Prime Number Program

[TrAnMu]

The objective of this program is to determine whether a number is prime or not. I've read the program through over and over but for whatever reason, the output is the same whether the number is prime or not. I figured out that the problem is in this snippet of code but I can't figure out the error.

while (ctr<=n)
    {
          rem=n%ctr;
          
          if (rem=0.0)
              mult++;
                    
          ctr++;
    }

Please let me know if you can help.

[TrAnMu]

Here's the full code if it helps.

#include <iostream>
using namespace std;

int main()
{
    int mult=0, ctr=1, n;
    double rem;
    
    do
    {
    cout<<"Input a number greater than 1: ";
    cin>>n;
    
    if (n<=1)
    {
             cout<<"\nInvalid input!"<<endl;
             cout<<"Please try again! :)\n"<<endl;;
    }
    
    }
    while (n<=1);    
    
    cout<<"\n";
    
    while (ctr<=n)
    {
          rem=n%ctr;
          
          if (rem=0.0)
              mult++;
                    
          ctr++;
    }
    
    if (mult>2)
        cout<<"The number is not prime.";
    else
        cout<<"The number is prime.";       
        
    cout<<"\n\n";
        
    system("pause");
    
    return 0;  
}

[long double main]

Could you use [ code ] [ / code ] (without the spaces) tags instead of the program output tag?
It makes the code look so much prettier...

Anyways, "rem" doesn't need to be a double.
You're never going to get anything other than an integer with the modulus operator....
And then
if (rem=0.0)
should be
if (rem == 0) // Only works if "rem" isn't a floating-point type!
because the comparison operator is == (two equal signs), not = (one equal sign), which is the assignment operator...

[TrAnMu]

Oh! Though I knew I was looking for remainder, I was thinking about decimals...

Anyway, you've been a great help! Thanks alot! :)

By the way, what do you mean by [ code ] [ / code ]? I'm sorry, I'm not very familiar with the formal names for these things...

[ResidentBiscuit]

When you go to post, on the right side (or bottom), there will be something that says format. Mouse over those things and you'll see one that says Source code

[Shinigami]

http://www.shinigami.lt/index.php/programing/c/1-prime-numbers-class

[cire]

I wouldn't call what you linked to a "good and efficient" program, Fahadmunir32. In fact it's not C++ (because main can't have a void return type in C++) and it is quite probably the most naive and inefficient method for checking a prime that I've seen. Not to mention the use of unnecessary and non-portable console functions and deprecated headers.

[fahadmunir32]

Cire i said "good and efficient program " about not the link i posted but about the program above..
The link i posted its easy to understand for me..
but this site is really helpful.. i bookmarked.. :)

[Shinigami]

fahadmunir32 your logic are good if you want to determine if one number are prime or not.
But you could change
for(int a=1;a<=number;a++)
to
for(int a=2;a<=number;a++)
and
if(count==2)
to
if(count==1)

because all the numbers will be divided by 1. Therefore unnecessary to check.