Print Prime Numbers(using for loop)

[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)

[Micko]

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

[john_hasan]

It is my while loop program:

#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();
}

[hollystyles]

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.

#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;
}

[Salem]

> 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

i = 0;
while ( i < 500 ) {
  // do stuff
  i++;
}

To

for ( i = 0 ; i < 500 ; i++ ) {
  // do stuff
}

[hollystyles]

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.

[Salem]

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.

[shavez_israr]

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)

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);
}

[Salem]

> 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.

[hollystyles]

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?

#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;
}

[hollystyles]

Ah it's just occured to me what Salem meant by square root !

A revised edition:

#include <iostream>
#include <cmath>

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)sqrt((double)n)); i++){
        if(n%i == 0){
            factors++;
            break;
        }
    }
            
    if(factors == 2)
        return true;
    
   return false;
}

Now if only I could make i++ step in primes!

[iamthwee]

>can anyone better it?

Yes, there are more faster prime number finding Al-Gore-it-hims out there.
http://en.wikipedia.org/wiki/Sieve_of_Atkin

Note I said faster as opposed to efficient. An efficient prime number list finding strategy would be rather elusive.

Also, I'm sure you can think of a much better way to pause the program than using system("PAUSE"); ?

:lol:

[hollystyles]

Also, I'm sure you can think of a much better way to pause the program than using system("PAUSE"); ?

:lol:

Ok whats wrong with system("PAUSE") ?

I

[dude543]

I dont know that high math, but I think I can do better.

bool isPrime(int n)
{
bool prime ;
int  lim;
 
if ( n == 0 || n == 1 || n % 2 == 0)
   return(false);

prime = true;
lim = (int)sqrt(n); 
 
for(int i=3; i<= lim && prime ; i+=2)
       prime =  n % i;
return(prime);
}

[Salem]

> for(int i=2; i<=((int)sqrt((double)n)); i++)
And it would be so much quicker if you didn't call sqrt() on every iteration of the loop!
n is constant (in this function), so it's root is constant also. Calculate it once and store in another variable to compare against.

Also, since 2 is prime, that's an easy case to get rid of, and you can start at 3.

Also, if you start at 3, then you can do i+=2 to only check all the odd numbers from there on.

[hollystyles]

Dude543 yes I like that very much.

Salem good feedback.

So system("PAUSE") then, what gives ? I havn't found anything negative on the internet yet.

Ok I've found it:

http://www.gidnetwork.com/b-61.html

[hollystyles]

Actually dud543 I think this is wrong: || n % 2 == 0) That would return false for 2 which is prime.

[dude543]

Well. I also think that it is wrong.
But poor me, does'nt know wheter 0,1,2 are prime.
I know that prime will divide by himself and one only.
I Guess you are right.
Anyway. is there someway for me to edit the post ?

[hollystyles]

you can't edit a post more than 30 minutes after you posted in this forum.

0 and 1 are not prime. Yes 1 is divisible by 1 and itself, but that's the same thing it's only one factor not two factors so it's not prime.

2 is a special case, it is the only even number that is a prime.

[Bench]

I wouldn't call 2 a special case - it fits the general description perfectly; A number evenly divisible only by 1 and itself. :)