Delete items from array and split it up into new arrays

Question

Bjælle 2022-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/1848043-delete-items-from-array-and-split-it-up-into-new-arrays

评论： Image Analyst 2022-11-10

Hi,

I have a long array consisting of a long section of negative values, then a long section of positive values, then a long section of negative values and so on. I would like to remove all the negative values from my array, but at the same time split up the array into the individual long sections of positive values.

Simplified example:

array = [-0.0016 -0.0017 0.1746 0.1834 -0.0017 -0.0018 0.1567 0.1456 0.1744 -0.0018 -0.0017 0.1243 0.1456]

And then I would like this:

answer = {0.1746 0.1834} {0.1567 0.1456 0.1744} {0.1243 0.1456}

So the negative values has been removed, and the array is split up into new arrays containing the positive values from each section.

I hope it makes sense!

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Answer 1

Image Analyst 2022-11-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1848043-delete-items-from-array-and-split-it-up-into-new-arrays#answer_1096403

在 MATLAB Online 中打开

You can easily do it if you have the Image Processing Toolbox.

array = [-0.0016 -0.0017 0.1746 0.1834 -0.0017 -0.0018 0.1567 0.1456 0.1744 -0.0018 -0.0017 0.1243 0.1456];
% Identify positive indexes
mask = array >= 0;
% Measure values at those indexes.
positiveRegions = regionprops(mask, array, 'PixelValues');
% Identify negative indexes
mask = array < 0;
% Measure values at those indexes.
negativeRegions = regionprops(mask, array, 'PixelValues');

positiveRegions and negativeRegions are structure arrays (rather than cell arrays). To extract the numbers for a particular region, say the 2nd region you can do

values = positiveRegions(2).PixelValues
values = 1×3
    0.1567    0.1456    0.1744

You can also get them in a table if you want:

array = [-0.0016 -0.0017 0.1746 0.1834 -0.0017 -0.0018 0.1567 0.1456 0.1744 -0.0018 -0.0017 0.1243 0.1456];
% Identify positive indexes
mask = array >= 0;
% Measure values at those indexes.
positiveRegions = regionprops('table', mask, array, 'PixelValues')
positiveRegions = 3×1 table
          PixelValues       
    ________________________

    {[       0.1746 0.1834]}
    {[0.1567 0.1456 0.1744]}
    {[       0.1243 0.1456]}
% Identify negative indexes
mask = array < 0;
% Measure values at those indexes.
negativeRegions = regionprops('table', mask, array, 'PixelValues')
negativeRegions = 3×1 table
       PixelValues    
    __________________

    -0.0016    -0.0017
    -0.0017    -0.0018
    -0.0018    -0.0017

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Bjælle 2022-11-10

That's perfect, thank you!

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Answer 2

Voss 2022-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/1848043-delete-items-from-array-and-split-it-up-into-new-arrays#answer_1096413

在 MATLAB Online 中打开

array = [-0.0016 -0.0017 0.1746 0.1834 -0.0017 -0.0018 0.1567 0.1456 0.1744 -0.0018 -0.0017 0.1243 0.1456];
is_negative = array < 0
is_negative = 1×13 logical array
   1   1   0   0   1   1   0   0   0   1   1   0   0
start_idx =  strfind([true is_negative],[true false])
start_idx = 1×3
     3     7    12
end_idx = strfind([is_negative true],[false true])
end_idx = 1×3
     4     9    13
answer = arrayfun(@(s,e)array(s:e),start_idx,end_idx,'UniformOutput',false)
answer = 1×3 cell array
    {[0.1746 0.1834]}    {[0.1567 0.1456 0.1744]}    {[0.1243 0.1456]}

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Bjælle 2022-11-10

Thank you so much!

Voss 2022-11-10

You're welcome!

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Answer 3

AH 2022-11-10

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1848043-delete-items-from-array-and-split-it-up-into-new-arrays#answer_1096423

在 MATLAB Online 中打开

You may find the code below doing the desired task

array = [-0.0016 -0.0017 0.1746 0.1834 -0.0017 -0.0018 0.1567 0.1456 0.1744 -0.0018 -0.0017 0.1243 0.1456]'
array = 13×1
   -0.0016
   -0.0017
    0.1746
    0.1834
   -0.0017
   -0.0018
    0.1567
    0.1456
    0.1744
   -0.0018
% find the mask where the array is non-negative
binMask = array >= 0;
% Find the start and end index of each region of interest
sidx = find(diff(binMask) == 1)+1
sidx = 3×1
     3
     7
    12
eidx = find(diff(binMask) == -1)
eidx = 2×1
     4
     9
if (binMask(1) == 1)
    sidx = [1;sidx];
end
if (binMask(end) == 1)    
    eidx = [eidx; length(array)];
end
% Split it into cells
ncell = min(length(sidx),length(eidx));
A = cell(ncell,1);
for i = 1:ncell
    A{i} = array(sidx(i):eidx(i));
end
A
A = 3×1 cell array
    {2×1 double}
    {3×1 double}
    {2×1 double}