Displaying Prime Number

A prime number is ...
any natural number greater than 1 that has the two divisors 1 and itself
(defined in: http://en.wikipedia.org/wiki/Prime_number )

Your algorithm is pretty bad, take a look at this C# code snippet at DaniWeb to help you out:
http://www.daniweb.com/code/snippet676.html

vegaseat

DaniWeb's Hypocrite

Moderator

5,989 posts since Oct 2004

Reputation Points: 1,345

Solved Threads: 1,417

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            bool current = false;
            int j;

            Console.WriteLine("Enter any integer");
            int num = Int32.Parse(Console.ReadLine());

            for (int i = 2; i <= num; i++)//2 is the first prime number.
                                          //I set i to 2 beacuse it has to print 2 firstly...  
            {
                for (j = 2; j < i; j++)
                {
                    if (i % j == 0)//Controls i is prime number or not...
                    {
                        current = true;
                        break;//breaks for not controlling anymore...
                    }
                }
                if (current == false)
                    Console.Write("{0} ", j);//if i is prime number, print it...
                else
                   current = false;   
            }

            Console.ReadLine();



        }
    }
}

FoX_

Junior Poster in Training

53 posts since Mar 2007

Reputation Points: 10

Solved Threads: 7

Hie guys!

Heres something i have created to display all the prime numbers from zero up to the number entered by user.

using System;
class pnumber
{
public static void Main()
{
int value; // Value Enter By User!
int count,count2; // For inner & outer loop
int prime; // used to check prime number

Console.WriteLine("Welcome To My Program\nPlease enter a number to check whether its a prime number or not!");
value=Convert.ToInt32(Console.ReadLine());
for(count=2;count<value;count++) // Outer loop
{
prime = 1; // to set prime value to 1 every time outer loop works
for(count2=2;count2<count;count2++) // Inner loop
{
if(count%count2 == 0) // If Value is divisible..
{
prime = 0; // Set prime to 0
}
}
if(prime==1) // If prime value is 1 then...
Console.WriteLine("{0} Is Prime Number",count); // display count variable
}
}
}

This code works fine for me!

puneetkay

Junior Poster

122 posts since Nov 2007

Reputation Points: 51

Solved Threads: 24

Some how the program is correct but the algorithm is not efficient of the large no.

see this link
http://www.daniweb.com/code/snippet809.html

Rhohitman

Junior Poster in Training

86 posts since Dec 2007

Reputation Points: 10

Solved Threads: 5

Some how the program is correct but the algorithm is not efficient of the large no.

see this link http://www.daniweb.com/code/snippet809.html

Hie Rhohitman,

Thanks buddy for snippet link but you are on C# section & thats a C++ snippet. :D

puneetkay

Junior Poster

122 posts since Nov 2007

Reputation Points: 51

Solved Threads: 24

Some how the program is correct but the algorithm is not efficient of the large no.

see this link http://www.daniweb.com/code/snippet809.html

I wrote something similar in C++, but the concept is the same... instead of using modulus with every ineger less than num, try this:

for (int i = 2; i <= sqrt(num); i++)

once i becomes larger than the square root of num, you're doing the same work over again!

you can also restrict it to only divide by ODD numbers, which theoretically cuts your processing time in half.

i ended up playing with an array, adding each new prime number to the array, adn only testing num against those values. worked pretty well, if i recall

slogfilet

Newbie Poster

1 post since May 2008

Reputation Points: 10

Solved Threads: 0

-2

using System;
class g9{
	public static void Main(){
			int num;
			Console.Write("Enter the number till you want Prime Number : ");
			num=Convert.ToInt32(Console.ReadLine());	
			for(int i=2;i<=num;i++){
				if(i<=5 && i!=4){
					Console.Write(i+" ");
				}
					if(i%2!=0 && i%3!=0 && i%5!=0)
					Console.Write(i+" ");
				
			}
		}
}

gauravm2k7

Newbie Poster

1 post since Nov 2009

Reputation Points: 10

Solved Threads: 0

Any time you are having troubles with iterations like this (especially nested) your first step should be to run through it step by step and track each variable at each stage:

num = 9:

i	j	i%j	output
3	2	1	3
3	3	0	
3	4	3	3
4	2	0	
4	3	1	4
4	4	0	
5	2	1	5
5	3	2	5
5	4	1	5
6	2	0	
6	3	0	
6	4	2	6

This allows you to see exactly whats going on and why the results arent what you expect. I wont tell you how to rewrite your algorythm as there are plenty of examples already here, just thought i'd chip in with some advice on solving similar issues in the future.Give a man a solution, and his problem is solved for today; give him the tools to trouble shoot his work, and his problems are solved for a lifetime ;)

Ryshad

Nearly a Posting Virtuoso

1,307 posts since Aug 2009

Reputation Points: 512

Solved Threads: 246

@Ryshad
Yes, I do this also when confronted wih this kind of "problems".
Although these days I prefer the wonderfull debugger that comes with VS!

ddanbe

Senior Poster

3,829 posts since Oct 2008

Reputation Points: 2,070

Solved Threads: 661

-1

My algorithm below is similar to what slogfilet suggested: I consider using anything beyond the square root of the integer candidate to be redundant and a waste of time to use as a test divisor.

I'm not accepting user input, nor am I writing the prime numbers to the screen, but it's easy enough to add that in (actually, the writing to the screen is simply commented out). I was more interested in seeing how fast I could get the program to run, hence the use of the stopwatch. It's under 300 milliseconds to get all primes up to 1,000,000 on my machine, just about 5 seconds to go up to 10,000,000, and of course will increase significantly from there. That's the price of using nested loops on growing lists. But I'm not done working on it, and I'm not going to assume I know enough about C# to say that I've thought of the best looping structure.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;

class Program
{
    static void Main()
    {
        Stopwatch stopwatch = new Stopwatch();
        stopwatch.Start();

        List<int> primes = new List<int>();
        primes.Add(2);

        for (int i = 3; i < 1000000; i += 2)
        {
            bool isPrime = true;
            int stoppingPoint = (int)Math.Pow(i, 0.5) + 1;
            foreach (int p in primes)
            {
                if (i % p == 0)
                {
                    isPrime = false;
                    break;
                }

                if (p > stoppingPoint)
                    break;
            }

            if (isPrime)
            {
                primes.Add(i);
                //Console.WriteLine(i);
            }
        }

        stopwatch.Stop();
        Console.WriteLine("-- {0}", primes.Count);
        Console.WriteLine(stopwatch.ElapsedMilliseconds);
        Console.Read();
    }
}

apegram

LINQ!

Team Colleague

552 posts since Jan 2010

Reputation Points: 327

Solved Threads: 135

-1

Thank you for going over this step by step. I am a self taught new programmer and this has helped me.

iajm

Newbie Poster

1 post since Mar 2010

Reputation Points: 10

Solved Threads: 0

Welcome iajm,

I'm glad you found it useful. Have you noticed that the current thread is two years old? Please do not resurrect old/solved threads. If you want to ask question, start your own thread.

Read before posting- http://www.daniweb.com/forums/thread78223.html

__avd

Posting Genius (adatapost)

Moderator

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