randomization of a vector by blocks

I am programming an experiment. Let's say that my experiment has 13 different conditions. Each condition is presented twice per block. There are two blocks in total. This makes a total of 52 trials.

conditions = repmat(1:13,1,4)

I can randomize the variable condition.

conditionsrand = conditions(randperm(length(conditions)))

However, I would like to do this randomization by block. That is every 26 trials there should be two instances of each condition. I think that this must be something easy, but I cannot find the solution.

Any idea?

Thanks!

Answer 1

[Jos (10584)]

C = repmat(1:13,1,2) % conditions per block
% [block1 block2]
rC = [C(randperm(numel(C))) C(randperm(numel(C)))] % *corrected

[Alejandro Estudillo]

Thanks for your reply! It gives me the following error

rC = [C(randperm(numel(C)) C(randperm(numel(C))] ↑ Error: Unexpected MATLAB expression.

In addition, each condition is presented 2 times per block, hence repmat(1:13,1,4). Also, I will have to increase the number of blocks to 100, so maybe the instructions rC = [C(randperm(numel(C)) C(randperm(numel(C))] will become too tedious?

THanks!

[Jos (10584)]

I forgot two closing brackets, which is now corrected. Do you want to create all trials at once? An easy alternative:

C = 1:5 
numC = numel(C)
numCperBlock = 2 
numBlocks = 3

[~,r] = sort(rand(numC, numCperBlock * numBlocks))
rC = reshape(C(r),1,[])

[Alejandro Estudillo]

thanks! that works perfectly!