Square root of all elements of Boost Ublas Matrix - c++

Question

I am trying to compute square root of all elements of a Boost Ublas matrix. So far, I have this, and it works.
#include <iostream>
#include "boost\numeric\ublas\matrix.hpp"
#include <Windows.h>
#include <math.h>
#include <cmath>
#include <algorithm>
typedef boost::numeric::ublas::matrix<float> matrix;
const size_t X_SIZE = 10;
const size_t Y_SIZE = 10;
void UblasExpr();
int main()
{
UblasExpr();
return 0;
}
void UblasExpr()
{
matrix m1, m2, m3;
m1.resize(X_SIZE, Y_SIZE);
m2.resize(X_SIZE, Y_SIZE);
m3.resize(X_SIZE, Y_SIZE);
for (int i = 0; i < X_SIZE; i++)
{
for (int j = 0; j < Y_SIZE; j++)
{
m1(i, j) = 2;
m2(i, j) = 10;
}
}
m3 = element_prod(m1, m2);
std::transform(m1.data().begin(), m1.data().end(), m3.data().begin(), std::sqrtf);
for (int i = 0; i < X_SIZE; i++)
{
for (int j = 0; j < Y_SIZE; j++)
{
std::cout << m3(i, j) << " ";
}
std::cout << std::endl;
}
}
But, I would like to not use the std::transform, and instead do something like this :
m3 = sqrtf(m1);
Is there a way to make it work? My application is very performance sensitive, so the alternative is only acceptable if it results in no loss of efficiency.
P.S. I would like to do this for a whole lot of other operations like log10f, cos, acos, sin, asin, pow. I need these all in my code.

Answer 1

You can define your own sqrt function with an appropriate signature:
typedef boost::numeric::ublas::matrix<float> matrix;
matrix sqrt_element(const matrix& a)
{
matrix result(a.size1(), a.size2());
std::transform(a.data().begin(), a.data().end(), result.data().begin(), std::sqrtf);
return result;
}
You could also define a general 'apply_elementwise' to take a callable object as an argument (untested/not compiled):
typedef boost::numeric::ublas::matrix<float> matrix;
template <typename CALLABLE>
matrix apply_elementwise(const CALLABLE& f, const matrix& a)
{
matrix result(a.size1(), a.size2());
std::transform(a.data().begin(), a.data().end(), result.data().begin(), f);
return result;
}
Then you could call this as:
matrix y(apply_elementwise(std::sqrt, x));
matrix z;
z = apply_elementwise(std::cos, x);
In these functions, we're returning a matrix by value. Ideally, you want to make sure you that the matrix class you're using employs rvalue-reference constructors and assignment operators to minimize copying of data.