Matlab-生成矩阵的随机坐标

Question

I need to create a list (of size n) of random, non-repeating set of coordinates on a matrix of predefined size.

Is there a fast way to generate this in Matlab?

My initial idea was to create a list of size n with permutations the size of (width x length) and to translate them back to Row and Col values, but it seems to me too much.

Thanks, Guy

Answer 1

You can use randperm to generate a linear index, and convert it to [row,col] if needed using ind2sub .

x = rand(7,9);
n = 20;
ndx = randperm(numel(x), n);
[row,col] = ind2sub(size(x), ndx);

Answer 2

As long as n is less than the number of elements in the matrix this is simple:

% A is the matrix to be sampled
% N is the number of coordinate pairs you want
numInMat = numel(A);

% sample from 1:N without replacement
ind = randperm(numInMat, N);

% convert ind to Row,Col pairs
[r, c] = ind2sub( size(A), ind )

Answer 3

Your idea is a good one, although you don't even have to convert your linear indices back to row and col indices, you can do linear indexing directly into a 2D array.

idx = randperm(prod(size(data)))

where data is your matrix. This will generate a vector of random integers between 1 and prod(size(data)) , ie one index for each element.

eg

n = 3;
data = magic(n);
idx = randperm(prod(size(data)));
reshape(data(idx), size(data)) %this gives you your randomly indexed data matrix back