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I am porting some MATLAB code to C++ using the Eigen 3 template library, and I am looking for a good mapping for this common MATLAB idiom:

K>> [1 2 3 4 5] <= 3

ans =

     1     1     1     0     0

So, compare an array and a scalar, returning an array of booleans that has the same shape.

I understand that Eigen's Array class has coefficient-wise comparison operators, but if I'm interpreting the docs correctly they only work with another array; not with scalar values.

Is there some option I've missed that will perform the comparison with a scalar? Or failing that, a nice idiomatic way to create an appropriately-shaped Array filled with the scalar value for the RHS of the expression?

Kaelin Colclasure
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1 Answers1

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With thanks to ChriSopht_ from the #eigen IRC channel:

VectorXd compareMat = ...;
double cutoff = 3;
Matrix<bool, Dynamic, 1> result = compareMat.array() <= cutoff;

So, the trick is using .array() to get at coefficient-wise operators, and of course then getting the return type right…

Kaelin Colclasure
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  • If you don't want to deal with the return type and you're using C++11, you can always use `auto result = compareMat.array() <= cutoff;` as well. – GJStein Jul 25 '16 at 01:17