prime numbers 1-100

[medosz]

I am trying to write a program to print the prime numbers between 1 and 100. Obviusly I am trying with the wrong logic but I can not find the way to fix it. Could someone please help me with it please?

#include <iostream>
#include <cmath>

using namespace std;

void prime();

int main(){
    cout << "Prime numbers: " << endl;
    prime();
}
void prime(){
  for(int number{1}; number <=100; number++){
    for(int i{2}; i<=sqrt(number); i++){
        if(number % i != 0){
            cout << number << endl;
        }
    }
}
}

[highwayman]

if(number % i != 0) {
  cout << number << endl;
}

This is where you went wrong. You only are testing one of the possible divisors, instead of all of them, before you cout. I suggest separating the function that determines whether a number is prime from the code that outputs the first 100 primes.

[medosz]

Thank you. I had to count the number of the divisors. It is prime if it has 0 divisor between 2 and the sqrt of the number.

#include <iostream>
#include <cmath>

using namespace std;

void prime();

int main(){
    cout << "Prime numbers: " << endl;
    prime();
}
void prime(){
  for(int number{1}; number <=100; number++){
    int counter{0};
    for(int i{2}; i<=sqrt(number); i++){
        if(number % i == 0){
            counter++;
        }
    }
    if(counter==0){
        cout << number << endl;
    }
  }
}

[highwayman]

👌 noice.

[TheToaster]

Something like this would be best done using templates and recursive metaprogramming at compile time. Especially when the number of primes you want to print is a compile-time constant.

[medosz]

If I want to do it with recursive can I do it with the same logic like I used above? Is it possible to change the nested for function to recursive?

[jonnin]

Something like this would be best done using templates and recursive metaprogramming at compile time. Especially when the number of primes you want to print is a compile-time constant.

Just make a const array table of them if you only want 100. There isnt any computation, recursion, etc. needed here. I would even argue that up to 1 million or so in a header file of a const table is fine. The path of least resistance is your friend.

recursion and loops are pretty much interchangeable with a lot of hand waving. I don't know if its been proven 100% but to date no problem has been presented that can't be written as a loop+extras that can be written recursively.