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arraysmatlabmatrixindexing
Captain
Captain

Reputation: 193

How to select rows of a matrix except of a list of indices

I am going to select some rows of a data set, except of a list of indices.

For example, in a given data_set I want to select all rows except for idx = [2,3,6,11,15].

How can we do this in MATLAB? Is there any command or logical indexing method?

Upvotes: 2

Views: 370

Answers (1)

Wolfie
Wolfie

Reputation: 30165

There are many ways to do this by comparing your exclusion list with a full list from 1:n, where n is the number of rows. Below I've listed 2 which use this logic. I've also shown the simplest way (removing rows) but that requires an intermediate step.

I'm not sure which is more performant of these:

% Setup
idx = [2,3,6,11,15]; % exclusion list
M = rand( 25, 10 );  % test matrix to index (25 rows)

Using ismember and logical indexing

K = M( ~ismember( 1:size(M,1), idx ), : );

Using setdiff to get the row numbers not listed

K = M( setdiff( 1:size(M,1), idx ), : );

Creating a temp matrix, then removing the excluded rows

K = M;
K( idx, : ) = [];

Error handling

Note that the final row-removing method will produce an error if any of the rows in your exclusion list are out of bounds (e.g. if 0 was in idx).

The setdiff and ismember methods won't give you any errors, the presence of out-of-bounds values in idx are simply redundant.

Upvotes: 3

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