Prime numbers

[Sebastien Zerbato]

Hello, I am trying to code this program that finds and prints out all the prime numbers between two numbers , it compiles fine but in the execution nothing happens, I am suspecting an infinite loop but cannot find the problem. Thank you for your time.

import java.lang.String;
import java.io.*;
import java.util.Arrays;
public class Erathostene1 {
    public static void main (String[] args) {
        String borneSuperieure = args[0];
        int n = Integer.parseInt(args[0]);
        if (!( n > 1)) { //Vérification du paramètre.
            System.out.println("Rentrer une valeure superieure a un.");
            System.exit(0);
        }
        int tailleTableau = (n - 2);
        boolean [] tableauParamètres = new boolean [tailleTableau];
        Arrays.fill(tableauParamètres, Boolean.TRUE);
        for (int i = 0; i < (tailleTableau) ; i++) {
            for (int j = i ; j < (tailleTableau); j=i+j) {
            tableauParamètres[j] = false;
            }
            if (tableauParamètres[i]) {
                System.out.print(i);
            }
        }
    }
}

[Paul Clapham]

I wouldn't expect that program to produce any output. Perhaps that's what is confusing you, the fact that you were expecting some output. I suggest you put in some debugging code to show you which array entries are being set to false.

[Sebastien Zerbato]

I did it found the error for the infinite loop thanks to you, : ) the code brings these number out for an input of 50: 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 4 8 12 16 20 24 28 32 36 40 44 5 10 15 20 25 30 35 40 45 6 12 18 24 30 36 42 7 14 21 28 35 42 8 16 24 32 40 9 18 27 36 45 10 20 30 40 11 22 33 44 12 24 36 13 26 39 14 28 42 15 30 45 16 32 17 34 18 36 19 38 20 40 21 42 22 44 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 so it appears to work, but I don't want it to take the first numbers for example 2,3,7. Do you have any idea of how to do that ?

[Paul Clapham]

So you are marking all of these numbers as non-primes:

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44

And also these:

3 6 9 12 15 18 21 24 27 30 33 36 39 42 45

But you already noticed that flagging the first one of each list as non-prime was incorrect, didn't you? Now look at the loop which does that. Notice where it starts. It shouldn't start there, should it?

[Sebastien Zerbato]

Ok, I get what you mean, Should it be ?

   for (int j = i+j ; j < (tailleTableau - 2); j=i+j) 

[Sebastien Zerbato]

ok not thinking right should it be more like this:

 for (int j = 2i ; j < (tailleTableau - 2); j=i+j) 

?

[Paul Clapham]

No, that isn't quite right.

You didn't get it right because your variable names "i" and "j" don't mean anything so it's easy to get them confused. Which you did.

I always find it easier to think of an example while I am working on code. In your case:

2 4 6 8 10 ...

you don't want to start at 2, you want to start at 4. Likewise with

3 6 9 12 15 ...

you don't want to start at 3, you want to start at 6. Does a pattern start to appear?

[Sebastien Zerbato]

Point taken, but then what would you call them ?

[Paul Clapham]

If you had used the name "nonPrime" instead of "j" I think you would have been less likely to start by assigning "i" to it.

[Sebastien Zerbato]

I now am officially brain-dead. I get what you mean by naming it nonPrime though but I don't see how that troubles my logic.

[Sebastien Zerbato]

Paul Clapham wrote:No, that isn't quite right.

You didn't get it right because your variable names "i" and "j" don't mean anything so it's easy to get them confused. Which you did.

I always find it easier to think of an example while I am working on code. In your case:

2 4 6 8 10 ...

you don't want to start at 2, you want to start at 4. Likewise with

3 6 9 12 15 ...

you don't want to start at 3, you want to start at 6. Does a pattern start to appear?

Ok yeah I didn't see that part of the comment before weird.
And yeah I agree and that was what I tried to do by saying "2*i".

[Sebastien Zerbato]

The thing is, I know it is wrong the right thing to do would be to directly start at the time you go into the loop. so I tried :

for (int nonPrime = i ; nonPrime < (tailleTableau - 2); nonPrime=i+nonPrime) {

but obviously the error "variable may not have been initialized appeared. Aaand I'm stuck.

[Paul Clapham]

Sebastien Zerbato wrote:Ok yeah I didn't see that part of the comment before weird.

That's because I edited the post after you had looked at it.

And yeah I agree and that was what I tried to do by saying "2*i".

Except that when I looked at your post it said "i + j" and not "2 * i". That's because you edited the post after I had looked at it.

[Sebastien Zerbato]

Oh ok,
so... Am I right ?

[Paul Clapham]

Sebastien Zerbato wrote:so... Am I right ?

Try it and see.

[Sebastien Zerbato]

it prints this out "2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 3 3 3 3 4 4 4 4 4 4 4 4 4 4 5 5 5 5 5 5 5 5 6 6 6 6 6 6 7 7 7 7 7 8 8 8 8 9 9 9 9 10 10 10 11 11 11 12 12 13 13 14 14 15 15 16 17 18 19 20 21 22 23 " so it doesn't work..

[Paul Clapham]

Then fix it.

Have a look at this FAQ article: ItDoesntWorkIsUseless -- and remember you are talking about code which nobody but you can see.

[Sebastien Zerbato]

hahaha. Point taken. I'll try and find out what the bug is..