# Problem with the use of strrep command

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Ivan Mich el 6 de Mzo. de 2023
Comentada: Steven Lord el 7 de Mzo. de 2023
I have a problem with strrep command. I have noticed that this command no use for cell array with square bracket character ([])
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Askic V el 6 de Mzo. de 2023
Here is one example with cell array with square brackets and use of strrep function:
aa = {['first'],['second']}
aa = 1×2 cell array
{'first'} {'second'}
aa
aa = 1×2 cell array
{'first'} {'second'}
aa(1) = strrep(aa(1),'first','replacement');
aa
aa = 1×2 cell array
{'replacement'} {'second'}
Can you explain what problem could be here?
DGM el 6 de Mzo. de 2023
Editada: DGM el 6 de Mzo. de 2023
In that case, the square brackets have no influence over the processing.
aa = {['first'],['second']}
is the same as
aa = {'first','second'}
Since we're guessing here, other interpretations might include:
% an unintended concatenation
aa = {['first','second']}
... or
% the brackets are within the char vectors
aa = {'first [subexpr]','second [other subexpr]'}
Where either the brackets themselves are to be replaced (shouldn't be a problem), or the subexpressions therin are to be replaced (strrep would be the wrong tool).
Alternatively, my guess is that this might not even be a cellchar
% a cell of strings or string arrays might cause problems
aa = {"first" "second"}
aa = {["first" "second"]}
in which case the solution really depends on what's in the cell array, how it's arranged, and whether it should even be in a cell array.
But we'll have to wait.

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### Respuestas (1)

Steven Lord el 6 de Mzo. de 2023
That is correct, when called with a cell array with some char vectors in the cells as input strrep requires each element of that cell array to be a char vector. I'm using try / catch here so I can run code after the section that throws the error.
a = {'apple', [], 'cherry'}
a = 1×3 cell array
{'apple'} {0×0 double} {'cherry'}
try
strrep(a, 'e', 'E')
catch ME
fprintf("This call threw error '%s'.\n", ME.message)
end
This call threw error 'Cell elements must be character vectors.'.
Use cellfun or a for loop to replace the empty numeric elements with an empty char array.
E = cellfun(@isempty, a);
a(E) = {''}
a = 1×3 cell array
{'apple'} {0×0 char} {'cherry'}
strrep(a, 'e', 'E')
ans = 1×3 cell array
{'applE'} {0×0 char} {'chErry'}
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Askic V el 7 de Mzo. de 2023
Cool, thank you for the insight @Steven Lord.
Important thing to watch for!
Steven Lord el 7 de Mzo. de 2023
More likely than not the code that generated the variable a did not explicitly assigned an empty vector into it, so it's easy to overlook that possibility.
a = {'apple'}
a = 1×1 cell array
{'apple'}
a{3} = 'cherry'
a = 1×3 cell array
{'apple'} {0×0 double} {'cherry'}
MATLAB needed to put something into the second cell in the variable a when I assigned 'cherry' into the third cell. It essentially chose []. In this case where I knew I wanted all the cells to have text data I might have preallocated the cell array with repmat to ensure all cells had some text data. That's not so much preallocating for memory (cells in a cell array are not required to be contiguous in memory like elements in a numeric array) but more preallocating with the expected types.
b = repmat({''}, 1, 3)
b = 1×3 cell array
{0×0 char} {0×0 char} {0×0 char}
b{1} = 'apple'
b = 1×3 cell array
{'apple'} {0×0 char} {0×0 char}
b{3} = 'cherry'
b = 1×3 cell array
{'apple'} {0×0 char} {'cherry'}
strrep(b, 'e', 'E')
ans = 1×3 cell array
{'applE'} {0×0 char} {'chErry'}

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