How to find vector elements indices for the first and last locations of a specific number?

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Tarek 2021-4-20

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https://ww2.mathworks.cn/matlabcentral/answers/807632-how-to-find-vector-elements-indices-for-the-first-and-last-locations-of-a-specific-number

评论： Tarek 2021-4-20

If I have a vector

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A=[0;0;0;0;20;20;20;20;0;0;0;0;62;62;62;0;0;0;112;112;112;112;0;0]

I want to find the index of the first 20 and the index of the last 20.

Same for the index of the first 62 and last 62; first 112 and last 112, etc.

Is there a way to do this, without loops prefereably?

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Answer 1

David Hill 2021-4-20

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https://ww2.mathworks.cn/matlabcentral/answers/807632-how-to-find-vector-elements-indices-for-the-first-and-last-locations-of-a-specific-number#answer_679937

在 MATLAB Online 中打开

[~,idxfirst]=ismember(unique(A),A);
[~,idxlast]=ismember(unique(A),flip(A));
idxlast=length(A)+1-idxlast;

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Tarek 2021-4-20

very nice and simple. thank you!

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Answer 2

Image Analyst 2021-4-20

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https://ww2.mathworks.cn/matlabcentral/answers/807632-how-to-find-vector-elements-indices-for-the-first-and-last-locations-of-a-specific-number#answer_679932

编辑：Image Analyst 2021-4-20

在 MATLAB Online 中打开

If you have just those specific numbers, you can use find(). Otherwise you'd have to pass A into unique() and then use ismember(), like in David's answer. But to find just one number, you can use find():

first20 = find(A == 20, 1, 'first');
last20 = find(A == 20, 1, 'last');
first62 = find(A == 62, 1, 'first');
last62 = find(A == 62, 1, 'last');

etc.

FYI, don't worry about loops, they will be fast, especially for some array as microscopic as this. Here is how to find all of the first and last locations:

A=[0;0;0;0;20;20;20;20;0;0;0;0;62;62;62;0;0;0;112;112;112;112;0;0]
ua = unique(A)
numUniqueElements = length(ua)
firstLocations = zeros(numUniqueElements, 1);
lastLocations = zeros(numUniqueElements, 1);
tic
for k = 1 : numUniqueElements
	firstLocations(k) = find(A == ua(k), 1, 'first');
	lastLocations(k) = find(A == ua(k), 1, 'last');
end
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