std::inner_product to calculate the standard deviation of a vector

Question

I'm trying to create a function to calculate the standard deviation. I tried using std::inner_product and lambda expressions for op1 and op2 of inner_product to modify the operations of the std::inner_product function.

Unfortunately I'm getting a compiler error when calling the function in the main loop:

error C2664: cannot convert parameter 1 from "std::vector<float,std::allocator<float>>" in "std::vector<std::vector<float,std::allocator<float>>,std::allocator<std::vector<float,std::allocator<float>>>> &"

This is my code:

#include <numeric>
#include <cmath>

float stdfunc(std::vector<float> const & invector) {
   float mean = std::accumulate(invector.begin(), invector.end(), 0.0) / invector.size();

   float sq_sum = std::inner_product(invector.begin(), invector.end(), invector.begin(), 0.0,
   [](float const & x, float const & y) {return x+y;},
   [mean](float const & x, float const & y) {return (x-mean)*(y-mean);});

   return std::sqrt(sq_sum / (invector.size() - 1));
}

Call in main:

int main(){
std::vector<float> testv {6,3,2,9,11,44,20};
float stdw = stdfunc(testv);
std::cout << "Standardabw: " << stdw << std::endl;
    return 0;
}

I hope you can help me.

Answer 1

The error message means that the type of the argument that is std::vector<float>

std::vector<float> testv {6,3,2,9,11,44,20};
float stdw = stdfunc(testv);

does not correspond to the type of the function parameter that is std::vector<std::vector<float>>.

So check the function declaration. It is not excluded that the function declaration and the functions definition can differ.