Hi,

is there any way to divide each row of a matrix by a vector or each row of matrix by the corresponding row of another?

I have this code in Matlab:

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mxn_matrix = some_mxn_matrix + another_mxn_matrix ./ some_other_mxn_matrix;


I have been trying to learn Eigen, but I don't know how to do the above.

I have a few questions:

1) How to do the division part in Eigen?

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another_mxn_matrix ./ some_other_mxn_matrix


2) Is it possible to do division of each row of a matrix by a vector?

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% won't work in Matlab, but I am curious about Eigen
another_mxn_matrix ./ some_1xn_vector;

I tried doing the following in Eigen:

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another_mxn_matrix / some_1xn_vector

where

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some_1xn_vector is Eigen::VectorXf
another_mxn_matrix is Eigen::Matrix<some_type, Eigen::Dynamic, Eigen::Dynamic>

But I get:

error: no operator "/" matches these operands

3) Is there any equivalent of Matlab's repmat in Eigen?

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%This WILL work in Matlab
another_mxn_matrix ./ repmat(some_1xn_vector, m, 1);

4) Is it possible to divide a Scalar by a vector/matrix?

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%Matlab
testv = 3 ./ some_1xn_vector;
testm = 4 ./ some_mxn_matrix;

5) In general, what are the Eigen equivalents of the Matlab operators?

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./
.*
.^


Thanks in advance

All these things are possible in Eigen and you should be able to find the syntax easily in the documentation. If you're using Eigen3 (which is currently only available as a beta version but very close to be released), here are some relevant links:

• http://eigen.tuxfamily.org/dox-devel/ - main page for the documentation: read through the tutorial, especially "Tutorial page 3 - The Array class and coefficient-wise operations"
• http://eigen.tuxfamily.org/dox-devel/AsciiQuickReference.txt - page comparing Matlab and Eigen syntax
• http://eigen.tuxfamily.org/dox-devel/QuickRefPage.html - another reference-style page for quickly looking up syntax

Sorry for the terse reply, but I don't have much time at the moment. If this does not help, let us know and I'll give more details later.

Hello,

jitseniesen wrote:All these things are possible in Eigen and you should be able to find the syntax easily in the documentation. If you're using Eigen3

No, I'm using the stable Eigen 2

jitseniesen wrote:

• http://eigen.tuxfamily.org/dox-devel/AsciiQuickReference.txt - page comparing Matlab and Eigen syntax

According to that link:

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#include <Eigen/Core>
#include <Eigen/Array>
...

Eigen::Matrix3f P, Q, R;
R = P.cwiseQuotient(Q);   // R = P ./ Q
R = P.array() / Q.array();


Should give me the result I wanted, but it fails to compile:

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1> error: class "Eigen::Matrix<float, 3, 3, 2, 3, 3>" has no member "cwiseQuotient"
1>        R = P.cwiseQuotient(Q);   // R = P ./ Q
1>              ^
1>
1> error: class "Eigen::Matrix<float, 3, 3, 2, 3, 3>" has no member "array"
1>        R = P.array() / Q.array();
1>              ^
1>
1> error: class "Eigen::Matrix<float, 3, 3, 2, 3, 3>" has no member "array"
1>        R = P.array() / Q.array();
1>                          ^


I'm using:

Intel(R) C++ Intel(R) 64 Compiler Professional for applications running on Intel(R) 64, Version 11.1, Windows 7 Pro x64

But don't blame it. My Eigen code has compiled and run fine up to now with that compiler.

So, my question remains:

How do I do R = P ./ Q ?

Thanks in advance

Doesn't work under Linux either:

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$ wget http://bitbucket.org/eigen/eigen/get/2.0.15.tar.gz
$ tar xzvf 2.0.15.tar.gz


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$ cat eigen_test.cpp

#include <Eigen/Core>
#include <Eigen/Array>

USING_PART_OF_NAMESPACE_EIGEN

int main(int argc, char** argv)
{
                Eigen::Matrix3f P, Q, R;
                R = P.cwiseQuotient(Q);   // R = P ./ Q
                R = P.array() / Q.array();

        return(0);
}


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$ g++ -c -I./eigen eigen_test.cpp
eigen_test.cpp: In function âint main(int, char**)â:
eigen_test.cpp:11: error: âclass Eigen::Matrix3fâ has no member named âcwiseQuotientâ
eigen_test.cpp:12: error: âclass Eigen::Matrix3fâ has no member named âarrayâ
eigen_test.cpp:12: error: âclass Eigen::Matrix3fâ has no member named âarrayâ


I'm using:

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$ uname -a
Linux machine_name 2.6.31-22-server #70-Ubuntu SMP Thu Dec 2 00:00:57 UTC 2010 x86_64 GNU/Linux


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$ g++ --version
g++ (Ubuntu 4.4.1-4ubuntu9) 4.4.1
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The array syntax is for Eigen3 only. In Eigen2 you have to use the cwise() prefix syntax:

R = P .cwise()/ Q;

See: http://eigen.tuxfamily.org/dox/Tutorial ... cOperators