# Filter matrix rows based on value of function in Matlab

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ARS on 18 Feb 2013
Commented: yair suari on 1 Sep 2015
This seems like a simple question but I have been unable to find an answer anywhere. If I have a Matlab matrix "A" consisting of an arbitrary number of rows, how would I filter these rows based on the value of some function "f" (the argument of which is a row vector)? In other words, how would I keep only the rows of matrix A for which f is true? I tried
A(f(A(:)), :)
but to no success. Any help would be greatly appreciated.
So for example, if I have the matrix:
A =
1 1
1 2
1 3
1 4
2 1
2 2
2 3
2 4
3 1
3 2
3 3
3 4
4 1
4 2
4 3
4 4
And a function f that returns true only for inputs [1 2], [1 3], [2 4] and [3 4], my desired output would be:
ans =
1 2
1 3
2 4
3 4
yair suari on 1 Sep 2015
seems like ismember will do the work

Mark Whirdy on 18 Feb 2013
Edited: Mark Whirdy on 18 Feb 2013
Shown below using an anonymous function for neatness, but the trick is just to make sure that your function (anonymous, nested, sub, or separate) returns a LOGICAL VECTOR (or a vector of linear indexes)
fn = @(x)(x>1);
a = randn(20,1);
fn(a); % this would return a logical vector if you ran it
myfilteredVector = a(fn(a));
If I've misunderstood, could you give me an example with an input & a required output.
------
So the bones of the required operation is actually a row-wise matching? I do this by transforming each row into a single unique element by 10*A(:,1) + A(:,2), as below. Then ismember() with its mex engine can then be put to work on matching individual element. There are other matching/indexing approaches though but the primary aim is to have the function outputting a logical vector.
A = [...
1, 1;
1, 2;
1, 3;
1, 4;
2, 1;
2, 2;
2, 3;
2, 4;
3, 1;
3, 2;
3, 3;
3, 4;
4, 1;
4, 2;
4, 3;
4, 4;];
fn = @(x)(ismember(x,[12;13;24;34]));
% fn(10*A(:,1) + A(:,2)) would return a logical vector
A(fn(10*A(:,1) + A(:,2)),:); % logical vector selects rows, & we want every column so (logic, :)
You can of course also specify the matchset a=[12;13;24;34] as an argument as below.
fn = @(x,a)(ismember(x,a));
a = [12;13;24;34];
A(fn(10*A(:,1) + A(:,2)),:);
To be honest, your function is kind of unnecessary really (unless there's a reason outside of the example you've given), as you can just use ismember with the unique encoder 10*A(:,1)+A(:,2).
a = [1,2;1,3;2,4;3,4];
A( ismember(10*A(:,1)+A(:,2), 10*a(:,1)+a(:,2)) , :)
Mark Whirdy on 18 Feb 2013

Azzi Abdelmalek on 18 Feb 2013
n=size(A,1);
A(arrayfun(@(x) expression(A(x,:)),(1:n)','un',0))=[]
Azzi Abdelmalek on 18 Feb 2013
Edited: Azzi Abdelmalek on 18 Feb 2013
Try
n=size(A,1)
A(cell2mat(arrayfun(@(x) f(A(x,:)),(1:n)','un',0)),:)=[]
Also this is not a general solution, each expression has to be treated individually.

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