MATLAB Answers

Selin Soguksu

How can I select randomly?

Asked by Selin Soguksu
on 12 Dec 2012

Hello, I have an 10000 rows and 10 columns matrix. I want to select randomly 500 rows from this matrix. I want to ask you, randperm function is true for this purpose. How can I select 500 rows randomly from this matrix?



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2 Answers

Answer by Jos (10584)
on 12 Dec 2012
 Accepted answer

Random can be defined in two ways:

% A is your original matrix
Nrows = size(A,1) ; % number of rows
% Option 1: randomly select 500 UNIQUE(!) rows
idx = randperm(Nrows) ; 
idx = idx(1:500) ;
% Option 2: randomly select 500 rows 
idx = randi(Nrows,[Nrows 1]) ;
% and then ...
B = A(idx,:) ;


Thank you very much for the answer. :)) It works great. But I want to ask you a question more. You wrote "randomly select 500 UNIQUE(!) rows", in this way for example when 550th row is selected, no more it can be selected. Is it true?

Jan Simon
on 12 Dec 2012

The RANDPERM approach creates unique indices, while the indices created by RANDI can be non-unique, e.g. [1, 503, 1017, 1, ...].

Answer by Jan Simon
on 12 Dec 2012
Edited by Jan Simon
on 12 Dec 2012

M = rand(10000, 10);
index = randperm(10000, 500);  % In modern Matlab versions
R = M(index, :);

In older Matlab versions randperm does not accept a 2nd input. Then:

index = randperm(10000);
index = index(1:500);

If this must be fast, use the C-Mex FEX: Shuffle.

index = Shuffle(10000, 'index', 500)


I could hear the sound of a hand slapping a forehead all the way across the Atlantic. ;-)

Matt Fig
on 14 Dec 2012

Apparently saying, "M is your original matrix" would have made all the difference ;-).

I understand :)))

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