Finding Strings

Over the years, MATLAB has become a friendlier environment for working with character information. MATLAB has a rich set of text handling functions, ranging from the simple, to the all-powerful regexp functionality (covered here). I'm going to cover a few of the simple and very useful string functions today.

Contents

Use strfind

Use strfind instead of findstr or find for string searches.

             strfind('abc','a')

             findstr('abc','a')

This usage is a bit slower potentially and may cause confusion since there is no way to know which string was found in the other one.

             find('abc'=='a')

This usage is about 5 times slower than strfind, and is not robust, since it only works if one of the arguments to == is scalar.

      - Speed improvement, less memory (no temporary for results of logical statement inside find
      - No ambiguity on which string to index into later, if desired
      - Code is robust compared to using FIND which can't handle as general a case, nor is FIND as fast.

Use strrep

Use strrep instead of replacing values via indexing.

             str = strrep(str,' ','')

             ind = find(str==' '); str(ind) = []
             str(str==' ') = []

             str = strrep(str,'&','')

             menuLabelStr(find(menuLabelStr=='&')) = []

      - speed
      - readability
      - more general, i.e., replacement strings don't need to be the same
        size (or empty) as the strings they replace

Use strncmp

Use strncmp instead of strmatch with literal second input.

             strncmp(str,'string',length(str))

             strmatch(str,'string')

             strmatch(str,'string','exact')

      - speed

      - strmatch returns indices where the string is found, while strncmp
        returns true/false, so upgrading code requires more than just copy/paste.

Use strcmpi

Use strcmpi instead of using strcmp with upper or lower.

             strcmpi(str,'lcstring')

             strcmp(lower(str),'lcstring')

      - speed
           - fewer function calls
           - fewer temporary variables
      - readability

Use ismember

Use ismember to vectorize string finding operations.

             pets = {'cat';'dog';'dog';'dog';'giraffe';'hamster'}
             species = {'cat' 'dog'}
             [tf, loc] = ismember(pets, species)

             locs = zeros(length(pets),1);
             for k = 1:length(species)
                 tf =  strcmp(pets, species(k));
                 locs(tf) = k;
             end

      - speed

      - strfind works on cell arrays of strings and returns results
        in a cell array, with relevant indices.  It does partial matching.
      - ismember requires an exact match.  The outputs are different
        than strfind's, so coding is not just a matter of direct
        substitution.

Summary

I've talked about a few simple string functions available in MATLAB. Do you have some simple string recommendations for users? Post your ideas here.

Get the MATLAB code

Published with MATLAB® 7.3