Strcmp for cell arrays of unequal length in MATLAB

Question

Is there an easy way to find a smaller cell array of strings within a larger one? I've got two lists, one with unique elements, and one with repeating elements. I want to find whole occurrences of the specific pattern of the smaller array within the larger. I'm aware that strcmp will compare two cell arrays, but only if they're equal in length. My first thought was to step through subsets of the larger array using a loop, but there's got to be a better solution.

For example, in the following:

smallcellarray={'string1',...
                'string2',...
                'string3'};
largecellarray={'string1',...
                'string2',...
                'string3',...
                'string1',...
                'string2',...
                'string1',...
                'string2',...
                'string3'};

index=myfunction(largecellarray,smallcellarray)

would return

index=[1 1 1 0 0 1 1 1]

Gilman

Answer 1

You could actually use the function ISMEMBER to get an index vector for where the cells in largecellarray occur in the smaller array smallcellarray, then use the function STRFIND (which works for both strings and numeric arrays) to find the starting indices of the smaller array within the larger:

>> nSmall = numel(smallcellarray);
>> [~, matchIndex] = ismember(largecellarray,...  %# Find the index of the 
                                smallcellarray);    %#   smallcellarray entry
                                                    %#   that each entry of
                                                    %#   largecellarray matches
>> startIndices = strfind(matchIndex,1:nSmall)  %# Starting indices where the
                                                %#   vector [1 2 3] occurs in
startIndices =                                  %#   matchIndex

     1     6

Then it's a matter of building the vector index from these starting indices. Here's one way you could create this vector:

>> nLarge = numel(largecellarray);
>> endIndices = startIndices+nSmall;  %# Get the indices immediately after
                                      %#   where the vector [1 2 3] ends
>> index = zeros(1,nLarge);           %# Initialize index to zero
>> index(startIndices) = 1;           %# Mark the start index with a 1
>> index(endIndices) = -1;            %# Mark one index after the end with a -1
>> index = cumsum(index(1:nLarge))    %# Take the cumulative sum, removing any
                                      %#   extra entry in index that may occur
index =

     1     1     1     0     0     1     1     1

Another way to create it using the function BSXFUN is given by Amro. Yet another way to create it is:

index = cumsum([startIndices; ones(nSmall-1,numel(startIndices))]);
index = ismember(1:numel(largecellarray),index);

Prognostication

Won't this fail to produce the correct result if largecellarray is {'string3'} ? – Thetic

@Jonas: I get index = 0 for that case, using the newest version of my solution above. – Prognostication

Oh, now I understand your solution. Clever! +1 – Thetic

I used this code for a similar issue. MATLAB complained that strfind needed inputs with one row only, so I typed this instead startIndices = strfind(matchIndex',1:nSmall') – Leacock

Answer 2

Here's my version (based on the answers of both @yuk and @gnovice):

g = grp2idx([S L])';
idx = strfind(g(numel(S)+1:end),g(1:numel(S)));
idx = bsxfun(@plus,idx',0:numel(S)-1);

index = zeros(size(L));
index(idx(:)) = 1;

Emphasis

+1: Very nice, although 2 things bear mentioning: 1) You need the Statistics Toolbox to use GRP2IDX. 2) The function FINDSTR appears to be slated for obsolescence in favor of STRFIND. – Prognostication

@gnovice: Fixed findstr/strfind (note the order of arguments is important now), I didnt realize it was a deprecated function.. thanks – Emphasis

Answer 3

In @gnovice answer the first part can be

l = grp2idx(largecellarray)';
s = grp2idx(smallcellarray)';
startIndices = strfind(l,s);

Choi

I didn't know grp2idx. Nice! But wouldn't this fail if there was a string0 in largecellarray? – Thetic

Unfortunately, this only works if the N entries in smallcellarray are exactly the same as the first N entries in largecellarray. – Prognostication

Yes, it will actually fail in many cases, since for grp2idx the order is important. Probably the ismember function is important here. – Choi

if you pass them to grp2idx combined as one cellarray, it will solve the problem.. – Emphasis

Answer 4

I got the following solution working, but I'm still wondering if there's a better way to do this:

function [output]=cellstrcmpi(largecell,smallcell)
output=zeros(size(largecell));
idx=1;
while idx<=length(largecell)-length(smallcell)+1
    if sum(strcmpi(largecell(idx:idx+length(smallcell)-1),smallcell))==length(smallcell)
       output(idx:idx+length(smallcell)-1)=1;
       idx=idx+length(smallcell);       
    else
        idx=idx+1;
    end
end

(I know, I know, no error checking - I'm a horrible person.)

Gilman