Vectorizing multiple string comparison

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Paolo Binetti
Paolo Binetti on 26 Jan 2017
Commented: Paolo Binetti on 28 Jan 2017
Is there a way to significantly speed up this loop, perhaps by vectorizing it? Inputs in attachment. I do not have a Matlab version with "string" functions.
d = a';
for i = 1:numel(a)
d{i} = c(strcmp(a{i}, b), :);
end
I tried working my way from the inner part with cellfun, but either I am not getting it right or it is not the good approach:
aux = cellfun(@strcmp, a, b); % does not work
  2 Comments
Paolo Binetti
Paolo Binetti on 27 Jan 2017
You are right. R2016 does not run on the PC I mostly use, and old beast which still works perfectly, but on XP. So until I buy a new computer, I am stuck with either a much older version of Matlab or Octave, which does run on XP. I could have generated the input with my older Matlab. And your answer below gives me one more motivation to buy a new computer soon!

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Accepted Answer

Guillaume
Guillaume on 26 Jan 2017
One obvious minor speed-up is to get rid of the find that serves absolutely no purpose. You can directly use the logical vector returned by strcmp:
d{i} = c(strcmp(a{i}, b)), :);
For some reason, I cannot load your mat file. I'm going to assume that a is a cell array of string, and so is b (otherwise the loop would not be needed). Assuming that there are no repeated strings in b:
assert(numel(unique(b)) == numel(b), 'This code does not work when there are duplicate values in b');
d = cell(size(a))';
[isfound, loc] = ismember(a, b);
d(isfound) = c(loc(isfound), :);
If it's guaranteed that all elements of a are found in b, then you can simplify even further to:
assert(numel(unique(b)) == numel(b), 'This code does not work when there are duplicate values in b');
[isfound, loc] = ismember(a, b);
assert(all(isfound), 'The next line only works if all elements of a are in b');
d = num2cell(c(loc, :), 2);
  2 Comments
Guillaume
Guillaume on 27 Jan 2017
According to Walter, your mat file is an octave file that matlab can't open.
If there are duplicate values in b, then you don't have a choice but to use a loop, either explicitly as you have done or with cellfun:
d = cellfun(@(aa) c(strcmp(aa, b), :), a, 'UniformOutput', false);
It's very possible that the cellfun may be slower than the explicit loop (due to the anonymous function call).
edit: in matlab R2016b there is a an extremely easy way to vectorise the string comparison, using the new string class:
string(a) == string(b)'
but you'd still need a loop or cellfun afterward to create the d cell array:
d = cellfun(@(r) c(r, :), num2cell(string(a) == string(b)', 1), 'UniformOutput', false)

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More Answers (1)

Walter Roberson
Walter Roberson on 27 Jan 2017
ismember can be used between cell arrays of strings. The two-output version can be used to find the indices, which you can then use to index into c.
  3 Comments
Paolo Binetti
Paolo Binetti on 28 Jan 2017
I had a feeling I was missing an obvious point. Thank you for pointing it out! The modified code, below, runs much faster. I tried to vectorize the remainder of the loop, to no avail, but the costly string comparison at least if out of the loop.
a = { 'AAG' 'AGA' 'ATT' 'CTA' 'CTC' 'GAT' 'TAA' 'TCT' 'TTC' };
b = { 'AAG' 'AGA' 'GAT' 'ATT' 'TTC' 'TCT' 'CTC' 'TCT' 'CTA' 'TAA' 'AAG' };
c = [ 'AGA';'GAT';'ATT';'TTC';'TCT';'CTC';'TCT';'CTA';'TAA';'AAG';'AGA' ];
[temp, idx] = ismember(b, a);
d = a';
for i = 1:numel(a)
d{i} = c(i == idx, :);
end

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