string startWith in Matlab

Question

I would like to get only the begining of string , is there an equivalent matlab that allows to say : startsWith('It-is') like in java?

Thanks

Kassab

Answer 1

You can use the strfind function to tell if one string starts with another. The function returns the starting index of each occurrence of the string you're looking for, or an empty array if the string is not found.

S = 'Find the starting indices of the pattern string';
strfind(S, 'It-is')

If the string started with 'It-is' then the first index of the array returned by strfind would be 1 (i.e. the index of the first character).

Demythologize

Answer 2

For long strings, it is faster to do this

s = (numel(a)>=numel(b)) && all(a(1:min(numel(a),numel(b)))==b(1:min(numel(a),numel(b))));

in order to have the equivalent to a.startsWith(b).

Florettaflorette

Answer 3

The option that works best for me is:

~isempty(regexp(s, '^It-is', 'once')) 

The ~isempty allows you to use the expression with logical ORs or ANDs like this:

if ~isempty(regexp(s, '^It-is', 'once')) || ~isempty(regexp(s, '^It-was', 'once')) 

The 'once' parameter is an optimization to ensure you don't keep scanning the string once you have found the match at the beginning.

Byerly

Answer 4

The trouble with strfind is that it returns a non-scalar result, which limits where you can use it. More straightfoward would be to use regexp like so,

s = 'It-is true.';
if regexp(s, '^It-is')
  disp('s starts with "It-is"')
end

Bisulfate

I do not agree with the comment--you can always wrap strfind in an isempty to get a boolean. Also good to keep in mind that the MATLAB implementation of regexp is much much slower than strfind. – Cambist

@mbroshi, no you can't. isempty(strfind(s, regexp) is always true, because strfind returns {[]} if it haven't found a match. – Profit

Downvoted because if regexp(s, '^It-is') gives a error: > "Conversion to logical from cell is not possible" – Profit

I'm not sure why you say it gives an error. I just confirmed this in the latest MATLAB and and it output as expected. >> s = 'It-is true.'; if regexp(s, '^It-is') disp('s starts with "It-is"') end outputs: s starts with "It-is" In the example s is not a cell. Can you provide the context in which you encountered the error? – Bisulfate

Answer 5

I want to add that if s is a cell then regexp and strfind return a cell array.
You can use one of two variants in this case:

pos = strfind(s, 'It-iss');
if (~isempty(pos{1,1}))
    disp('s starts with "It-is"')
end

or

pos = regexp(s, '^It-is');
if (~isempty(pos{1,1}))
    disp('s starts with "It-is"')
end

You can't directly cast the return value of regexp or strfind to bool, because they return If there are no matches, regexp and strfind return a cell array with empty cell {[]}. To access the first cell you need to us ecurled braces operator pos{1,1}.

Profit

Answer 6

Starting in Matlab2016b, there is a startsWith function:

startsWith(your_string, 'It-is')

In previous versions, you can use Bill the Lizard's answer to make your own function:

matches = strfind(your_string, 'It-is');
string_starts_with_pattern = ~isempty(matches) && matches(1) == 1;

Brighton