How can I select randomly?

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Selin Soguksu
Selin Soguksu 2012-12-12
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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Jos (10584)
Jos (10584) 2012-12-12
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,:) ;
  2 个评论
Selin Soguksu
Selin Soguksu 2012-12-12
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
Jan 2012-12-12
编辑：Jan 2012-12-12
The RANDPERM approach creates unique indices, while the indices created by RANDI can be non-unique, e.g. [1, 503, 1017, 1, ...].

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Jan
Jan 2012-12-12
编辑：Jan 2012-12-12
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)
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Matt Fig
Matt Fig 2012-12-14
Apparently saying, "M is your original matrix" would have made all the difference ;-).

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