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matlabrandom
Name Name
Name Name

Reputation: 55

How do I randomly pick an element in each column of a matrix in MATLAB?

Question title explains what I would like. For example, if there are 6 elements in a particular column, how do I randomly pick 1 element from that column. Please keep it simple if possible.

Thanks for the help.

Upvotes: 3

Views: 2647

Answers (3)

Dennis Jaheruddin
Dennis Jaheruddin

Reputation: 21563

Here is an easy way to do it. Note that a version without loops should be more efficient.

Assuming your variable is x loop over its n columns:

selected = zeros(1,n);
for c = 1:n
    selected(c) =  x(randperm(6,1),n);
end

Upvotes: 0

Mohsen Nosratinia
Mohsen Nosratinia

Reputation: 9864

You can use randi if your version of MATLAB is > R2008a

samples = A(sparse(randi(size(A,1),size(A,2),1), 1:size(A,2), true));

or,

[m, n] = size(A);
samples = A(sparse(randi(m,n,1), 1:n, true));

However for older versions you can replace randi with randsample but that requires Statistics Toolbox. Or introduce:

randi = @(imax, m, n) floor(1+rand(m,n)*imax);

Upvotes: 3

Shai
Shai

Reputation: 114786

Suppose you have a matrix A of size m-by-n. You wish to pick one element from each of the n columns at random:

>> rows = randsample( m, n ); % sample n times from integers 1:m

Now rows has n values, each represent a random entry at the corresponding column.
To access those values

>> sampledValues = A( sub2ind( size(A), rows, 1:n ) );

For more information see the doc on randsample and sub2ind.

Upvotes: 5

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