How do I fix this compiler error - c++

Question

Been working on this for the past 5 hrs or so. I have done the research and tried to see what was wrong but I couldn't grasp what was missing. If anyone can help me solve this minor issue I would be grateful
Here's the code
#include <iostream>
using namespace std;
bool isPrime (int num);
int main()
{
int num=0;
cout << "Enter a number and I'll tell you whether it is prime: ";
cin >> num;
if (isPrime(num)==true)
cout << num << " is prime.";
else
cout << num << " is NOT prime.";
return 0;
}
bool isPrime(int input)
{
if(input<1)
return false;
else if (input == 1||input ==2 ||input==3)
{
return true;
}
else
{
for(int i=2; i<input; i++)
{
if(input%i==0)
return false;
}
return true;
}
}
It gives me this compiler error
./main.cpp: In function ‘int main()’:
./main.cpp:31:6: error: redefinition of ‘int main()’
int main()
^
./main.cpp:10:5: error: ‘int main()’ previously defined here
int main() {
^
I'm not sure what it means, but from where I researched, it means there are two mains but I have only one.

Answer 1

The following code compiles well with TurboC++ on windows and gives the appropriate result according to as required:
#include <iostream.h>
enum bool {true, false};
bool isPrime (int num);
int main()
{
int num=0;
cout << "Enter a number and I'll tell you whether it is prime: ";
cin >> num;
if (isPrime(num)==true)
cout << num << " is prime.";
else
cout << num << " is NOT prime.";
return 0;
}
bool isPrime(int input)
{
if(input<1) {
cout<<"Negatives not allowed"<<endl;
return false;
}
else if(input == 1) {
return false;
}
else if (input ==2 ||input==3) {
return true;
}
else {
for(int i=2; i<input; i++) {
if(input%i==0)
return false;
}
return true;
}
}

Answer 2

Make sure you're not including any additional files that also define main()
If you're working in visual studio, you should check the solution explorer for other .cpp files, especially if you've moved this over from an old solution, where you could've forgotten to get rid of old references. If that fails, you could just copy and paste the code that you do have into a new project, since what you have shouldn't be producing the error that you're getting.

Answer 3

Are you including this file from multiple others without an include guard? Or you could possibly have another main written in another cpp file in your project that you forgot about, in which case make sure you do not build that one with the project by excluding, or creating an entirely new project and adding each file you need individually.
I have found that with most IDE's, and even with command-line compiling, when you get really weird errors and cannot fix them creating a new project can solve many of them.

Answer 4

Your exact code runs perfectly on my compiler. So there is no problem on your code. This kind of problem always happens for you compiler. I think you are using an IDE in which you have created a project and added another code, that has a main() function too. So you can easily resolve this by creating a new project from scratch and copy paste this code.