在质数 C++ 程序中不断得到浮点异常

Question

I am trying to create a program that takes a user integer and determines whether or not the integer is a prime number. However, when I try to enter a number I get a floating point exception. Why is that? I made an array in order to go through every number up until the integer to check its divisibility Here is the code:

#include <iostream>
using namespace std;
bool isPrime(int n)
{
    int arr[n];
    if (n == 1 or n == 2) {
        return true;
    }
    for (int i = 1; i < n; i++){
        if (n % arr[i] == 0) {
            return false;
        }
        else {
             return true;
        }
    }
}
int main()
{
    int user_input;
    cout << "Enter an integer to test if it's prime: ";
    cin >> user_input;
    bool value = isPrime(user_input);
    string true_false;
    if (value == 1) {
        true_false = "true";
    }
    else {
        true_false = "false";
    }
    cout << true_false;
    return 0;
}

Answer 1

The fatal exception occurs in the line if (n % arr[i] == 0) { . This array points to uninitialized memory, which will occasionally contain 0 resulting in a divide by zero error. Thanks to nm . for pointing out my error.

Using C idiom such as int arr[n]; where n is not a know constant at compile time is going to result in common C programming issues, like this. C++ has features that can be used which avoid these problems. I recommend you study and use modern C++.