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arraysmatlab
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Reputation: 2299

Reordering a vector in Matlab?

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)

Upvotes: 4

Views: 391

Answers (4)

Luis Mendo
Luis Mendo

Reputation: 112759

This requires just one sort and indexing:

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

Upvotes: 3

Santhan Salai
Santhan Salai

Reputation: 3898

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

Upvotes: 2

Divakar
Divakar

Reputation: 221674

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))))

Upvotes: 5

learnvst
learnvst

Reputation: 16193

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

Upvotes: 0

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