Prime Number - System print all the prime numbers ...

[pinkdreammsss]

Guys , it's me again !
How to write a java program for following question :

" A prime number (or a prime) is a natural number which has exactly two distinct natural number divisors: 1 and itself. Alex have problem with Prime Number. He would like system to print all the prime number until the number he entered. "

[CJSLMAN]

Who's Alex? Anyway, again, this is not "CODE-R-US". Somebody, either you or Alex, has to show some effort, so what have you done so far? What ideas do you have? What have you tried?

Luck,
CJSL

[pinkdreammsss]

I said so in another topic , i've just started with java and i have no idea about this language .

[CJSLMAN]

OK... here's a good place to start...

The Java™ Tutorials

Luck,
CJSL

[pinkdreammsss]

guy this is what i got for prime numbers

import java.util.Scanner;
public class prime{
public static void main (String args[]){
int num,prime;
Scanner input=new Scanner(System.in);
System.out.print("enter the number=");
num=input.nextInt();
num=
System.out.print(num);
}
}
}

my question is that i wanna tell to java that as user enters a number lets say "10" , display all prime number till "10" .
i think the formula goes after num =
however , idont know what's the formula , could guide me ?
by the way , do i need to use Do,while command for this program ?

[pinkdreammsss]

take a look at this code :

[CODE]import java.util.Scanner;
public class prime{
public static void main (String args[]){
int num;
Scanner input=new Scanner(System.in);
System.out.print("enter the number=");
num=input.nextInt();
for ( int i = 2; i <= num; i++ )
    
      if ( num % i == 0 )
    { 	 System.out.print();
}
}
}

[/CODE]

but the answer is not prime numbers till entered number.
why should i do with it ?

[pinkdreammsss]

i dont know what's the purpose of this forum!!! where i cann't get my answer :(

[corlettk]

> i dont know what's the purpose of this forum!!! where i cann't get my answer

If you went to a lumberjack forum to learn how to cut down trees, would you expect the people there to volunteer to cut-down your trees for you FOR FREE just because you're to lazy and stupid to workout how to use an axe, or saw, or a chain-saw, or a bulldozer?

Show some effort on your own behalf or take a hike buster.

Have a go yourself... We'll help, but nobody is going to do your homework for you.

Cheers. Keith.

[pinkdreammsss]

WTF ?!?!!?! please take a look at my last post , it's done by myself ! i stuck !!! ya'll want money for a little guidance? feel so sorry .

[logicbug]

You are on the right track, your for loop needs to change as such.

for ( int i = 1; i <= num; i++ ){
            if ( num % i == 0 )
                 	 System.out.println(i);
        }

[pinkdreammsss]

thank for your help !
i thought nobody is gonna help me !
dude , i've made the changes , as i enter 12 the prime numbers will be 1,2,3,4,6,12 ! i think 4 ,6 and 12 aren't prime . am i right ?

[logicbug]

woops, there is a problem with the math in the if statement.

[pinkdreammsss]

i've change the if statement to :
if ( num%2!=0||num==0)
but no result !

[corlettk]

If you aren't sure that 2, 4 and 6 aren't prime (and don't know how to find out for yourself) do you really think you're ready to try your hand at computer programming... I mean programming get's pretty complicated, even to do "basic stuff"... It's not a persute oft recommended to folks who are yet to master grade three math.

[tomiu]

u must use twise the for loop...once to check all number from 2 to num, and the second time u do the dividing...but this requires the labeled continue..i dont know if u have alredy used continue before

[logicbug]

Normally I wouldn't do this but I'm bored.

     
public static void main (String args[]){
        int num;
        boolean isPrime = true;
            Scanner input=new Scanner(System.in);
            System.out.print("enter the number=");
            num=input.nextInt();
            for ( int i = 1; i <= num; i++ ){
               isPrime = true;
               for(int y=2; y <= i/2; y++) {
                if ((i%y == 0))
                     isPrime = false;             
              }
              if (isPrime)
                  System.out.println(i);
         }
}

[corlettk]

i've change the if statement to :
if ( num%2!=0||num==0)
but no result !

Getting warmer... what do you think num%2!=0 does?

[CJSLMAN]

... i think 4 ,6 and 12 aren't prime . am i right ?

oopsss... Houston, we have a problem...
huh, yes, you are right. If your going to program, you have to completely understand the requirements (a suggestion for future projects).
Useful links...

Prime number - Wikipedia, the free encyclopedia
http://primes.utm.edu/lists/small/1000.txt

Luck,
CJSL

[sandeepsai39]

for ( int i = 1; i <= num; i++ ){
            if ( num % i == 0 )
                 	 System.out.println(i);
        }

hi change your code like this , declare another int varible say 'j' and intilise it to '0' now modified code is

for( int i = 1; i <= num; i++ ){
         for(int k = 1; k <= num; k++){
         if ( i % k == 0 )
            {
            j++;
            if(j>2){
                break;
            }
          }
        }
         if(j==2){
            System.out.println(i);
         }
          j=0;
     }

but this not good one when given is very high .It is possible to change it to make it faster [for example we need not to check
'num%4 ' bcs if a num is divided by 2 it should also divisiable by 4 ,if a num is divided by 3 it should also divisiable by 6 etc..]

[pbrockway2]

OP: begin with an algorithm, not with code. Google about for one: although I doubt you need go further than Wikipedia.

Then once you have a strategy which you can understand - and therefore be able to explain to others - you can ask serious questions about how to implement it.

Or, less seriously, try this

import java.math.BigInteger;
import java.util.Scanner;

public class Prime {
    public static void main (String args[]){
        Scanner input=new Scanner(System.in);
        System.out.print("enter the number=");
        int num=input.nextInt();
        for(int i = 0; i < num; i++) {
            if(new BigInteger("" + i).isProbablePrime(100)) {
                System.out.println(i);
            }
        }
    }
}

No algorithm required! Don't be put off by "probable" the chance of being wrong is less than 1 in 2^100 which is as close to certainty as you'll get in this lifetime.