Is it a prime?

[CodeNoobster]

Hi guys, I'm embarrassed to ask but, I'm struggling with this problem. I've been instructed to check if a number (inputted by a user) is a prime number or not. So far I've managed to show the fact that for example, 7/7=1 and 7/1=7 and that those are the only operations that have no remainder.

Now the trick is, getting the program to check and make sure that those two operations must be the only ones to have no remainder in order for it to tell the user that 7 is a prime number rather than the user judging for him/herself.

here's what I've done so far

        Dim input As Integer
        Dim test As Double
        Dim i As Integer = 1
 
        Console.WriteLine("Enter a number to see if it is a prime number")
 
        input = Console.ReadLine()
 
        Console.WriteLine(" ")
 
        While (i <= input)
            test = input Mod i
            Console.WriteLine(test)
            i = i + 1
        End While
 

ANY help would be greatly appreciated guys ;)

[Rabbit]

I think you're approaching this backwards. I would skip checking 1 and n because the results are always the same. Instead, you should check everything in between 1 and n. If any of those results in a division with no remainder, then you know it's not a prime. Full stop, no need to check the rest.

[CodeNoobster]

hmmmmmm true rabbit, ohk I have remixed the program

        Dim num1 As Integer
        Dim test As Integer
        Dim finalnum As Integer = (num1 - (num1 - 1)) + 1
 
        Console.WriteLine("Enter a number to check if it is a prime")
        num1 = Console.ReadLine()
 
        While (finalnum <= (num1 - 1))
            test = num1 Mod finalnum
            If (test = 0) Then
                Console.WriteLine("number is not a prime number")
                Exit While
            ElseIf (test <> 0) Then
                Console.WriteLine("number is a prime number")
                Exit While
            End If
            finalnum = finalnum + 1
        End While       
 

it works, but now there is an accuracy problem. any number entered that's higher than 15 is an anomaly. Non prime numbers are prime numbers and vice verso. Any suggestions?

[Rabbit]

Lines 11 and 13 to 15 shouldn't be there. That's making it print out every iteration in the loop. Instead you should be setting a flag within the loop and decide what to print out once you're out of the loop.

[jack003]

Try this code :
C++ program to enter a no and print it is Prime number or not

#include <iostream>
#include <math.h>
using namespace std;
int main()
{
int no, i;
 
cout << "Enter an integer : ";
cin >> no;
int limit = sqrt(no);
i = 2;
while(no%i != 0 && i <= limit)
i++;
if(i > limit)
cout<<no<<" is Prime\n";
return 0;
} 
 

Check the no is Prime or not And compile code online:
http://cbtsam.com/cppl1/cbtsam-cppl1-051.php

[CodeNoobster]

@ Rabbit : Sweet thanks man, i did another remix. It works perfectly. Did it as a console to get a working version of the program. Now that its working, gonna make an interface for it now.

@ jack 003 : thanks man. Though i got a working version, there's no harm in looking for an easier way to do it.

[Rabbit]

No problem, good luck with the rest of your project.