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I have matrix and need to subtract another matrix element by element on each row. Something like this:

$ \begin{pmatrix} x_{1} & x_{2}\\ x_{3} & x_{4}\\ \vdots & \vdots\\ x_{n-1} & x_{n}\\ \end{pmatrix} - \begin{pmatrix} y_{1} & y_{2}\\ \end{pmatrix} $

So end result should be something like:

$ \begin{pmatrix} x_{1} - y_{1} & x_{2} - y_{2}\\ x_{3} - y_{1} & x_{4} - y_{2}\\ \vdots & \vdots\\ x_{n-1} - y_{1} & x_{n} - y_{2}\\ \end{pmatrix} $

How to do this? How to do this in Octave, Matlab?

Sorry for noob question. Also would be very kind if you pint me where to read about this.

  • 0
    How about x - repmat (y, [n 1]) ;2016-05-24

5 Answers 5

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With the current version (3.6) of Octave, simply subtracting will work

> a = [1 2; 3 4; 5 6; 7 8] > b = [1 -1] > a - b ans =     0   3    2   5    4   7    6   9 

Edit: Apparently this will also work in Matlab starting with the upcoming release (2016b).

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    This depends if the feature called "Automatic Broadcasting" is turned on or not. You can turn it off to make make sure your code is more compatible with Matlab if you want to.2016-05-24
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Solution from Stackoveflow - https://stackoverflow.com/a/1773119/38975

bsxfun(@minus, X, y); 
  • 0
    T$h$is works also in Octave 3.42013-10-27
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If your matrices are only two columns, here's a nasty way to do it:

>> a = [1 2; 3 4; 5 6; 7 8] >> b = [1 -1]  >> [a(:,1)-b(1),a(:,2)-b(2)] ans =     0   3    2   5    4   7    6   9 

I suspect there's a better way though ...

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The following is also a Kronecker product shortcut and is quite general: Suppose your $X,y_1,y_2$ is in the workspace, then

result = X - kron(ones(size(X,1),1),[y1 y2]); 

gives you the ... result :)

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Another way to do it is by outer product:

> a = [1 2; 3 4; 5 6; 7 8] > b = [1 -1] > a - ones(4,1)*b 

This easily gets nastier to express with outer products if you start having 3,4 or even more dimensions and want to replicate any subset of them though.