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.
- Preferred
strfind('abc','a')
- Not recommended
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.
- Not recommended
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.
- Benefits
- 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.
- Preferred (removing blanks from a string)
str = strrep(str,' ','')
- Not recommended
ind = find(str==' '); str(ind) = [] str(str==' ') = []
- Preferred (remove & from strings, e.g., menu accelerators)
str = strrep(str,'&','')
- Not recommended
menuLabelStr(find(menuLabelStr=='&')) = []
- Benefits
- 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.
- Preferred
strncmp(str,'string',length(str))
- Not recommended
strmatch(str,'string')
- Not recommended
strmatch(str,'string','exact')
- Benefits
- speed
- Note
- 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.
- Preferred
strcmpi(str,'lcstring')
- Not recommended
strcmp(lower(str),'lcstring')
- Benefits
- speed - fewer function calls - fewer temporary variables - readability
Use ismember
Use ismember to vectorize string finding operations.
- Preferred
pets = {'cat';'dog';'dog';'dog';'giraffe';'hamster'} species = {'cat' 'dog'} [tf, loc] = ismember(pets, species)
- Not recommended
locs = zeros(length(pets),1); for k = 1:length(species) tf = strcmp(pets, species(k)); locs(tf) = k; end
- Benefits
- speed
- Note
- 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.
- Category:
- Strings
Comments
To leave a comment, please click here to sign in to your MathWorks Account or create a new one.