如何从main内调用函数

Question

I am trying to call the function "isPrime" from main , but as soon as I enter a number the program exits. I don't know what the problem is? If anyone can see where am I doing wrong, I will appreciate it. Thanks.

Code:

#include <iostream>
#include <string>
#include <fstream>

using namespace std;


ofstream myfile;
    int num;

int main(){

    cout << "Please Enter a number " << endl;
    cin >> num;

        while (num > 3001){
            cout << "Your input integer should be less than 3001. Try again, or -1 to exit" << endl;
            cin >> num;
            if (num == -1){
        break;
            }
        }

    bool isPrime(num);


    //cout << "CMSC 140 CRN <your course CRN> Project 5: Prime Numbers Written by a student <YourName> Due Date : DD / MM / YYYY>" <<endl;

}

bool isprime(int a){
    myfile.open("primelist.txt");
    a = num;
    int prime;

    if (a <= 1 || (a % 2 == 0)){        // check if the number is even
        cout << " that is not a prime number" << endl;
        return false;
    }
    else if (a == 2){
        cout << "tht is a prime number" << endl;
        return true;
    }
    else{
        int divisor = 3;
        int top = a - 1;
        while (divisor <= top)
        {
            if (a % divisor == 0)
                return false;
        }
        divisor += 2;  //check the odd divisions 
        return true;
    }

    for (int i = 4; i < a; i++){
        if (i % 2 != 0 || i % 3 != 0){ 
            myfile << "2, 3, " << i << endl;
        }
    }
}

Answer 1

 bool isPrime(num); 

Honestly, I don't think this should compile. The nearest match, syntactically, though would be a function declaration...which pragmatically does nothing.

So as was said in comments, omit the bool in that statement.

Answer 2

I think you are miss something like this: 1. You replace

bool isPrime(num);

To

bool ret = isPrime(num);

Or

(void) isPrime(num);

2. You declare function bool isPrime(Int a); before main function.