Reordering a vector in Matlab?

Question

I have a vector in Matlab B of dimension nx1 that contains the integers from 1 to n in a certain order, e.g. n=6 B=(2;4;5;1;6;3).

I have a vector A of dimension mx1 with m>1 that contains the same integers in ascending order each one repeated an arbitrary number of times, e.g. m=13 A=(1;1;1;2;3;3;3;4;5;5;5;5;6).

I want to get C of dimension mx1 in which the integers in A are reordered following the order in B. In the example, C=(2;4;5;5;5;5;1;1;1;6;3;3;3)

Divakar · Accepted Answer · 2015-04-23T08:53:26.187

5

One approach with ismember and sort -

[~,idx] = ismember(A,B)
[~,sorted_idx] = sort(idx)
C = B(idx(sorted_idx))

If you are into one-liners, then another with bsxfun -

C = B(nonzeros(bsxfun(@times,bsxfun(@eq,A,B.'),1:numel(B))))

edited Apr 23 '15 at 08:53

answered Apr 23 '15 at 08:12

Divakar

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score 3 · Answer 2 · answered Apr 23 '15 at 10:18

3

This requires just one sort and indexing:

ind = 1:numel(B);
ind(B) = ind;
C = B(sort(ind(A)));

answered Apr 23 '15 at 10:18

Luis Mendo

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score 2 · Answer 3 · edited May 23 '17 at 10:24

2

Another approach using repelem, accumarray, unique

B=[2;4;5;1;6;3];
A=[1;1;1;2;3;3;3;4;5;5;5;5;6];

counts = accumarray(A,A)./unique(A);
repelem(B,counts(B));

%// or as suggested by Divakar 
%// counts = accumarray(A,1);
%// repelem(B,counts(B));

PS: repelem was introduced in R2015a. If you are using a prior version, refer here

edited May 23 '17 at 10:24

Community

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answered Apr 23 '15 at 08:45

Santhan Salai

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In case OP doesn't have access to 2015a for `repelem`, link it to [this one](http://stackoverflow.com/q/1975772/3293881)? – Divakar Apr 23 '15 at 08:58
@Divakar y not? even i am using R2014a, i didn't test the last line LOL. – Santhan Salai Apr 23 '15 at 09:02
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Think, you can do it in two steps: `counts = accumarray(A,1); C= repelem(B,counts(B))`. – Divakar Apr 23 '15 at 09:13
@Divakar, Thanks.. just noticed both `i` and `B` were same *face palm* – Santhan Salai Apr 23 '15 at 09:21

score 0 · Answer 4 · answered Apr 23 '15 at 08:23

0

Another solution using hist, but with a loop and expanding memory :(

y = hist(A, max(A))
reps = y(B);
C = [];
for nn = 1:numel(reps)
    C = [C; repmat(B(nn), reps(nn), 1)];
end

answered Apr 23 '15 at 08:23

learnvst

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Reordering a vector in Matlab?

4 Answers4