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c++armadillo

Complementary indexes to find() in Armadillo

Basic question from a newcomer to Armadillo and C++ from R.

I have a vector X and I want to set the entries below 0 to a given value and the ones larger than 0 to another. Armadillo has the find function for returning indices of elements of X that are non-zero or satisfy a relational condition (not logical!?) so I can do:

arma::uvec ind0 = find(X < 0);
arma::uvec ind1 = find(X >= 0);

X(ind0).zeros();
X(ind1).fill(1);

This is clearly not the best solution. What would be a better way that does not involve calling find two times?

Solution

  • You can use the .transform() member function. Requires C++11 compiler.

    mat X(100,100,fill::randu);

X -= 0.5;

X.transform( [](double val) { return (val < 0) ? double(0) : double(1); } );