Why is the call to the function “sample_mean” ambiguous?

Question

I'm writing a program with xcode to do some statistical analysis. However, when I'm building the function for computing the sample variance, xcode keeps telling me that my call the function sample_mean (which is also function defined by me) is ambiguous.

Here's the code:

#include <iostream>
#include <fstream>
#include <cmath>
#include <vector>


//sample_mean
double sample_mean (std::vector<double> v);


//sample_variance
double sample_variance (std::vector<double> v);



int main()
{
    std::cout<<"hello world";
}


//sample_mean
double sample_mean (const std::vector<double>& v)
{
    double sum= 0;
    int i=0; for (; i<v.size(); ++i)
    {
        sum= sum+ v[i];
    }
    return sum/v.size();
}



//sample_variance
double sample_variance (const std::vector<double>& v)
{
    double average= sample_mean(v);
    double sum=0;
    int i=0; for (; i<v.size(); ++i)
    {
        sum= sum+ std::pow((v[i]-average), 2);
    }
    return sum/v.size();
}

This code contains only the definition of sample_mean and sample_variance , but the compiler keeps telling me that the function call of sample_mean inside the definition of sample_variance (which is double average= sample_mean(v); ) is ambiguous. What is wrong with my program?

Answer 1

Your definitions have different signatures than the declarations.

Simple cure: move the definitions above main , and forget about forward-declaring the functions.

Forward-declaring the functions is a C-ism, not a particularly bright idea in C++. It just adds work, and creates problems. As you discovered.

Answer 2

You have 2 methods called sample_mean , not just one, and both can be called:

double sample_mean (const std::vector<double>& v)

and

double sample_mean (std::vector<double> v)

Answer 3

First you have a function declaration double sample_mean (std::vector<double>) and then you have a function definition double sample_mean (const std::vector<double>&) . These are treated as identifiers of two different functions.