Ambiguity error with double as parameter type

For the following code I'm getting the following error: line 15

"Ambiguity between 'std::sqrt(double)' and 'sqrt(double)' in function main()"

I compared the code with my friend's and they were practically identically. Yet, his got no errors. I copied and pasted his code, compiled it, and ran it. It got the same ambiguity error! The assignment itself is just a square root function and that's not the problem. The code is below:

Code:

#include<iostream>
using namespace std;
double sqrt(double x){
        double counter = 1;
        double guess = 1;
        while (counter < 20){
                guess = (guess + (x/guess))/2;
                counter = counter + 1;
        }
        return guess;
}
int main(void){
        double a;
        cin >> a;
        cout << sqrt(a);
        return 0;
}

Your friend is probably using a different compiler.

The problem is that <cmath>, apparently indirectly included through <iostream> on your compiler, defines a function called sqrt already, and due to your using statement, the compiler doesn't know whether you want to call your function or std::sqrt.

There are two solutions.
1) Give your own function a different name.
2) Remove the using statement and either import cin and cout selectively or fully qualify them. But you will probably run afoul of the global sqrt in <math.h> soon. So, it's best if you just don't use names used by the standard library.

The header <cmath> already has a sqrt function. It seems that each compiler treats this kind of thing differently (MingW compiles and calls custom sqrt) and your safest solution might be to rename the function.