newbie woes: 21 line C++ code crashes

[Caffiend]

Hi there! I'm brand new to programming, save for some adventuring with BASIC 10 years ago. I have been learning from "C++ Without Fear" by Brian Overland, and it is really well written, but I still have dificulty grasping many concepts. I am trying to write code that will take an input number, then print all the prime numbers between 1 and that number. At this point, the code compiles in Dev C++ without problem, but after inputting the number it crashes. Here's my code:

#include <iostream>
#include <math.h>
using namespace std;
 
int knum; // Highes number
int i = 0; // loop counter
 
int main() {
    cout << endl;
    cout << "Please enter a number: ";
    cin >> knum; // this is the last time I see something happen
 
    while (i <= sqrt(static_cast<double>(knum))) {
          if (knum % i != 0) {
                   cout << i << " ";
                   }
          i++;
                   }
 
int getch();
} 
 

Can anyone help me figure out what I am doing wrong? I will admit I am almost completely ignorant with respect to C++, so please forgive me if my code makes no sense whatever!

BTW, I look forward to getting to know everyone here and increasing once more the proportion of my time spent on the couch staring blankly at the LCD!

[horace1]

the problem is probably in

if (if (knum % i != 0) {
 

the modulus operator % in the expression knum % i gives the remainder of knum / i and as i has been initialised to 0 so you get a division by 0

what you trying to caculate?

[Caffiend]

I'm trying to calculate all the prime numbers between 1 and knum.... BTW, thanks for point out initializing i = 0, i changed it to i = 1, and it doesn't crash, which is excellent, but now when I entered 100 as knum, it prints "3 6 7 8 9" and then closes... I'm probably trying to do this completely the wrong way...!

[horace1]

I'm trying to calculate all the prime numbers between 1 and knum.... BTW, thanks for point out initializing i = 0, i changed it to i = 1, and it doesn't crash, which is excellent, but now when I entered 100 as knum, it prints "3 6 7 8 9" and then closes... I'm probably trying to do this completely the wrong way...!

you should start divsions (i) at 2 and test knum % i for 0, i.e. if the remanider is 0 the number is not a prime, e.g.

#include <iostream>
#include <math.h>
using namespace std;
 
int knum; // Highes number
int i = 2; // loop counter
 
int main() {
cout << endl;
cout << "Please enter a number: ";
cin >> knum; // this is the last time I see something happen
 
while (i < knum) {
      // should only be divisable by inself and 1
      if (knum % i == 0) {
          cout << "divisable by " << i << " not prime ";
          getchar(); getchar();
          return -1;
         }
      i++;
}
cout << i << " prime ";
getchar(); getchar();
} 
 

[Caffiend]

Thanks Horace, I'm getting closer... what I'm trying to do specifically is not just to determine if a number is prime or not, but to actually print onscreen all the prime numbers between 1 and knum, so if knum is 10, then I want to print onscreen "1 3 5 7"... Here is my revised code:

#include <iostream>
#include <math.h>
using namespace std;

int knum;
int i = 2;

int main() {
cout << endl;
cout << "Please enter a number: ";
cin >> knum;

while (i < knum) {
if (knum % i == 0) {
i++;
}
else {
cout << i << ", ";
i++;
}
}
getchar(); getchar();
}

But now I am getting a compiler error: "Permission denied Id returned 1 exit status" I have no idea what that means, other than it doesn't work....

[Caffiend]

Silly me, that error was from having the file running while trying to compile, but now I am getting a display of numbers, mostly sequential, but every so often it skips one. Most of the numbers are certainly NOT prime... where have I gone wrong?