Selecting random cells in a matrix without repetition

Question

Lorson Blair 2021-10-1

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https://ww2.mathworks.cn/matlabcentral/answers/1464959-selecting-random-cells-in-a-matrix-without-repetition

评论： Lorson Blair 2021-10-1

Good day guys. This may be a simple question, but I cannot seem to find an answer on here for it. I am trying to select random cells in a matrix without repeating the cells. For example. I have a 10x10 matrix, and I'm trying to select 20 cells randomly without repeating the cells. Any help will be appreciated.

Thanks.

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Answer 1

dpb 2021-10-1

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https://ww2.mathworks.cn/matlabcentral/answers/1464959-selecting-random-cells-in-a-matrix-without-repetition#answer_799654

编辑：dpb 2021-10-1

在 MATLAB Online 中打开

N2Choose=20;
R=M(randperm(numel(M),N2Choose));

ERRATUM

Stupid auto-complete put the silly parens in instead of letting me when/where wanted...my story and I'm stickin' to it!!! :)

ADDENDUM

As far as the comment re: positions, just wrap the indices when generate them if those are the desired return values instead of the values.

[r,c]=ind2sub(randperm(numel(M),N2Choose),size(M));

In some ways this is less useful than the linear indices; you'll need arrayfun or similar construct to pull the elements that way as MATLAB will expand the two vectors to all combinations of each instead of just accessing the two vector elements pairwise.

I just answered another earlier this AM that is similar issue -- https://www.mathworks.com/matlabcentral/answers/1464664-feeding-index-returned-by-find-in-another-matrix#answer_799384

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Kelly Kearney 2021-10-1

在 MATLAB Online 中打开

One parentheses was out of place on dpb's example:

M = rand(10);
N2Choose=20;
idx = randperm(numel(M), N2Choose) % the indices
idx = 1×20
    31    46    11    16    79    30    40    90     3    36    47     6    81    94    96    66     2    89    83    42
[r,c] = ind2sub(size(M), idx) % if you need row/column
r = 1×20
     1     6     1     6     9    10    10    10     3     6     7     6     1     4     6     6     2     9     3     2
c = 1×20
     4     5     2     2     8     3     4     9     1     4     5     1     9    10    10     7     1     9     9     5
R=M(idx) % the values
R = 1×20
    0.0584    0.3064    0.4200    0.4720    0.8760    0.4465    0.7343    0.7369    0.6275    0.2472    0.9182    0.7630    0.1024    0.7336    0.3009    0.4893    0.5159    0.3713    0.5391    0.0316