Eigen boolean array slicing

Question

In MATLAB it is common to slice out values that satisfy some condition from a matrix/array (called logical indexing).

vec = [1 2 3 4 5];
condition = vec > 3;
vec(condition) = 3;

How do I do this in Eigen? So far I have:

Eigen::Matrix<bool, 1, 5> condition = vec.array() > 3;

Inspan

dont have much experience with Eigen, but looks like you're looking for the select feature – Permeance

Possible duplicate of Submatrices and indices using Eigen – Cohort

Answer 1

As pointed out in the answer to an similar question here: Submatrices and indices using Eigen, libigl adds this functionality to Eigen.

igl::slice(A,indices,B);

Is equivalent to

B = A(indices)

Cohort

while it may be useful, it doesn't answer the question here. OP asked for the equivalent of MATLAB's A(A>3)=3, not how to extract a submatrix... The solution I showed is basically an element-wise ternary operator equivalent to: m(i) = (m(i) > 3) ? 3 : m(i). – Permeance

Browsing the docs, igl::slice_into, is a closer match, but as far as I can tell, it only works for a list of indices, not a vector of logicals... Even their MATLAB-to-eigen/igl conversion table suggests using Eigen::select: libigl.github.io/libigl/matlab-to-eigen.html (see the A(B == 0) = C(B==0) statement). – Permeance

igl::slice_mask(vec,condition,output) – Isidoro

Answer 2

Try this:

#include <iostream>
#include <Eigen/Dense>

int main()
{
    Eigen::MatrixXi m(1, 5);
    m << 1, 2, 3, 4, 5;
    m = (m.array() > 3).select(3, m);
    std::cout << m << std::endl;

    return 0;
}

Permeance

@srsci: what do you mean? The example above is working fine, it's practically straight out of the documentation.. – Permeance

Actually, for the given problem (i.e., capping of the values) just m.cwiseMin(3) should work (and is usually faster). – Groggery