{"id":552,"date":"2025-11-21T08:53:09","date_gmt":"2025-11-21T03:23:09","guid":{"rendered":"https:\/\/codexplained.in\/?p=552"},"modified":"2025-11-21T08:53:09","modified_gmt":"2025-11-21T03:23:09","slug":"check-prime-no-use-by-looping","status":"publish","type":"post","link":"https:\/\/codexplained.in\/?p=552","title":{"rendered":"Check Prime no. Use by looping"},"content":{"rendered":"
\n#include <stdio.h>\n\n\/\/ Function to check if a number is prime using recursion\nint isPrime(int num, int i) \n{\n    \/\/ Base cases\n    if (num <= 2) \n{\n        if (num == 2) \n{\n            return 1;  \/\/ 2 is a prime number\n        } else {\n            return 0;  \/\/ Numbers less than 2 are not prime\n        }\n    }\n    \n    if (num % i == 0) \n{\n        return 0;  \/\/ If num is divisible by i, it's not prime\n    }\n\n    if (i * i > num) \n{\n        return 1;  \/\/ If no divisors are found, num is prime\n    }\n\n    \/\/ Recursive call: increment i to check the next possible divisor\n    return isPrime(num, i + 1);\n}\n\nint main() \n{\n    int num;\n\n    \/\/ Input the number from user\n    printf("Enter a positive integer: ");\n    scanf("%d", &num);\n\n    \/\/ Check if the number is prime and print result\n    if (isPrime(num, 2)) {\n        printf("%d is a prime number.\\n", num);\n    } \n\telse \n    {\n        printf("%d is not a prime number.\\n", num);\n    }\n\n    return 0;\n}\n\n\n<\/pre><\/div>\n\n\n
*example<\/strong>–<\/p>\n\n\n\n
->Enter a positive integer: 29
=29 is a prime number<\/p>\n\n\n\n
*explanation<\/strong><\/p>\n\n\n\n
1) isPrime Function:<\/p>\n\n\n\n
=This function takes two arguments: the number num and a divisor i that starts from 2.
=Base Case 1: If the number num is less than or equal to 2:
=If num is 2, it’s a prime number, so return 1 (true).
=If num is less than 2 (such as 0 or 1), it’s not prime, so return 0 (false).
=Base Case 2: If num is divisible by i, meaning num % i == 0, then num is not prime, and we return 0 (false).
=Base Case 3: If i * i exceeds num, it means we’ve checked all possible divisors up to the square root of num, and since no divisors were found, we return 1 (true), declaring num as prime.
=Recursive Call: The function calls itself, incrementing i by 1 each time to check the next possible divisor.<\/p>\n\n\n\n
2) Main Function:<\/p>\n\n\n\n
=The user is prompted to input a number.
=The is Prime function is called with the user input and 2 as the starting divisor.
=Based on the result of the is Prime function, a message is printed to indicate whether the number is prime or not.<\/p>\n\n\n\n
*How Recursion Works<\/strong><\/p>\n\n\n\n
Recursion means that a function calls itself until a specific condition is met. In this program, we recursively check for divisors of the input number. The base cases ensure that the recursion terminates when either:<\/p>\n\n\n\n