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Print Prime Numbers(using for loop)

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Print Prime Numbers(using for loop)

Apr 22nd, 2006

Friends i have little bit problem in making a c language program.
I have to make a program which prints prime numbers from 1 to 500.
I make this program but with while loop then i was told to make it using for loop, which i tried but failed.
So kindly help me. Thanks
from,
john(pakistan)

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Re: Print Prime Numbers(using for loop)

Apr 22nd, 2006

No problem, just show us what you manage to do so far...

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Re: Print Prime Numbers(using for loop)

-1

Apr 22nd, 2006

It is my while loop program:

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include<stdio.h>
#include<conio.h>
void main()
{
int count,i=1;
int a;
clrscr();
while(i<=500)
{
count=0;
a=1;
while(a<=i)
{
if(i%a==0)
count++;
a++;
}
if(count==2)
printf("%d\t",i);
i++;
}
getch();
}
#include<stdio.h>
#include<conio.h>
void main()
{
int count,i=1;
int a;
clrscr();
while(i<=500)
{
count=0;
a=1;
while(a<=i)
{
if(i%a==0)
count++;
a++;
}
if(count==2)
printf("%d\t",i);
i++;
}
getch();
}

Last edited by Dave Sinkula; Apr 23rd, 2006 at 5:35 pm.

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Re: Print Prime Numbers(using for loop)

Apr 22nd, 2006

Here you go. Added for loop in place of while, and commented out the i++ at end of while loop.

Also added int variable col, to print a newline every ten numbers printed to standard out.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include<stdio.h>
#include<conio.h>
 
int main(int argc, char *argv[])
{
    int count,i=1;
    int a;
    int col=0;
 
    //while(i<=500)
    for(i=1; i<501; i++)
    {
        count=0;
        a=1;
 
        while(a<=i)
        {
            if(i%a==0)
                    count++;
            a++;
        }
 
        if(count==2){
            printf("%d\t",i);
            col++;
            if(col%10==0)
                printf("\n");
        }  
        //i++;
    }
 
  system("PAUSE");	
  return 0;
}
#include<stdio.h>
#include<conio.h>

int main(int argc, char *argv[])
{
    int count,i=1;
    int a;
    int col=0;
    
    //while(i<=500)
    for(i=1; i<501; i++)
    {
        count=0;
        a=1;
        
        while(a<=i)
        {
            if(i%a==0)
                    count++;
            a++;
        }
        
        if(count==2){
            printf("%d\t",i);
            col++;
            if(col%10==0)
                printf("\n");
        }  
        //i++;
    }
 
  system("PAUSE");	
  return 0;
}

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Re: Print Prime Numbers(using for loop)

Apr 22nd, 2006

> I make this program but with while loop then i was told to make it using for loop, which i tried but failed.

So turn

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
i = 0;
while ( i < 500 ) {
  // do stuff
  i++;
}
i = 0;
while ( i < 500 ) {
  // do stuff
  i++;
}

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
for ( i = 0 ; i < 500 ; i++ ) {
  // do stuff
}
for ( i = 0 ; i < 500 ; i++ ) {
  // do stuff
}

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Re: Print Prime Numbers(using for loop)

Apr 22nd, 2006

DOH! Salem

Johns' while loop was: less than OR EQUAL to 500.

So change to for i < 501 NOT 500.

This is Johns' future your playing with, pay attention.

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Re: Print Prime Numbers(using for loop)

Apr 22nd, 2006

I didn't pay any attention to his program, it wasn't formatted with code tags.

All I did was illustrate the relationship between for loops and while loops, and how ridiculously easy it is to convert one into the other.

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Re: Print Prime Numbers(using for loop)

Apr 25th, 2006

•

Originally Posted by john_hasan

Friends i have little bit problem in making a c language program.
I have to make a program which prints prime numbers from 1 to 500.
I make this program but with while loop then i was told to make it using for loop, which i tried but failed.
So kindly help me. Thanks
from,
john(pakistan)

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
int flag=1;
for(int i=2;i<500;i++)      //since 1 and 500 are not prime
{                  
flag=1;
for(int j=2;j<i/2;j++)   //since a no. cannot be divisible by a 
{                                //no. greater than its half.
if(i%j==0)                 //if i is divisible by any no. between 2 & i/2
{
flag=0;
break;
}
}
if(flag==1)
printf("\n",i);
}
int flag=1;
for(int i=2;i<500;i++)      //since 1 and 500 are not prime
{                  
flag=1;
for(int j=2;j<i/2;j++)   //since a no. cannot be divisible by a 
{                                //no. greater than its half.
if(i%j==0)                 //if i is divisible by any no. between 2 & i/2
{
flag=0;
break;
}
}
if(flag==1)
printf("\n",i);
}

Last edited by Dave Sinkula; Apr 25th, 2006 at 10:50 am.

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Re: Print Prime Numbers(using for loop)

Apr 25th, 2006

> for(int j=2;j<i/2;j++) //since a no. cannot be divisible by a
> { //no. greater than its half.
It's square root of actually, but hey what the heck.

At least it's not the only mistake in your code.

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Re: Print Prime Numbers(using for loop)

#10

Apr 25th, 2006

Ok here's my best shot. The key rule for a prime number is:

It has precisely two positive integer factors.

So the neatest solution I can think of counts those and quits the for loop as soon as it's found more than 2 or reached half way to n.

Can anyone better it?

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include <iostream>
 
using namespace std;
 
static const int MAX = 500;
static const int MIN = 0;
static const int MAXCOL = 10;
 
bool isPrime(int r);
 
int main(int argc, char *argv[])
{
    int col = 0;
    int n;
 
    for(n=MIN; n<=MAX; n++)
      if(isPrime(n)){
          cout << n << '\t';
          col++;
          if(col == MAXCOL){
              cout << '\n';
              col = 0;
          }
      }  
 
  system("PAUSE");	
  return 0;
 
}
 
bool isPrime(int n)
{
    if((n == 0)||(n == 1))
        return false;
 
    int factors = 2;
 
    for(int i=2; i<=((int)n/2); i++){
        if(n%i == 0){
            factors++;
            break;
        }
    }
 
    if(factors == 2)
        return true;
 
   return false;
}
#include <iostream>

using namespace std;

static const int MAX = 500;
static const int MIN = 0;
static const int MAXCOL = 10;

bool isPrime(int r);

int main(int argc, char *argv[])
{
    int col = 0;
    int n;
    
    for(n=MIN; n<=MAX; n++)
      if(isPrime(n)){
          cout << n << '\t';
          col++;
          if(col == MAXCOL){
              cout << '\n';
              col = 0;
          }
      }  
      
  system("PAUSE");	
  return 0;
  
}

bool isPrime(int n)
{
    if((n == 0)||(n == 1))
        return false;
        
    int factors = 2;
            
    for(int i=2; i<=((int)n/2); i++){
        if(n%i == 0){
            factors++;
            break;
        }
    }
            
    if(factors == 2)
        return true;
    
   return false;
}