# choose one out of every 8 elements randomly

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### Accepted Answer

goerk
on 8 Jan 2016

Edited: goerk
on 8 Jan 2016

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)

##### 11 Comments

goerk
on 12 Jan 2016

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
on 12 Jan 2016

Edited: Guillaume
on 12 Jan 2016

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(:)

### More Answers (2)

Guillaume
on 8 Jan 2016

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
on 8 Jan 2016

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

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