Doug's MATLAB Video Tutorials

December 31st, 2009

Basics: Finding a subset of a matrix

Imagine you have a matrix, and you want to make a subset of that matrix that has only the rows that have a 2 in the first column. This is how you would do that.

By Doug Hull

14:33 UTC | Posted in Level: Basic | Permalink | 9 Comments »

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9 Responses to “Basics: Finding a subset of a matrix”

matt fig replied on January 1st, 2010 at 23:24 UTC :

Unless the vi is needed for something else, I usually forgo the find function in favor of logical indexing.

vi = m(:,1) == target;
Jimmy replied on February 28th, 2010 at 16:35 UTC :

Hey doug,

can this sort of approach be used to find the smallest value within a row or column of a matrix? if so how?

Thanks.
dhull replied on March 1st, 2010 at 16:00 UTC :

@Jimmy,

You can use

[C,I] = min(…)

C will be the value and I will be the index.

Doug
Rommy replied on August 10th, 2010 at 19:39 UTC :

Hey doug

Say if you have a 3×3 matrix and want to create a subset based on 2 key values, eg. column 1 must match value x and column column 2 must match y, and your index should only return rows where both criteria are true. Is this possible using the same simple approach?

dhull replied on August 10th, 2010 at 19:44 UTC :

>> a = magic(3)

a =

     8     1     6
     3     5     7
     4     9     2

>> vi = (a(:,1) == 3) & (a(:,3) == 7)

vi =

     0
     1
     0

Aparna replied on August 11th, 2010 at 09:54 UTC :

I have 5 sub matrices with labels 1-5. I am using a loop and i want to delete the ith label at each iteration. Anyone knows how to do it?
c= mat2cell(exlData, count, cols+1);
display (c);
for i = 1 : k
display (c{i,:});
end
my matrix is stored in c{i,:}

thanks!
dhull replied on August 11th, 2010 at 14:41 UTC :

@Aparna,

What do you mean you have submatrices? Are they stored in their own variable names? What is a label in MATLAB?

Doug
claire replied on January 17th, 2012 at 16:26 UTC :

Hi Doug

Thanks for this, have found it to be really useful. Is there a way to target a range of values within the column?

For example instead of returning all those that contain 2 in column 1, returning all those that contain 1, 2, and 3 in column 1?
Doug replied on January 19th, 2012 at 09:33 UTC :

@Claire,

You can use two statements ((X<3) & (X>1))

Doug