Finding the primary numbers in an array of pointer

问题

This piece of code which works fine it tells you to enter a number, then it puts the number in a for loop and it checks if it's dividable by i, if true it prints not prime if not prints prime.

#include <iostream>
using namespace std;

int main() {
  int x;
  cin >> x;

  bool f = true;
  for (int i = 2; i < x; i++) {
    f = false;
    if (i % x == 0)
      f = true;
    if (f)
      cout << "not primary";
    else
      cout << "primary";
  }
}

but when i convert it to an array like so:

#include <iostream>
using namespace std;

int main() {
  cout << "the number of array:" << endl;
  int n;
  cin >> n;

  cout << "enter them = \n";
  int *p = new int[n];
  for (int i = 0; i < n; i++)
    cin >> p[i];

  bool f = true;
  for (int i = 0; i < n; i++)
    for (int j = 2; j < p[i]; j++) {
      f = false;
      if (p[i] % j == 0)
        f = true;
      if (f)
        cout << "This is not a primary number!\n";
      else
        cout << "this is a primary number!\n";
    }

  delete p;
}

it stores just the first number and i get that but how to increment it lets say n =3 so p[3] = {4,6,7}; my question is how tell the compiler in the j condition if (p[0] % j) then(p[1] %j) it seems that it just takes p[0]

回答1:

This will work much better

#include <iostream>
using namespace std;

int main() {
  cout << "the number of array:" << endl;
  int n;
  cin >> n;

  cout << "enter them = \n";
  int *p = new int[n];
  for (int i = 0; i < n; i++)
    cin >> p[i];

  for (int i = 0; i < n; i++) {
    bool f = false; // we set f to false for each number
    for (int j = 2; j < p[i]; j++) {
      if (p[i] % j == 0) {
        f = true;
        break; // we break the loop if it's a prime number
      }
    }
    if (f)
      cout << p[i] << " is not a primary number!\n";
    else
      cout << p[i] << " is a primary number!\n";
  }

  delete[] p; // Here you forget brackets [], when you use new[] you must use delete[].
}

Doc for delete operator.

I let some problem like int. You should not use signed number for iteration or stock a size. Because you are a beginner, I don't want to confuse you. So I let it.

来源：https://stackoverflow.com/questions/40945872/finding-the-primary-numbers-in-an-array-of-pointer

标签

c++

primes