Why do these conditions not work?

Question

3.00/5 (1 vote)

See more:

I started programing Java about a week ago, and this is my first post here.

I came up with this method for determining if a number is prime. It has been working flawlessly except for the case of 1 or 0. I want to exclude 1 and 0 from being considered prime. However it doesn't seem to have any effect.

Java

//determines if number in form 'long' is prime. Returns boolean.
public static boolean isPrime(final long a){ 
		
	boolean is = true;
		
	for (double i = 2; i <= (.5*a); i++) {
			
		if ((a%i==0)||(a==0)||(a==1)){
			is = false;
		}//end if
	}//end for loop
		return is;
}//end of isPrime.

Posted 10-Mar-14 16:08pm

CarlPayne

Add a Solution

Comments

Sergey Alexandrovich Kryukov 10-Mar-14 22:13pm

If you used the debugger, you would see the problem immediately. So, use the debugger, in case of any doubt of you runtime, any at all...
—SA

CarlPayne 10-Mar-14 22:56pm

I tried the debugger and can quickly tell that it is over my head at the moment. I will do some research on it immediately.
I did get it to work by moving the comparison with 1 and 0 to an if statement before the loop.
And changed "i <= (.5*a)" to "i*i <= a"

But still, from a syntax standpoint, why didn't the || statements work as is?

Sergey Alexandrovich Kryukov 11-Mar-14 2:24am

I told you what you have to do in my answer. Way too simple.
—SA

2 solutions

Solution 2

Firstly don't use double, use modulo arithmetic to check.
Secondly, you need to loop until the square root.
Thirdly, what about negative numbers?
Finally, If you have special cases for 0, 1, 2 and 3 then your loop can be shorted if you read this[^]

Luckily I'm bored today. The method is STATIC because it does not use any member variables. The special cases shorten the run time considerably for any reasonably large number; say greater than 437;

Java

static boolean isPrime(long number)
{
	if (number < 0)
	{
		// negatives are prime based on the absolute value
		return isPrime(-number);
	}
	if (number == 0 || number == 1)
	{
		// first two cases, these are not PRIME
		return false;
	}
	if (number % 2 == 0 || number % 3 == 0)
	{
		// we just have the simple prime:
		return true;
	}
	for (long divisor = 5;
			divisor <= number/divisor;
			divisor = divisor + 6)
	{
		if ((number % divisor) == 0 || (number % (divisor  + 2)) == 0 )
		{
			// exit the loop, we're done:
			return false;
		}
	}
	return true;
}

Posted 12-Mar-14 2:17am

Nagy Vilmos

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Sergey Alexandrovich Kryukov · Accepted Answer · 2014-03-10T16:23:00

Solution 1

To start with, this is bad, redundant code. Why would you need to compare a with 0 and 1 in each iteration of your loop? Check it before loop and return immediately. Always use the debugger, in case of any doubt in your runtime, any at all.

—SA

Posted 10-Mar-14 16:23pm

Sergey Alexandrovich Kryukov