How to I iterate a random selection among given elements without selecting the same element twice?

Hello,

I am conducting an experiment and would like to present different stimuli (stimpair1 thru stimpair8) across 8 blocks (Nblocks = 8). The presented stimulus within each block shall be the same though. I would like to randomize the stimulus selection without selecting the same stimulus twice, such that a differerent stimulus is presented in every block. I have already randomized stimulus selection using the following code.

stimlist = [list.stimpair1; list.stimpair2; list.stimpair3; list.stimpair4; list.stimpair5; list.stimpair6; list.stimpair7; list.stimpair8];

for bnum = 1:Nblocks

stimpairpresented = [];

stimpairpresented = [stimpairpresented,randi(8)];

switch stimpairpresented

case 1

chosenstim.s1 = list.stimpair1

case 2

chosenstim.s2 = list.stimpair2

case 3

chosenstim.s3 = list.stimpair3

case 4

chosenstim.s4 = list.stimpair4

case 5

chosenstim.s5 = list.stimpair5

case 6

chosenstim.s6 = list.stimpair6

case 7

chosenstim.s7 = list.stimpair7

otherwise

chosenstim.s8 = list.stimpair8;

end

end

Yet, I am stuck at how to implement the condition that the presented stimulus is not the same as one that was presented in the previous blocks. Can someone help me with that?

Thank you for your support in advance.

Best

Answer 1

[Jan]

By the way, your code wouzld be much cleaner and shorter, if you use arrays instead of hiding indices in the field names: Define list.stimpair as nan(1, NBlock) and your code becomes:

for bnum = 1:Nblocks
   index = randi(8);
   chosenstim.s(index) = list.stimpair(index);
end

To avoid repetition use randperm:

index = randperm(8, 8)
chosenstim.s(index) = list.stimpair(index);

No loop, no switch, no bunch of case blocks, and even:

stimlist = [list.stimpair1; list.stimpair2; list.stimpair3; list.stimpair4; list.stimpair5; list.stimpair6; list.stimpair7; list.stimpair8];

becomes:

stimlist = list.stimpair;

Do you see the power of arrays instead of numbered field names?

A hint:

stimpairpresented = [];
stimpairpresented = [stimpairpresented,randi(8)];
% is exactly the same as:
stimpairpresented = randi(8);

[Kristin]

Dear Jan, thank you very much for your answer as well as your tips concerning the simplification of my code. Could you clarify how to define list.stimpair as nan(1,NBlock)? Do you mean that the list of stimulus pairs ranging from block 1 to 8 is not a number i.e. list.stimpair = NaN(1,Nblocks) ?

I am afraid that matlab concatenates all my stimulus pairs (each containing two elements) if I list them in the vector list.stimpair. Do I need to create nested lists/vectors to be able to select both elements of a stimulus pair together?

Best

[Jan]

I used nan(1, NBlock) as an example of how to create a vector with 8 elements.

If the used data contain 2 elements, you can store them in a matrix.

Currently it is not clear, what your input data are and what you want to achieve. Maybe this example helps:

stimuli = [1, 2; ...
           3, 4; ...
           5, 6; ...
           7, 8; ...
           9, 10];
mixedStimuli = stimuli(randperm(5), :)

mixedStimuli = 5×2

7 8 9 10 3 4 1 2 5 6