Extracting from vector to multidimensional array using indices provided

Question

Andris Cerins 2020-4-14

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https://ww2.mathworks.cn/matlabcentral/answers/517829-extracting-from-vector-to-multidimensional-array-using-indices-provided

评论： Andris Cerins 2020-4-18

Can this extraction be performed better without loops?

% data vector
data = [1 1 1 1 1 4 4 4 4 4 7 7 7 7 7 2 2 2 2 2 5 5 5 5 5 8 8 8 8 8 3 3 3 3 3 6 6 6 6 6 6 9 9 9 9 9]

% Desired output

data3d(:,:,1)

1 1 1 1 1

2 2 2 2 2

3 3 3 3 3

data3d(:,:,2)

4 4 4 4 4

5 5 5 5 5

6 6 6 6 6

data3d(:,:,3)

7 7 7 7 7

8 8 8 8 8

9 9 9 9 9

% indices for starts and ends of trials, rows are channels

dstart = [1 16 31; 6 21 36; 11 26 42]
dend = [5 20 35; 10 25 41; 15 30 46]

% note the annoying extra 6 in data

% can truncate now, dend_tr = dstart+4, or just use dstart:dstart+4

% or if target matrix pads then truncate later?

[numchannels,numsweeps] = size(dstart) %in case it is useful

% Here is how I think it would work using loops

% For all channels in that file
for ich = 1:numchannels 
    data_sweeps = [];
    % For all sweeps in that file
    for isw = 1:numsweeps
        sweepdata = [];
        % Get that sweep, truncate it to 5 samples (because some have extras) 
        sweepdata = data(dstart(ich,isw):dstart(ich,isw)+4);
        % Make sweeps x samples matrix     
        data_sweeps(isw,:) = sweepdata;    
        
    end
    % Adds each channel as a page in a 3d matrix
    data3d(:,:,ich) = data_sweeps
    
end

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Answer 1

Stephen23 2020-4-17

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编辑：Stephen23 2020-4-17

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In three lines (could be just one), no loops, and yes that extra 6 is taken into account!

>> D = [1,1,1,1,1,4,4,4,4,4,7,7,7,7,7,2,2,2,2,2,5,5,5,5,5,8,8,8,8,8,3,3,3,3,3,6,6,6,6,6,6,9,9,9,9,9];
>> S = [1,16,31;6,21,36;11,26,42];
>> E = [5,20,35;10,25,41;15,30,46];
>> F = @(s)reshape(s:s+4,[],1);
>> X = cell2mat(arrayfun(F,sort(S(:)),'uniformoutput',false));
>> A = permute(reshape(D(X),[5,3,3]),[3,1,2])
A(:,:,1) =
   1   1   1   1   1
   2   2   2   2   2
   3   3   3   3   3
A(:,:,2) =
   4   4   4   4   4
   5   5   5   5   5
   6   6   6   6   6
A(:,:,3) =
   7   7   7   7   7
   8   8   8   8   8
   9   9   9   9   9

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Andris Cerins 2020-4-18

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Thank you Stephen, much appreciated!

Tested as below on a part of my actual data where data is 1x7005392 and datastart is a 4x350

Your code is 5x faster than my loop

trunc = 5000 % where I choose the truncation length
[numchannels, numsweeps] = size(datastart);
F = @(s)reshape(s:s+(trunc-1),[],1);
X = cell2mat(arrayfun(F,sort(datastart(:)),'uniformoutput',false));
data3d = permute(reshape(data(X),[trunc,numchannels,numsweeps]),[3,1,2]);

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Answer 2

Guru Mohanty 2020-4-17

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I understand you are trying to extract data from a vector to multidimensional array without for loops.

It can be possible using MATLAB Array indexing. Here is a sample code for it.

% data vector
tic;
data = [1 1 1 1 1 ...
        4 4 4 4 4 ...
        7 7 7 7 7 ...
        2 2 2 2 2 ...
        5 5 5 5 5 ...
        8 8 8 8 8 ...
        3 3 3 3 3 ...
        6 6 6 6 6 ...
        9 9 9 9 9];
data=reshape(data,5,9);
dstart = [1 4 7;
          2 5 8;
          3 6 9];
data3d(:,:,1) = [data(:,dstart(1,1))';data(:,dstart(1,2))';data(:,dstart(1,3))'];
data3d(:,:,2) = [data(:,dstart(2,1))';data(:,dstart(2,2))';data(:,dstart(2,3))'];
data3d(:,:,3) = [data(:,dstart(3,1))';data(:,dstart(3,2))';data(:,dstart(3,3))'];
toc;

Using MATLAB Indexing instead of for loop will improve your Code performance. You can also check Execution time using tic & toc command.

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Stephen23 2020-4-17

编辑：Stephen23 2020-4-17

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@Guru Mohanty: your data vector is missing a 6 (as the original question points out, there are more of them). So your data vector should actually be defined as:

data = [1 1 1 1 1 ...
        4 4 4 4 4 ...
        7 7 7 7 7 ...
        2 2 2 2 2 ...
        5 5 5 5 5 ...
        8 8 8 8 8 ...
        3 3 3 3 3 ...
        6 6 6 6 6 6 ... <- "note the annoying extra 6 in data"
        9 9 9 9 9];