Get the positions of the first logicals (multiple sets) in an array

Question

BobbyRoberts 2022-6-16

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1741890-get-the-positions-of-the-first-logicals-multiple-sets-in-an-array

评论： Voss 2022-6-16

Hi. I am trying to find where the value of an array increases to a particular threshold (thresh=5). For example, I started off with:

x = [0.5,1.6,1.5,5.2,8.5,9.3,6.3,2.2,1.9,1.8,7.4,6.4,6.5,2.3,2.4,8.5,7.6,5.3,2.0,2.4]

I converted this to a logical :

thresh = 5; 
x_log = x>thresh;

which gives me:

x_log = [0   0   0   1   1   1   1   0   0   0   1   1   1   0   0   1   1   1   0   0]

What I actually need is a logical that just has the first 1 of each group. So the output should be [0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 1 0 0 0 0] ... is there a function I don't know of that can do this? I can't find it. Or do you suggest another way to do this using the original array?

Thanks a lot in advance!

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BobbyRoberts 2022-6-16

x = [0.5,1.6,1.5,5.2,8.5,9.3,6.3,2.2,1.9,1.8,7.4,6.4,6.5,2.3,2.4,8.5,7.6,5.3,2.0,2.4]

BobbyRoberts 2022-6-16

if you use the original x array, sorry. Not the logical. I'm converting it to logical here using x_idx = x>5.

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Answer 1

Voss 2022-6-16

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1741890-get-the-positions-of-the-first-logicals-multiple-sets-in-an-array#answer_987095

在 MATLAB Online 中打开

x = [0.5,1.6,1.5,5.2,8.5,9.3,6.3,2.2,1.9,1.8,7.4,6.4,6.5,2.3,2.4,8.5,7.6,5.3,2.0,2.4];
thresh = 5; 
x_log = x>thresh
x_log = 1×20 logical array
   0   0   0   1   1   1   1   0   0   0   1   1   1   0   0   1   1   1   0   0

One way to get the indices of the first 1s:

idx = find(diff([false x_log]) == 1)
idx = 1×3
     4    11    16

Another way:

idx = strfind([false x_log],[false true])
idx = 1×3
     4    11    16

(In either case, a false/0 element is prepended to x_log to correctly handle the case that x_log starts with a true/1.)

Then, regardless of how you get idx, build x_new from idx:

x_new = false(1,numel(x));
x_new(idx) = true
x_new = 1×20 logical array
   0   0   0   1   0   0   0   0   0   0   1   0   0   0   0   1   0   0   0   0

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BobbyRoberts 2022-6-16

nice! thank you. very similar to what i did, but more logical (pun maybe intended).

Voss 2022-6-16

You're welcome!

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Answer 2

Image Analyst 2022-6-16

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1741890-get-the-positions-of-the-first-logicals-multiple-sets-in-an-array#answer_987085

在 MATLAB Online 中打开

Try this:

x = [0.5,1.6,1.5,5.2,8.5,9.3,6.3,2.2,1.9,1.8,7.4,6.4,6.5,2.3,2.4,8.5,7.6,5.3,2.0,2.4];
mask = x > 5
mask = 1×20 logical array
   0   0   0   1   1   1   1   0   0   0   1   1   1   0   0   1   1   1   0   0
props = regionprops(mask, 'PixelIdxList');
for k = 1 : length(props)
    startingIndexes(k) = props(k).PixelIdxList(1);
end
% Show in command window:
startingIndexes
startingIndexes = 1×3
     4    11    16