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Maddy
Maddy

Reputation: 2570

Sort a matrix based on the order given in a second matrix (MATLAB)

I have a bigmatrix that is sorted. However, I need to re-order it as per the order of IDs in another matrix (col. 1 here for both matrices). How can I do this using vectorization? Eg:

bigmat = [ ...
            1 10 ; 
            1 30 ; 
            1 40 ; 
            2 1  ; 
            2 11 ; 
            3 58 ; 
            4 2  ;
            4 5 ] ;

ordermat = [ 2 ; 1 ; 4 ; 3 ; 6] ;       % Integer IDs

finalans = [ ...
            2 1  ; 
            2 11 ; 
            1 10 ; 
            1 30 ; 
            1 40 ;
            4 2  ;
            4 5  ;
            3 58 ; ] ;

It is possible that all some IDs (integers here) in ordermat may not be present in bigmat. They can be ignored as shown above with id = 6. Thanks!

Upvotes: 6

Views: 2253

Answers (3)

nibot
nibot

Reputation: 14928

%# Input values:
bigmat = [1 10; 1 30; 1 40; 2 1; 2 11; 3 58;  4 2; 4 5];
ordermat = [ 2 ; 1 ; 4 ; 3 ; 6] ;   

%# Make a look-up table that tells us the relative order for each order id
sortmat(ordermat) = 1:length(ordermat);

%# Extract the order ID's from the big matrix
keys = bigmat(:,1);

%# Get the new ordering
new_key = sortmat(keys);

%# Sort the new ordering, remembering the permutation necessary
[~, permutation] = sort(new_key);

%# Apply the permutation to the big matrix
finalans = bigmat(permutation, :);

Upvotes: 1

Amro
Amro

Reputation: 124563

Here is my solution:

ordermat = [2; 1; 4; 3; 6];
bigmat = [
    1 10
    1 30
    1 40
    2 1
    2 11
    3 58
    4 2
    4 5
];
%#bigmat = sortrows(bigmat,1);

%# keep valid IDs, 
ord = ordermat( ismember(ordermat,bigmat(:,1)) );
ord = grp2idx(ord);

%# starting/ending locations of the different IDs in bigmat
startInd = find( diff([0;bigmat(:,1)]) );
endInd = [startInd(2:end)-1; size(bigmat,1)];

%# generate startInd(i):endInd(i) intervals
ind = arrayfun(@colon, startInd, endInd, 'UniformOutput',false);

%# order then combine the intervals of indices
ind = [ind{ord}];

%# get final sorted result
finalans = bigmat(ind,:);

I made sure it handles different cases like:

  • ordermat contains IDs not found in bigmat: ordermat = [2;1;4;3;6]
  • not all IDs of bigmat are represented in ordermat: ordermat = [2;1]
  • IDs not sequential and/or not starting at 1: ordermat=ordermat+10; bigmat=bigmat+10;

Upvotes: 2

Jonas
Jonas

Reputation: 74940

I'd split bigmat into chunks according to the unique identifiers, and then reorder the chunks, like this:

%# this assumes bigmat is sorted, btw
%# i.e. that the different ids are grouped together

%# find unique ids
dd = [true;diff(bigmat(:,1))~=0];

uniqueIDs = bigmat(dd,1);

%# reorder uniqueIDs
[~,newOrder] = ismember(uniqueIDs,ordermat);

%# if there are uniqueIDs that are not in ordermat
%# we'd need to remove those. I assume this won't
%# be the case

%# chop up bigmat
numUniqueIds = diff([find(dd);length(dd)+1]);
bigChunks = mat2cell(bigmat,numUniqueIds,size(bigmat,2));

%# reorder chunks
finalans = cat(1,bigChunks{newOrder});

Upvotes: 0

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