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)

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();
}
```

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;
}
```

> 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
}
```

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.

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.

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);
}
```

> 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.

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;
}
```

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!

>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:

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

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);
}
```

> 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.

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:

Actually dud543 I think this is wrong: `|| n % 2 == 0)`

That would return false for 2 which is prime.

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 ?

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.