How can I find index of element in array?
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Mykhailo Yaroshenko
on 8 Nov 2017
Edited: MathWorks Support Team
on 5 Jun 2024
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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Accepted Answer
James Tursa
on 15 May 2024
Edited: MathWorks Support Team
on 5 Jun 2024
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
on 2 Oct 2021
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
on 29 Jun 2023
@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)
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)
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