Prime numbers using loop

[theeeeeyyaaaa]

Hi. We were asked to make a program which displays the prime numbers within the range you inputted... like for example i entered 20 as the upper limit and 1 as the lower, then the program must display all prime numbers within 20 and 1..

and so my problem is, i get to display the prime numbers, but 2, 3, 5, and 7 can't because it think it's with the if statement i made within the loop? any help regarding this?? thanks. (code below)

#include<iostream.h>
#include<conio.h>

void prime (int up, int low);

main()
{
 clrscr();
 int Upper, Lower, i;

 cout<<"Enter the upper limit: ";
 cin>>Upper;
 cout<<"Enter the lower limit: ";
 cin>>Lower;

 prime(Upper, Lower);

 getch();
 return 0;
}

void prime(int up, int low)
{
  int i;

  for(i=low;i<up;i++) {
    if(i%2!=0&&i%3!=0&&i!=1&&i%5!=0&&i%7!=0) {
      cout<<i<<" "; } }
}
 

[hoity]

Do you want to show 2,3,5 and 7 or not. Your question doesn't make any sense to me. If you don't want to show 2 to 7 ,then code given below works well:

#include<iostream>

using namespace std;

void prime (int up, int low);

main()
{
 int Upper, Lower, i;

 cout<<"Enter the upper limit: ";
 cin>>Upper;
 cout<<"Enter the lower limit: ";
 cin>>Lower;

 prime(Upper, Lower);


 return 0;
}

void prime(int up, int low)
{
  int i;

  for(i=low;i<up;i++) {
    if(i%2!=0&&i%3!=0&&i!=1&&i%5!=0&&i%7!=0) {
      cout<<i<<" "; } }
}

[theeeeeyyaaaa]

i wanted to show the numbers 2, 3, 5, and 7 but it doesn't show up.. that's what my question is all about.

[theranga]

The big problem with this code is that it will output numbers that aren't prime, eg the program will think that 121 is prime but it isn't.

[theranga]

Otherwise, the reason it isn't outputting 2, 3,5 or 7 is because when i is 2, 3, 5 or 7 then i%2, i%3, i%5 or i%7 is equal to 0, so it skips that value of i

[theeeeeyyaaaa]

yep.. that's my problem.. i used i%2, i%3, i%5 and i%7 seven that's why 2, 3,, 5 and 7 cannot be displayed.. i'm also out of ideas on how to do this...

[hoity]

Try this.

#include <cstdio>
#include <iostream>
bool isPrime( int i ) {
for( int j = 2; j < i; j++ ) {
if( i % j == 0 ) // i is divisible by j, so i is not a prime
return false;
}
// No integer less than i, divides i, so, i is a prime
return true;
}
int main() {
    int up,low;
    scanf("%d ",&up);
    scanf("%d",&low);
for( int i = low; i <= up; i++ ) {
    if(i<=1)
    {
        continue;
    }
if( isPrime(i) == true )
printf("%d ", i);
}
return 0;
}

for more about generating prime , see this document.

http://lightoj.com/article_show.php?article=1001