Prime or not (Java) (doubt)

Hey guys it have been around 8 - 10 months since I started learning Java.I understand most of the basic concepts and programs written by others. But I can't figure out how to findout that a number is prime or not. So I surfed net mainly Stackoverflow to find out . But after several attempts I wasn't able to. Please can anybody give the answer as well as explanation.

PS - It may sound stupid sorry for that...

#Primes #ComputerScience #JAVA

Easy Math Editor

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Comments

To find out whether a number is prime or not, you need to find out whether it is divisible by natural numbers greater than 1 and less than the number. Here is the easiest algorithm to find out if a number is prime or not -

private boolean prime(int n){
    for(int i = 2; i < n; i++){
        if(n % i == 0){
            return false;
        }
    }
    return true;
}

Consider any natural number $n$ . In the above code, you loop from $2$ to $n-1$ . If any number divides $n$ completely, you return false as the number is divisible by some number and it is composite. If no number divides $n$ then you return true as the number is prime.

Note : The % sign means modulo. It finds out the remainder when a number is divided by another number. For eg, 9 mod 3 or 9 % 3 is 0 as when 9 is divided by 3, remainder is 0.

The code given by me is certainly very inefficient. Can you figure out a way to make the code faster? Also remember to read this post - Prime Test.

Arulx Z - 6 years ago

Thank You so much that helped me a lot !!

Tanmay Jain - 6 years ago

You are welcome

Arulx Z - 6 years ago

Math	Appears as
Remember to wrap math in `\(` ... `\)` or `\[` ... `\]` to ensure proper formatting.
`2 \times 3`	$2 \times 3$
`2^{34}`	$2^{34}$
`a_{i-1}`	$a_{i-1}$
`\frac{2}{3}`	$\frac{2}{3}$
`\sqrt{2}`	$\sqrt{2}$
`\sum_{i=1}^3`	$\sum_{i=1}^3$
`\sin \theta`	$\sin \theta$
`\boxed{123}`	$\boxed{123}$