Having troubles with resizing matrices in 'for loop' and index search.

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Madina Makhmutova 2018-4-1

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评论： Madina Makhmutova 2018-4-1

I have a logical array of 0s an 1s. I would like to extract indices of all the cells with the value of 1, but without loosing information on columns. This gives a single column of indices.

idx = find(logical_array)

I tried for loop, but I'm not sure how to define my array before the for loop. Since the number of values varies between the columns, it generates a dimension mismatch error.

data = logical(randi(2,[10 3])-1);
idx = zeros(x,y);
for i = 1:y
    r_idx(:,i) = find(data(:,i));
end

It works when I generate separate vectors and concatenate them with padcat function. But is there an easier way?

   data = logical(randi(2,[10 3])-1);
    idx1 = find(data(:,1));
    idx2 = find(data(:,2));
    idx3 = find(data(:,3));
    idx = padcat(idx1, idx2, idx3);

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Answer 1

Walter Roberson 2018-4-1

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[r, c] = find(logical_array)

This gives vectors of row and column indices.

I cannot tell what output you are hoping for.

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Madina Makhmutova 2018-4-1

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Dear Walter and David,

I do have a follow up question. Lets assume we do the following:

v = randi(50,100,10);   
logical_array = logical(randi(2,[10 3])-1);
    [r, c] = find(logical_array)

Now I have 2 vectors. Vector (c) contains information on the categories 1:3 and vector (r) contains indices of the vectors (v) that fall within the category (c). I need to generate a 3D array, where (1D = r), (2D = v), (3D = c). where numbers of (v) and (c) are constant while number of (r) varies for each (c). How can I do that? 1)generate a vector of all the r-values that have matching c-values (this is what I have been trying to do initially with my logical array); 2)assign corresponding v-vectors to the r-values. This will generate a 2D array with r-columns and 100 rows; 3)Add a 3rd dimension of c-values (1:3 in this case). This will generate a 3D array with c-sheets.

I recognize that perhaps there are way to many questions combined here. But I'm just starting to learn Matlab while trying to complete this analysis for the deadline. I would greatly appreciate if you can help me or refer me to the source where I can get help.

Walter Roberson 2018-4-1

在 MATLAB Online 中打开

I think you will find you do not need to do that when you use accumarray. But if you insist:

splits = splitapply(@(V) {V}, r, c );

Madina Makhmutova 2018-4-1

Dear David and Walter, thank you so much for your help! David, you are right. As I started thinking about it more, I do not need to put it back together (i'm still getting used to the "code" type of thinking:-). Here is the whole idea briefly. I have multiple traces of (r) over time, and I apply several conditions (c) at certain time points. I calculate AUC following each condition for each trace and generate a logical array, which tells me which traces changed at which condition. At the end I want to make a box plot of AUCs for the responding traces only. So I guess I just need to extract the indices from the logical array and assign them an AUC value that was calculated before. I do not need to go back to the original trace and recalculate it again. Anyway, just by typing it out, I understand it better. Thank you very much for your help!

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