choose one out of every 8 elements randomly

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bug bug 2016-1-8

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编辑： Guillaume 2016-1-12

I have a 1xm matrix and I want to choose one out of every 8 elements randomly, I can think of ways using for loops but I was hoping for a solution without for loops and efficient in terms of speed. Thanks

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采纳的回答

goerk 2016-1-8

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编辑：goerk 2016-1-8

在 MATLAB Online 中打开

m = 21;
A = (1:m)+0.314;
nof8 = ceil(m/8);
randSelect = randi([1 8],1,nof8);
inds = randSelect+(0:nof8-1)*8;
% if m is not a multiple of 8 the last samples must get special treatment
% the simplest solution is to limit the indices in inds to the m
inds(end)=min(inds(end),m);
% but this leads to a non-uniform distribution for the last selection.
selectedElements = A(inds)

A solution with an uniform distribution for all cases was suggested from Guillaume:

 randselect = randi(8, 1, floor(numel(A)/8));
 if mod(numel(A), 8) > 0 
   randselect = [randselect, randi(mod(numel(A), 8))];
 end
 selectedvalues = A(randselect + (0:ceil(numel(A)/8)-1)*8)

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goerk 2016-1-12

在 MATLAB Online 中打开

Select multiple (unique) Samples:

m = 21;
A = (1:m)+0.314;
nof8 = floor(m/8); %floor-> if m ~= x*8 ignore last samples
nofSamples = 2;
randSelect = zeros(nofSamples,nof8);
for i=1:nof8
    randSelect(:,i)= randperm(8,nofSamples);
end
inds = randSelect+repmat((0:nof8-1)*8,nofSamples,1);
selectedElements = A(inds)

Guillaume 2016-1-12

编辑：Guillaume 2016-1-12

在 MATLAB Online 中打开

Since the number of elements is always multiple of 8, this is a case where using reshape is a lot more elegant.

A = 1:104;
nofsamples = 3;
A8 = reshape(A, 8, []);
selectedelements = A8(sub2ind(size(A8), randperm(8, nofsamples), 1:size(m8, 2)))
%if required as a vector:
selectedelements = selectedelements(:)

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更多回答（2 个）

Guillaume 2016-1-8

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在 MATLAB Online 中打开

One possible way:

m = 1:104; %input
assert(mod(numel(m), 8) == 0, 'the number of elements in m must be a multiple of 8');
m8 = reshape(m, 8, []);
m8(sub2ind(size(m8), randi(8, 1, size(m8, 2)), 1:size(m8, 2)))

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Nobel Mondal 2016-1-8

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/263088-choose-one-out-of-every-8-elements-randomly#answer_205504

在 MATLAB Online 中打开

m = 100;
spacedIds = [0:8:m];
rands = randi(8, 1, size(spacedIds,2)-1);
randIds = spacedIds(1:end-1)+ rands;
% check if m is not a multiple of 8
% add another number from the remaining tail-section
if ~(spacedIds(end) == m) 
  randIds(end+1) = spacedIds(end) + randi(m-spacedIds(end), 1,1);
end