Create a join on two cell-Arrays

Hi,

I've got two cell-arrays:

A = {'id1','H20';
     'idc','O2';
     'id3','CO2'};

B = {'idc';'id1';'id3';'id1';'id1'};

After running my script, I want B modified that it contains:

 B = {'idc','O2';
      'id1','H2O';
      'id3','CO2';
      'id1','H2O';
      'id1','H2O'};

Has anyone an idea avoiding loops? Thanks :)

Answer 1

[Walter Roberson]

[tf, aidx] = ismember(B, A(:,1));
B = [B, A(aidx,2)];

[Jan]

Or B=A(aidx, :);

Answer 2

[Vincent]

Works like a charm, thanks! This very useful option for ismember is way to hidden :-/

I'm working since two months now with Matlab and used a dirty workaround until now...

Answer 3

[Jan]

ISMEMBER, INTERSECT and SETDIFF are very powerful and optimized to work on huge data sets. But if you operate on cell strings with < 1000 elements, some simple loops are usually much faster:

A = {'id1','H20';
   'idc','O2';
   'id3','CO2'};
B = {'idc';'id1';'id3';'id1';'id1'};

function B = myFunc(A, B)
index = zeros(numel(B), 1);
for i = 1:size(A, 1)
  index(strcmp(B, A{i})) = i;  % A{i} == A{i,1}  
end
B = A(index, :);

This is 27 times faster than the ISMEMBER approach for your tiny dataset. Therefore I would not call such FOR loops "dirty workaround".

[EDITED] Using the index method instead of the former method to create the column cell directly is 40% faster.

Answer 4

[Vincent]

Nice, this leveled up my self-esteem :)

but the code takes much more time to be written and doesn't stay readable. And of course, I gave just a tiny example above; my datasets are usually betwenn 600x40 and 2000x40 entries big

But thank you anyway for this hint

[Jan]

This function vanishes inside a subfunction such that the actual program contains "B=myFunc(A,B)" only, which is easy to read.

I'd be very interested in a speed comparison of the two methods for the larger dataset. I modify my example to work for cells with 40 columns in a few minutes.

I've converted an equivalent function into a C-Mex: http://www.mathworks.com/matlabcentral/fileexchange/24380-cstrainbp