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Finding equal members in a vector

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mr mo
mr mo on 17 Dec 2017
Commented: Star Strider on 17 Dec 2017
Hi. My question is how can I find equal members in a vector with their indices.
For example I have this vector.
V = [ 10 13 10 20 10 95 70 13];
Thanks a lot.

  1 Comment

Image Analyst
Image Analyst on 17 Dec 2017
This has nothing to do with the MATLAB compiler product or the Coder product, or a GUI or 3-D plots, or nearly all the tags you applied. Save yourself some time and only put the relevant tags that will help people.

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Answers (4)

jean claude
jean claude on 17 Dec 2017
find(x==10)

  2 Comments

mr mo
mr mo on 17 Dec 2017
Assume that I don't know the values of the vector.
jean claude
jean claude on 17 Dec 2017
[a] = histc(x,unique(x));
t=unique(x);
c=find(a>1);
d=0;
for i=1:length(c);
d=[d find(x==t(c(i)))];
end
d(1)=[]; % d is the output

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YT
YT on 17 Dec 2017
Edited: YT on 17 Dec 2017
Found your answer in this post

  2 Comments

mr mo
mr mo on 17 Dec 2017
In my question I want to have the below form of output
1 3 5
2 8
that indicates
V(1) = V(3) = V(5)
and
V(2) = V(8)
YT
YT on 17 Dec 2017
Something like this then?
clear C
V = [ 10 13 10 20 10 95 70 13];
unqV = unique(V);
for i = 1:length(unqV);
C{i,1} = unqV(i)
C{i,2} = find(V==unqV(i));
end
%C = {10,[1 3 5];
% 13,[2 8];
% 20,4;
% 70,7;
% 95,6}

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Star Strider
Star Strider on 17 Dec 2017
If you only want the repeated values in the vector and their indices:
V = [ 10 13 10 20 10 95 70 13]; % Original Vector
[Vu,~,I] = unique(V(:), 'stable'); % Unique Values
Tally = accumarray(I, 1); % Count Occurrences
Idx = bsxfun(@eq, V, Vu(Tally > 1)) .* (1:length(V)); % Determine Indices Of Multiple Values Only
OutD = [Vu(Tally > 1) Idx] % Double Matrix Result
for k1 = 1:size(Idx,1)
IdxC{k1} = Idx(k1,(Idx(k1,:)>0)); % Keep Only Non-Zero Values
OutC{k1} = {OutD(k1,1) IdxC{k1}}; % Cell Array Result
end
celldisp(OutC) % View Cell Array (Optional)
OutD =
10 1 0 3 0 5 0 0 0
13 0 2 0 0 0 0 0 8
The ‘OutC’ cell array result eliminates the zeros in the ‘(2:end)’ columns of ‘OutD’.

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Star Strider
Star Strider on 17 Dec 2017
You did not say the line that threw that error.
My code ran for me without error in R2017b, with the default ‘automatic expansion’.
Use these nested bsxfun calls instead:
Idx = bsxfun(@times, bsxfun(@eq, V, Vu(Tally > 1)), (1:length(V)));
mr mo
mr mo on 17 Dec 2017
Is there any possibility to use other command except bsxfun and accumarray ?
Star Strider
Star Strider on 17 Dec 2017
Not that I am aware of. It is necessary to accumarray (or one of the histogram functions) to count the occurrences. Using bsxfun is necessary to do the array calculations.

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Jos (10584)
Jos (10584) on 17 Dec 2017
V = [ 10 13 10 20 10 95 70 13];
[VU, ~, j] = unique(V)
P = accumarray(j, 1:numel(V), [] ,@(x) {x})
% P{k} holds all the indices where V equals VU(k)

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