CC-3 : Check if number is prime.

Description:

Given an input number check if it is a prime number.

Test cases and expected outputs:

Input Parameters	Expected outputs
Number: 10	Number is not prime
Number: 7	Number is prime

Pseudocode:

checkPrime(num, divisor):

The number to be checked is received as input parameter. The number divisor is also received as input parameter.

We will divide number by divisor to check if it prime.

Execute following steps if divisor is > num:

If remainder after dividing number by divisor is not equal to 0:

Increment divisor by 1.

Recursively call checkPrime() with num and updated value of divisor as parameters.

Else if remainder is 0, return 0, indicating input number is not prime.

Else return 1, indicating number is prime, as we could not find any divisor which returns 0 after dividing num.

Code:

public int checkPrime(int num, int divisor){
	if (divisor < num) {
		if (num % divisor != 0) {
			divisor++;
			return checkPrime(num, divisor);
		}else {
			return 0;
		}
	}
	return 1;
}

Click here to download and run code and test cases !

Below fully running code can be copied and run on Eclipse or other Java IDEs. Refer the classname in code below. If the class name below is "A", save the code below to a file named A.java before running it. Be sure to try your own test cases to enhance your understanding ! You can also tweak the code to optimize or add enhancements and custom features.

public class RecursionCheckPrime {

public int checkPrime(int num, int divisor){
	if (divisor < num) {
		if (num % divisor != 0) {
			divisor++;
			return checkPrime(num, divisor);
		}else {
			return 0;
		}
	}
	return 1;
}


public static void main(String[] args) {
	/***********************
	 Test cases given below:
	 **********************/
	int retVal;
	try {
	
	RecursionCheckPrime rcn=new RecursionCheckPrime();
	int num=10;
	retVal=rcn.checkPrime(num, 2);
	System.out.println("Number: "+num);
	if (retVal==1) {
		System.out.println("Number is prime ");	
	} else {
		System.out.println("Number is not prime ");
	}
	System.out.println();
	
	num=6;
	retVal=rcn.checkPrime(num, 2);
	System.out.println("Number: "+num);
	if (retVal==1) {
		System.out.println("Number is prime ");	
	} else {
		System.out.println("Number is not prime ");
	}
	System.out.println();
	
	
	num=7;
	retVal=rcn.checkPrime(num, 2);
	System.out.println("Number: "+num);
	if (retVal==1) {
		System.out.println("Number is prime ");	
	} else {
		System.out.println("Number is not prime ");
	}
	System.out.println();
	
	}catch (Exception exception) {
		System.out.print("Exception: "+ exception);
		exception.printStackTrace();
	}
		
}

}