How can I find index of element in array?

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Mykhailo Yaroshenko 2017-11-8

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编辑： MathWorks Support Team 2024-6-5

I know that I have a number 5 as an element in array X, but I do not know its index. Does MATLAB have a built-in function similar to Python's "index" method for finding the index of an element in an array?

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James Tursa 2024-5-15

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编辑：MathWorks Support Team 2024-6-5

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To find the index of a specific integer value (without roundoff error) in an array of integers, use the "find" function and == operator. For example, find the index of an element equal to 5 in a 1-by-11 vector of integers.

x = 0:1:10; 
k = find(x==5) 

To find a numeric value in an array of floating-point numbers, use a tolerance value based on your data. Otherwise, the result is sometimes an empty matrix due to floating-point roundoff errors. For example, find the index of an element equal to 0.5 within a roundoff error of 1e-6.

y = 0:0.1:1; 
k = find(abs(y-0.5) < 1e-6) 

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Ehsan Partovi 2021-10-2

The function find() is useful as far as matrices (2-D tensors) are concerned. I cannot, however, find a useful function for nd-arrays where, for instance, the index could be an array on its own. See example below:

M = reshape(1:24, [2,3,4]);

indices = index_finder(M==20); % indices = vector of indices

It would be very useful if there was a function which worked for tensors of any dimensionality.

Jesse Ivers 2023-6-29

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@Ehsan Partovi I couldn't agree with you more; this is a problem I seem to run into often, and here is my solution:

% Example ND-array
arr = reshape([1:6000], [5 5 10 4 6]);
numberOfInterest = 99;
% Get the linear index of the 
linearIndex = find(arr==numberOfInterest);
% Convert linear index to subscript
[row, col, depth, channel, time] = ind2sub(size(arr), linearIndex)
row = 4
col = 5
depth = 4
channel = 1
time = 1

The only drawbacks are the reuirement that you know how many dimensions. YOu can get around this with CSLs like so:

% Use CSL to get all the outputs
[idicies{1:ndims(arr)}] = ind2sub(size(arr), linearIndex)
idicies = 1×5 cell array
    {[4]}    {[5]}    {[4]}    {[1]}    {[1]}

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