# How I can find index of element in array?

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Mykhailo Yaroshenko il 8 Nov 2017
Commentato: Jesse Ivers il 29 Giu 2023
I know, that number, for example, 5, is an element in array X, but I don't know it's index. In Python, I can use:
X.index(5)
I realized this function, using for loop and if statement, but did Matlab doesn't have build-in similar function?
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James Tursa il 8 Nov 2017
Modificato: James Tursa il 8 Nov 2017
If you know the number exactly, then you can use:
result = find(X==5);
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Ehsan Partovi il 2 Ott 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 il 29 Giu 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)
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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