Find the indices of numbers that occur first

X is an array. Find(X, n, 1) gives the index of the first "n" that occurs. Now what of instead of "n" I have a vector like V = [n l m o ...]? The elements of V are unique.

[Steven Lord]

Find(X, n, 1) gives the index of the first "n" that occurs.

No, it doesn't.

help find

 find - Find indices and values of nonzero elements
    This MATLAB function returns a vector containing the linear indices of
    each nonzero element in array X.

    Syntax
      k = find(X)
      k = find(X,n)
      k = find(X,n,direction)

      [row,col] = find(___)
      [row,col,v] = find(___)

    Input Arguments
      X - Input array
        scalar | vector | matrix | multidimensional array
      n - Number of nonzeros to find
        positive integer scalar
      direction - Search direction
        'first' (default) | 'last'

    Output Arguments
      k - Indices to nonzero elements
        vector
      row - Row subscripts
        vector
      col - Column subscripts
        vector
      v - Nonzero elements of X
        vector

    Examples
      openExample('matlab/ZeroAndNonzeroElementsInMatrixExample')
      openExample('matlab/ElementsSatisfyingAConditionExample')
      openExample('matlab/ElementsEqualToSpecificValuesExample')
      openExample('matlab/LastSeveralNonzeroElementsExample')
      openExample('matlab/ElementsSatisfyingMultipleConditionsExample')
      openExample('matlab/SubscriptsAndValuesForNonzeroElementsExample')
      openExample('matlab/SubscriptsOfMultidimensionalArrayExample')

    See also ind2sub, nonzeros, strfind, sub2ind, Short-Circuit AND,
      Short-Circuit OR, ismember

    Introduced in MATLAB before R2006a
    Documentation for find
       doc find

    Other uses of find

       cgddnode/find
       cgprojconnections/find
       coder.asap2.getEcuDescriptions/find
       coder.dictionary.Section/find
       coder.mapping.api.CodeMapping/find
       coder.mapping.api.CodeMappingCPP/find
       codistributed/find
       coninputfactor/find
       database.mongo.connection/find
       gpuArray/find
       linearize.advisor.LinearizationAdvisor/find
       mlreportgen.finder.AxesFinder/find
       mlreportgen.ppt.Presentation/find
       mlreportgen.ppt.Slide/find
       sdo.SimulationTest/find
       Simulink.data.dictionary.Entry/find
       Simulink.data.dictionary.Section/find
       Simulink.ScenarioSimulation/find
       Simulink.SimulationData.Dataset/find
       Simulink.SimulationOutput/find
       slreportgen.finder.DataDictionaryFinder/find
       slreportgen.finder.EnumerationType/find
       slreportgen.finder.ModelVariableFinder/find
       slreportgen.finder.SignalFinder/find
       slreq.Justification/find
       slreq.LinkSet/find
       slreq.modeling/find
       slreq.Reference/find
       slreq.ReqSet/find
       slreq.Requirement/find
       slreq/find
       sltest.harness/find
       sltest.testmanager/find
       sltest.testsequence/find
       Stateflow.Chart/find
       sweepset/find
       sweepsetfilter/find
       tall/find

find(X, n) would find the first n non-zero elements in X. 1 is not a valid value for the direction input argument.

Now if n is an integer or something that you know for a fact is in X you could use find(X == n, 1). If you're using release R2024b and have an n that is in or "close to" a value in X you could use find(isapprox(X, n), 1).

x = 0:0.1:1;
y = x(randi(numel(x), 100, 1));
loc = find(isapprox(y, 0.3), 5) % find the first 5 values in y that are approximately 0.3

loc = 1×5

16 18 40 43 48

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y(loc)

ans = 1×5

0.3000 0.3000 0.3000 0.3000 0.3000

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Answer 1

[Walter Roberson]

[found, idxV] = ismember(X, V);

This will return the idx of elements of X within V; found(K) will be false if X(K) does not match any elements of V, and otherwise idxV(K) will be the index within V for X(K)

Closely related to this is

[found, idxX] = ismember(V, X);

which returns the index within X of the elements of V

[DGM]

e.g.

% inputs
v = [0 1 2];
x = randi([0 3],1,10)

x = 1×10

3 3 0 3 3 2 2 3 3 2

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[vinx,idx] = ismember(v,x)

vinx = 1x3 logical array
   1   0   1

idx = 1×3

3 0 6

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Answer 2

[Matt J]

You won't be able to do better than a for-loop over the V(i).

[Amir Mahmoudi]

Thanks. Actually, I wanted to avoid a for loop. So, I thought it would be possible to do the coding in a single line.

[Matt J]

You can do it in a single line, either with the code below or with your own wrapper function, but it won't execute faster than a for-loop:

out = arrayfun(@(n) find(X,n,1), V,'uni',0)

[Steven Lord]

Actually, I wanted to avoid a for loop.

Why? [If the answer is that you've been told "for loops are slow in MATLAB", tell whoever told you that that it is not really true anymore.]

Answer 3

[Star Strider]

That is not an appropriate find call. The syntax is incorrect.

X = randperm(9)                                 % Unique Random Vector

X = 1×9

1 6 9 4 3 7 2 8 5

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idx = find(X, 2, 1)

Error using find
Option must be of char type.

What do you want to do?

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