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# Thread: PrimeNumbers

- 07-19-2009, 12:39 AM #1Member
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## PrimeNumbers

So this is my problem

Create a program that calculates the prime numbers from any inputted start

and end range of values (e.g. 5 and 28) and print the prime numbers in rows

of 10. Hint: Use a counter to count how many values are printed and then

print a blank line and re-initialize the counter back to 0.

and I have this:

import java.awt.*;

import hsa.Console;

public class Activity3ad

{

static Console c;

public static void main (String[] args)

{

c = new Console ();

int start, last;

c.println ("Please enter the first number");

first = c.readInt ();

c.println ("Please enter the last number");

last = c.readInt ();

boolean isPrime[];

isPrime = new boolean[last+1];

for (int i=start; i <=last; i++)

{

isPrime[i] = true;

}

for (int i=first; i<=last; i++)

{

if (isPrime[i])

{

for (int j=first*i; j<=last; j+=i)

{

isPrime[j] = false;

}

}

}

c.println ("between" + first +" and " + last + " the prime numbers are ");

for (int i=first; i<last;i++)

{

if (isPrime[i])

{

c.print (i,5);

}

}

}

}//end

I have no idea what to do to fix and what to do next...

- 07-20-2009, 10:51 PM #2Member
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I am guessing you have another method called isPrime, so maybe your problem is there. What exactly are you having problems with? Helps to know what I am looking for.

- 07-23-2009, 10:41 AM #3Member
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try this code, i wrote it years ago:

Main Class:

public static void main(String[] args) {

int start = 0;

int end = 51;

ArrayList al = PrimeNumber.extractPrime(start, end);

System.out.println("prime beetwen " + start + " and " + end + " are: ");

// loop to reveal result

for (int counter = 0; counter < al.size(); ++counter) {

System.out.println((Integer) al.get(counter));

}

}

public class PrimeNumber {

public static ArrayList extractPrime(int _start, int _end) throws IllegalArgumentException {

// validation

if (_end < _start) {

throw new IllegalArgumentException("invalid argument");

}

// validation

if ((_start < 0) || (_end < 0)) {

throw new IllegalArgumentException("invalid argument");

}

// this ArrayList contains all primary numbers

ArrayList arrayList = new ArrayList();

int factor = 0;

int counter = 0;

int number = 0;

for (number = _start; number < _end; ++number) {

if (number > 1) {

for (counter = 1; counter <= number; ++counter) {

// find the factor

if (number % counter == 0) {

++factor;

}

}

// prime number has only two factors, 1 and the number it self

if (factor == 2) {

arrayList.add(number);

}

// reset the factor

factor = 0;

}

}

return (arrayList);

}

}

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