Can I use ismember to compare a vector with a matrix

Question

Gayan Lankeshwara 2020-8-28

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评论： Gayan Lankeshwara 2020-8-29

Hi there,

Imagine I have an array and a matrix as below.

array_sample = [10;20;30]
matrix_sample = [15,35,10;22,33,44;55,25,30]

both array_sample and array_sample have the same number of rows.

Can anyone please tell me how to use ismember to compare each row in array_sample with each row in matrix_sample and output 1 if the array_sample element is present in the corresponding row of the matrix_sample ?

For the example given above the output should be

%% if I try something like this,
sample_out = ismemeber(array_sample, matrix_sample)
%% the output should be like this 
sample_out  = [1,0,1]' ; %% Because 10 and 30 are found respectively in 1st and 3rd row of matrix_sample.

Thank you.

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Answer 1

Walter Roberson 2020-8-28

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any(array_sample(:) == matrix_sample,2)

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Gayan Lankeshwara 2020-8-28

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Thanks a lot for your response.

This actually works perfect.

But my overall intention is something different.

Actually I am formulating constraints for an optimisation problem (using YALMIP)

So, array_sample is my decision variable (which I want to optimise). So I do not know the values of it.

%% assume I have loaded YALMIP to matlab path
nu = 3; %% matched with the previous MWE
N =  10 ; %% arbitrary value
%% matrix_sample
matrix_sample = [15,35,10;22,33,44;55,25,30];
%% decision variable
array_sample = sdpvar(repmat(nu,1,N), repmat(1,1,N)); %% spdvar is a YALMIP function.
constraints = [];
for k = 1 : N
    
    %% for each iteration of k, I need array_sample to take any value from matrix_sample
    constraints = [constraints, array_sample{k}, matrix_sample]; %% this line is wrong, only tells what I want
    
end

Please mind that the above code is not a MWE. (since it includes YALMIP toolbox plugin)

But would really appreciate if you could at least give me hint on how the assignment of array_sample to take values from matrix_sample can be achieved ?

Thank you.

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Answer 2

Steven Lord 2020-8-28

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Since you're using release R2019a (which supports implicit expansion) and your data is compatibly sized this is a job for the == operator and the any function.

array_sample = [10;20;30];
matrix_sample = [15,35,10;22,33,44;55,25,30];
sample_out = any(array_sample == matrix_sample, 2)

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Gayan Lankeshwara 2020-8-28

在 MATLAB Online 中打开

Thanks a lot for your response.

This actually works perfect.

But my overall intention is something different.

Actually I am formulating constraints for an optimisation problem (using YALMIP)

So, array_sample is my decision variable (which I want to optimise). So I do not know the values of it.

%% assume I have loaded YALMIP to matlab path
nu = 3; %% matched with the previous MWE
N =  10 ; %% arbitrary value
%% matrix_sample
matrix_sample = [15,35,10;22,33,44;55,25,30];
%% decision variable
array_sample = sdpvar(repmat(nu,1,N), repmat(1,1,N)); %% spdvar is a YALMIP function.
constraints = [];
for k = 1 : N
    
    %% for each iteration of k, I need array_sample to take any value from matrix_sample
    constraints = [constraints, array_sample{k}, matrix_sample]; %% this line is wrong, only tells what I want
    
end

Please mind that the above code is not a MWE. (since it includes YALMIP toolbox plugin)

But would really appreciate if you could at least give me hint on how the assignment of array_sample to take values from matrix_sample can be achieved ?

Thank you.

Steven Lord 2020-8-28

So you want to evaluate your objective function on a column vector where each element must come from the corresponding row of the matrix? So for your example matrix_sample your optimization routine would be allowed to evaluate the objective function with input [15; 33; 55] but not [15; 33; 54] (since 54 is not in the third row of matrix_sample)?

That's a very different question than the one you asked.

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