Exits while loop before condition is false? - c++

Question

This is a pretty specific question but my program seems to exiting its while loop before the condition is false. I added in quite a few memory checks for safety when I was debugging and it prints to screen that counter is 4 and SqRoot is 6 at the end which means it should still be looping through (TestNum=32). I definitely know it's getting past the loop with counter<=SqRoot because it prints both "The integer 32 is composite" and "The integer 32 is prime". Any help is very appreciated! Thanks so much
EDIT: I changed the overall logic of program and it is working now. Thanks!
#include <iostream>
#include <cmath>
using namespace std;
//Declare variables.
int TestNum, DivInt, SqRoot, PrintCounter(0), oppcounter;
float DivFloat, counter(2);
int main()
{
//Prompt user for input.
cout << "Input an positive integer to test its primality.";
cin >> TestNum;
//Check if input is positive.
while (TestNum < 0)
{
cout << "Please input a *positive* integer.";
cin >> TestNum;
}
//Square root.
SqRoot = sqrt(TestNum)+1;
//Loop to test if prime.
while (counter<=SqRoot)
{
++counter;
DivFloat = TestNum/counter;
DivInt = TestNum/counter;
oppcounter = TestNum/counter;
if (DivFloat-DivInt == 0)
{
++PrintCounter;
if (PrintCounter==1)
{
cout << "The integer " << TestNum << " is composite.\n " \
<< TestNum << " is divisible by\n";
};
cout << counter << " " << oppcounter;
cout << "counter* " << counter;
cout << " TestNum " << TestNum;
cout << " DivInt " << DivInt;
cout << " SqRoot " << SqRoot;
cout << " DivFloat " << DivFloat;
}
}
if (counter<=SqRoot)
{
cout << "The integer " << TestNum << " is prime.\n";
}
cout << "counter " << counter;
cout << " TestNum " << TestNum;
cout << " DivInt " << DivInt;
cout << " SqRoot " << SqRoot;
cout << " DivFloat " << DivFloat;
//End main.
return (0);
}

Answer 1

I am seeing the opposite behavior of what you are describing, and I can see why. It's possible that the code you have posted is different than the code you are executing.
As an aside, I added the line
cout << endl;
after the line
cout << " DivFloat " << DivFloat;
at couple of places to make the output more readable.
When I enter 32, I see the following output:
The integer 32 is composite.
32 is divisible by
4 8
counter* 4 TestNum 32 DivInt 8 SqRoot 6 DivFloat 8
counter 7 TestNum 32 DivInt 4 SqRoot 6 DivFloat 4.57143
When I enter 17, I see the following output:
counter 6 TestNum 17 DivInt 2 SqRoot 5 DivFloat 2.83333
The reasons for that:
You don't break out of the while loop when you have detected that a number is a composite.
As a result of that, you always break out of the while loop only when counter<=SqRoot evaluates to false. As a result, in the code below,
if (counter<=SqRoot)
{
cout << "The integer " << TestNum << " is prime.\n";
}
you never execute the line in the if block.
The program should behave correctly if you break out of the while loop when you detect a composite and change the logic in the last if block to:
if (counter > SqRoot)
{
cout << "The integer " << TestNum << " is prime.\n";
}

Answer 2

Why so strange check for prime?
for(int i = 2; i*i <= n; ++i)
{
if (n % i == 0)
{
cout << "not prime";
break;
}
}