works
+
+The program checks divisibility up to the square root of the number. If nothing divides it evenly, it's prime. Numbers less than 2 aren't considered prime.
+
+## Test it with these
+
+Some primes to try: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97
+
+Some non-primes: 4, 6, 8, 9, 10, 12, 15, 16, 18, 20, 21, 24, 25, 27, 28, 30
+

C/prime_checker/prime_checker.c

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+#include <stdio.h>
+#include <stdbool.h>
+#include <math.h>
+
+bool isPrime(int num) {
+    if (num <= 1) return false;
+    if (num == 2) return true;
+    if (num % 2 == 0) return false;
+    
+    int limit = (int)sqrt(num);
+    for (int i = 3; i <= limit; i += 2) {
+        if (num % i == 0) return false;
+    }
+    
+    return true;
+}
+
+int main() {
+    int number;
+    
+    printf("Prime Number Checker\n");
+    printf("Enter a number: ");
+    
+    if (scanf("%d", &number) != 1) {
+        printf("Invalid input!\n");
+        return 1;
+    }
+    
+    if (isPrime(number)) {
+        printf("%d is Prime\n", number);
+    } else {
+        printf("%d is Not Prime\n", number);
+    }
+    
+    return 0;
+}
+