Collect response value with two for loops nested

Hello everybody 'I'm new and this is my first approach to matlab, I have a matrix problem; maybe it is trivial but I have tried to search for it and couldn't find anything useful. This is a simplified version of my code. I used psychtoolbox to present visual stimuli and i want that, for each block, it shows the same number of trials in the right side of the screen and then in the left one. For each side I need it to record the response (the key pressed, the reaction time and other information). I was able to concatenate in horizontal the two matrix "left and right", but when the program saves my results it only keeps the last "block" of results( when you run it creates a response matrix 5x4 and not a 20x4 one). Can someone please help me to concatenate in vertical the results in a single matrix? Thank you in advance I hope I made myself clear

Cheers

numBlocks=4;

 dx=2;
 sx=1;%1=dx 0=sx
side=repmat( [ dx sx],numBlocks, 1);%1=dx 0=sx
numTrials=5;
responsesDx= zeros(numTrials,2);
responsesSx= zeros(numTrials,2);
responses=zeros(numTrials*numBlocks,4);
for i=1:numBlocks %how many time it has to repeat the loop
    for j=1:size(side,2)
        if j==side(sx)
           for j=1:numTrials
               key_pressed= 3;
               reaction_time=5;
               responsesDx(j,1)= key_pressed;
               responsesDx(j,2)= reaction_time;

           end
        else
            for j=1:numTrials
            key_prressed= 4;
               reaction_time=6;
               responsesSx(j,1)= key_pressed;
               responsesSx(j,2)= reaction_time;
               end
        end
         responses=[ responsesDx responsesSx];
    end

end

[Andy L]

First of all I noticed that in your second loop key_pressed is spelt key_prressed - is this ok? Also Have you tried sticking a keyboard into your loop to test why it isn't behaving as you expect? This pauses the program each time it comes to that line, allowing you to see what the values are on each iteration and help with your debug. The program resumes execution when you type continue in the workspace.

[alice]

yes is only a typing error, by the way thi is a simplified version in reality i use athe psychtoolbox to present stimulus in 4 different conditions and I have testes the whole code but it saved only the last "run" block. on this code I will expect a response table of a matrix of 20 rows (number of trials repeated for 4 blocks) and 4 columns(2 for each condition), but if you try to run the script you will see only a response matrix of 5x4. I want a final summary matrix to store every response of the subject

Answer 1

[Andy L]

It is only in the loops that the variable responses is changed to 5 x 4. If we deconstruct your code...

numBlocks = 4;

dx = 2;
sx = 1;  %1=dx 0=sx
side        = repmat( [ dx sx],numBlocks, 1); % 1 = dx 0 = sx
numTrials   = 5;
responsesDx = zeros(numTrials,2);
responsesSx = zeros(numTrials,2);
responses   = zeros(numTrials*numBlocks,4);

• responsesDx (5 x 2)
• responsesSx (5 x 2)
• responses (20 x 4)

for i = 1:numBlocks               % for i = 1:4.
    for j = 1:size(side,2)        % size(side,2) = 2.
        if j == side(sx)          % logical indexing, sx = 1, side(1) = 2.
            for j = 1:numTrials

j has now been reset to 1, and will be 5 when it exits this loop - suggest a new indexing variable here!

                  key_pressed      = 3;
                  reaction_time    = 5;
                  responsesDx(j,1) = key_pressed;
                  responsesDx(j,2) = reaction_time;
              end

When this loop has completed, responsesDx/Sx are 5 x 1.

          else
              for j = 1:numTrials

j has now been reset to 1, and will be 5 when it exits this loop - suggest a new indexing variable here!

                  key_pressed     = 4;
                  reaction_time    = 6;
                  responsesSx(j,1) = key_pressed;
                  responsesSx(j,2) = reaction_time;
              end

When this loop has completed, responsesDx/Sx are 5 x 1.

          end
          responses = [ responsesDx responsesSx];

responses is not indexed and so is rewritten on each loop. This is why your code provides you a 5x4 output. I would use a separate variable (block response) here, that then feeds into responses outside this loop.

      end
  end

Try the loop below!

for i = 1:numBlocks % how many time it has to repeat the loop
    for j = 1:size(side,2)
        if j == side(sx)
            for k = 1:numTrials
                key_pressed      = 3;
                reaction_time    = 5;
                responsesDx(k,1) = key_pressed;
                responsesDx(k,2) = reaction_time;
            end
        else
            for k = 1:numTrials
                key_pressed     = 4;
                reaction_time    = 6;
                responsesSx(k,1) = key_pressed;
                responsesSx(k,2) = reaction_time;
            end
        end
        blockResponse = [responsesDx responsesSx];
    end
    p = ((i-1)*numTrials) + 1;
    disp(['iteration ',num2str(i)]);
    responses(p:p+4,:) = blockResponse
end

[alice]

thank you so much, but can you explain why the rows in the array go from p to p+4? sorry I'm not expert matlab user.

[Andy L]

Not a problem Alice!

Your Matrix responses is already defined as a 20x4 matrix of zeros. At the end of each loop, blockResponse is a 5x4 matrix made up of two 5x2 matrices. The variable p is calculated to find out which area of the matrix responses, blockResponse should be written to - it is your index variable.

responses(p:p+4,:)

is just a 5x4 area of the matrix responses that is being written to on each loop. (The colon operator is used to index for all columns - can also be used for rows).

Lets look at the first two iterations:

p = ((i-1)*numTrials) + 1;
%%Loop 1
%  numTrials = 5; (fixed in this case)
%  i         = 1;
%  p         = ((1-1)*5) + 1
%            = 1;
%  p+4       = 5;
%  responses(1:5,:) = blockResponse;

%%Loop 2
%  i   = 2;
%  p   = ((2-1)*5) + 1
%      = 6;
%  p+4 = 10;
%  responses(6:10,:) = blockResponse;

and so on.

However, upon thinking about it, it would be more appropriate to index as such:

p_end                = p +(numTrials-1);
responses(p:p_end,:) = blockResponse;

This will account for different lengths of numTrials - which is intrinsically linked to the size of blockResponse .

I hope this is helpful in clearing things up?

Answer 2

[Iain]

Change:

 responsesDx= zeros(numTrials,2);
 responsesSx= zeros(numTrials,2);
 responses=zeros(numTrials*numBlocks,4);

 responsesDx(j,1)= key_pressed;
 responsesDx(j,2)= reaction_time;
 (and the like)

to

 responsesDx= zeros(numBlocks,numTrials,2);
 responsesSx= zeros(numBlocks,numTrials,2);
 responses=zeros(numBlocks,numTrials,4);

 responsesDx(i,j,1)= key_pressed;
 responsesDx(i,j,2)= reaction_time;
 (and the like)

[alice]

Thank you for your answer, but i would like have only one matrix with each condition rapresents a column and each trial a row, I need this because it is important for the eeg analysis that i will do after.

[Iain]

Ok, then, your answer is to either reshape (and or permute) the output of what I suggested into what you want, or you simply need to retune the indexing to include i & j, like (i-1)*maxj + j

[alice]

ok I got it , thank you for your time and for being so helpful