Find the position of numbers in array

Question

Marco Piazza 2022-9-30

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https://ww2.mathworks.cn/matlabcentral/answers/1815020-find-the-position-of-numbers-in-array

编辑： Ergin Sezgin 2022-9-30

Hello everybody, I'm trying to find the position of some values in an array. The values i'm interested in are in a 1x182 vector and I want to find they're position in a 1x1820 vector (practically I'm just looking for a percentile). Can you guess how I can find them?

PS: i did it with a for loop using the find function but i think it is a non-efficient way, there must be an easier and fast way!

Thank a lot in advance,

Marco

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Answer 1

Walter Roberson 2022-9-30

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percent_present = mean(ismember(SmallVector, LargeVector)) * 100

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Answer 2

Star Strider 2022-9-30

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I am not certain what you are asking.

Perhaps using the ismember (or ismembertol) function will do what you want.

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Marco Piazza 2022-9-30

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Thnaks for answering, I'll try to explain it better with an example

a = [4 5 7 2 1 6 3 9 8]
b = [2 7 8]

Can MATLAB find the position of each numbers in vector b in vector a? So the answer I need is

ans = [4 3 9]

Star Strider 2022-9-30

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Getting them is straightforward. Getting them in that exact order requires a bit of coding gymnastics —

a = [4 5 7 2 1 6 3 9 8];
b = [2 7 8];
[~,idx] = ismember(a, b);
[idx_sorted,sortidx] = sort(idx(idx>0));
idx2 = find(idx);
Result = idx2(sortidx)
Result = 1×3
     4     3     9

.

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Answer 3

Ergin Sezgin 2022-9-30

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编辑：Ergin Sezgin 2022-9-30

在 MATLAB Online 中打开

Hello Marco,

Following code might help with your task:

rng(2022)
inputArray = 100*rand(1,1820); % you should use your array
selectedElems = randperm(1820,182); % random indexing for subset, 
yourVector = inputArray(selectedElems); % you should use your vector instead
elemPositions = zeros(size(yourVector)); % result vector for determined positions
for i=1:size(yourVector,2)
    elemPositions(1,i) = find(inputArray==yourVector(1,i)); 
end
isequal(selectedElems, elemPositions) % Compare real and determined positions
ans = logical
   1