How can I select randomly?

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?

Answer 1

[Jos (10584)]

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,:) ;

[Selin Soguksu]

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]

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

Answer 2

[Jan]

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)

[Selin Soguksu]

Thank you for the answer, but it didn't solve my question.

[Jan]

@Beril: That's interesting. Why does it not solve your problem although it seems to be the same code as in the accepted answer?

[Selin Soguksu]

Because when I wrote M=rand(10000,10), MATLAB generates numbers between 0 and 1. But I didn't want this. I had an matrix with datas.

[Jan]

@Beril: Ouch. In my example M is only created to have any test data. In the real program this line should be omitted, of course. Your question contains only the description "I have an 10000 rows and 10 columns matrix" and I filled it with random data. This is a usual method in a forum, because it allows to test the code before posting it.

[Image Analyst]

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

[Matt Fig]

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

[Selin Soguksu]

I understand :)))