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index of an array multiple same element

Asked by Arkanra Kadhum on 25 Feb 2019
Latest activity Answered by Stephen Cobeldick on 26 Feb 2019
How can find the index of an array element if it have element have the same value for example a=[7 8 8 2 5 6 6 2 6 8] what is the index of third 8 and 6. I used find(A==8,1, firs) this gave the first one only, and how can I know the element have more than one value.

  2 Comments

what’s your expected output?
the index of 7 is 1
first 8 is 2, second 8 is 3, third 8 is 10
first 6 is 6, second 6 is 7, third 8 is 9
i

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3 Answers

Answer by Stephen Cobeldick on 26 Feb 2019

>> a = [7,8,8,2,5,6,6,2,6,8];
>> U = unique(a) % the order is the same as C.
U =
2 5 6 7 8
>> [R,C] = find(U(:)==a);
>> C = accumarray(R,C,[],@(v){v});
>> C{:} % same order as U.
ans =
4
8
ans =
5
ans =
6
7
9
ans =
1
ans =
2
3
10

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Answer by Andrei Bobrov
on 25 Feb 2019
Edited by Andrei Bobrov
on 26 Feb 2019

[a1,~,ii] = unique(a,'stable');
out = [num2cell(a1(:)),accumarray(ii,(1:numel(ii))',[],@(x){sort(x)})];

  2 Comments

Hi Andrei
I try this code
a1=7; a2=8; a3=8; a4=2; a5=5 ;a6=6; a7=6; a8=2;a9=6; a10=8;
A=[ a1, a2, a3, a5, a4, a6,a7,a8,a9,a10];
[a1,~,ii] = unique(A,'stable');
out = [num2cell,a1(:)accumarray(ii,(1:numel(ii))',[],@(x){x})];
[a2,~,ii] = unique(A,'stable');
out = [num2cell,a2(:)accumarray(ii,(1:numel(ii))',[],@(x){x})];
I gate this error
out = [num2cell,a1(:)accumarray(ii,(1:numel(ii))',[],@(x){x})];
Error: Unexpected MATLAB expression.
I'm fixed my typo in answer:
[a1,~,ii] = unique(a,'stable');
out = [num2cell(a1(:)),accumarray(ii,(1:numel(ii))',[],@(x){sort(x)})];

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Answer by madhan ravi
on 25 Feb 2019
Edited by madhan ravi
on 25 Feb 2019

u=unique(a);
C=arrayfun(@(x) find(a==u(x)),1:numel(u),'un',0);
%celldisp(C)

  1 Comment

If you want the order to be preserved then use 'stable' as an option in unique.

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