Select 100 random samples from a 50000x9 matrix and perform 50 iterations to save the choices.

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Alex 2018-9-25

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https://ww2.mathworks.cn/matlabcentral/answers/420676-select-100-random-samples-from-a-50000x9-matrix-and-perform-50-iterations-to-save-the-choices

评论： Image Analyst 2018-9-25

I have a matrix of size 50000 x 9. I want to select 100 random samples (like a series of 100nos) from different columns in 50 different iterations. Later I want to save the results of these 50 iterations in a matrix. Can someone shed any light?

Thanks

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Stephen23 2018-9-25

编辑：Stephen23 2018-9-25

"Later I want to save the results of these 50 iterations in 50 different variables."

Dynamically creating/accessing variable names in a loop is one way that beginners force themselves into writing slow, complex, buggy code that is hard to debug. Read this to know why:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

It is simpler and much more efficient to use indexing and one array, just as the MATLAB documentation recommends.

"Can someone shed any light?"

Write a loop and use indexing. What have you tried so far?

Alex 2018-9-25

Thanks Stephen for the fruitful comment. I would rather save it to a matrix now. And accordingly, I edited the question.

I know how to extract random columns from a matrix using randperm function but I am not getting any idea how to repeat it for different columns.

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Answer 1

Image Analyst 2018-9-25

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https://ww2.mathworks.cn/matlabcentral/answers/420676-select-100-random-samples-from-a-50000x9-matrix-and-perform-50-iterations-to-save-the-choices#answer_338366

在 MATLAB Online 中打开

Looks like this code should do it:

m = rand(50000, 9);
numIterations = 50;
results = zeros(1, numIterations); % Preallocate results.
for k = 1 : numIterations  % 50 iterations
  % Get 100 locations randomly.
  randomIndexes = randperm(numel(m), 100);
  % Extract those locations
  selectedValues = m(randomIndexes);
  % Do some operation on them, like summing for example.
  % Change this to do whatever operation(s) YOU want to do.
  results(k) = sum(selectedValues);
end
plot(results, 'b-', 'LineWidth', 2);
grid on;
xlabel('Iteration Number', 'FontSize', fontSize);
ylabel('Result', 'FontSize', fontSize);
caption = sprintf('Results over %d iterations', numIterations);
title(caption, 'FontSize', fontSize);

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Alex 2018-9-25

Thanks IA for the detailed script. Unfortunately, it is not what I was looking for (it gave some idea though :))

I need to randomly select columns and then select a data series (like 100nos.). And I need to repeat it for 50 times. Guess my question was unclear. I will edit now.

Image Analyst 2018-9-25

在 MATLAB Online 中打开

OK, please go ahead and edit. You haven't yet. Be sure to say how many of the 9 columns to you want to randomly select. Let's say you want to select 100 numbers from 4 of the 9 columns. You could use code like this:

rows = 50000;
columns = 9;
m = rand(rows, columns);
numIterations = 50;
columnsToSample = 4;
% Get the columns
cols = sort(randperm(columns, columnsToSample))
results = zeros(numIterations, columnsToSample); % Preallocate results.
for k = 1 : numIterations  % 50 iterations
  for k2 = 1 : length(cols)
    % For each selected column...
    thisColumn = cols(k2);
    % Get 100 locations randomly.
    randomIndexes = randperm(rows, 100);
    % Extract those locations
    selectedValues = m(randomIndexes, thisColumn);
    % Do some operation on them, like summing for example.
    % Change this to do whatever operation(s) YOU want to do.
    results(k, k2) = sum(selectedValues);
  end
end
plot(results, '-', 'LineWidth', 2);
grid on;
xlabel('Iteration Number', 'FontSize', fontSize);
ylabel('Result', 'FontSize', fontSize);
caption = sprintf('Results over %d iterations', numIterations);
title(caption, 'FontSize', fontSize);