# Comparing lists of strings

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Neil on 17 Nov 2012
OK so I have two lists of, say, names, List_one and List_two.
- List_one is smaller than List_two: they are both cell arrays.
I want to find the indices where List_two contains elements from List_one. So far, I've been using strcmpi, but because the lists are different sizes (and in different orders) I need to do it element by element, which I can't believe is the most efficient solution. Any tips?
Just now I'm doing something like:
for i = 1:length(List_one) tf = strcmpi(List_one(i),List_two); ind = ind + tf; % so that the end result List_two(ind) = List_one. end
and I just can't imagine that's the best way, though I've spent a long time reading about ways to compare lists, I still haven't found anything satisfactory. One more, related, question:
If I have a list of names, and someone permutes it (they don't give me the indices for the new ordering, only the new list) - what is the best way to uncover the indices?
As in, List_two(indices) = List_two_permuted; - how do I find the permutation?

Jan on 18 Nov 2012
Edited: Jan on 18 Nov 2012
Both tasks can be achieved by the fast C-Mex FEX: CStrAinBP:
List1 = {'A', 'b', 'cd', 'eFG', 'Miss'};
List2 = {'b', 'A', 'eFG', 'cd', 'Q', 'A'};
[Ex, Seq] = CStrAinBP(List1, List2);
% >> Ex = 1 2 3 4
% >> Seq = 2 1 4 3
% Now: isequal(List1(Ex), List2(Seq))
Repeated strings are considered. The 3rd input "i" triggers a case-insensitive comparison.
Speed (measured with R2009a, 184 folder names, Core2Duo 2.3 GHz):
List1 = regexp(path, pathsep, 'split');
List2 = List1(randperm(length(List1)));
tic; for k=1:1000, [Tf, Loc] = ismember(List1, List2); end; toc
% >> 1.747856 sec
tic; for k=1:1000, [Ex, Seq] = CStrAinBP(List1, List2); end; toc
% >> 0.071419 sec
For large strings lists, e.g. 100'000 strings, the sorting of ismember has benefits, because the binary search is cheaper than the linear search of CStrAinBP.

Azzi Abdelmalek on 17 Nov 2012
Edited: Azzi Abdelmalek on 17 Nov 2012
List_one='is smaller than'
List_two='they are both cell arrays'
idx=find(ismember(List_one,List_two))

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