Create a join on two cell-Arrays

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Vincent
Vincent 2011-9-22
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 :)

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采纳的回答

Walter Roberson
Walter Roberson 2011-9-22
[tf, aidx] = ismember(B, A(:,1));
B = [B, A(aidx,2)];

更多回答（3 个）

Vincent
Vincent 2011-9-22
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...
Jan
Jan 2011-9-22
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.
Vincent
Vincent 2011-9-22
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
  1 个评论
Jan
Jan 2011-9-22
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

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