'for' loops in app designer methods have a parsing problem

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Michael Cole
Michael Cole 2022-11-25
回答： Image Analyst 2022-11-26
I've been debugging a simple app designer project. I have a list of strings that are country names. I want to delete all duplicates so I have a simple property 'ii' that is an index for traversing the list so I referred to it as 'app.ii' in a 'for' loop that is inside a working method. Matlab won't let me do it and says there is a parsing error at '.' It will let me use 'app.ii' in a 'while' loop or 'if' statement but as soon as I type it in a 'for' loop a red line appears under the period. 'ii' is a declared property of the app. Anyone have an idea what this seemingly straightforward 'for' loop is doing wrong?

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Walter Roberson
Walter Roberson 2022-11-26
It is a language limitation. for has never supported any kind of indexing for the loop control variable. The limitation saves defining a bunch of edge cases.
For example,
S(1).ii = 1;
S(2).ii = 7;
K = 1;
for S(K).ii = -2:2
S(K).ii
K = K + 1;
end
Does that record the value of K at the start of the loop and thereafter loop over S(1).ii ? Or should the second iteration be working with S(2).ii ?
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Walter Roberson
Walter Roberson 2022-11-26
Under what circumstances would it be relevant for a different part of the application need to know what the current for loop index is, such that making it a property of the application as a whole would be appropriate ?
Creating a loop index as a property of the application as a whole would give a tendency for people to want to change the loop control variable. If you have a different method that can change your active loop control index, then that is seldom a good design strategy.

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Image Analyst
Image Analyst 2022-11-26
Ran in:
Try this to remove duplicates
% Remove duplicates from arrays of countries.
% Using character arrays.
countries = {'USA', 'Belgium', 'UK', 'USA', 'Japan'}
countries = 1×5 cell array
{'USA'} {'Belgium'} {'UK'} {'USA'} {'Japan'}
uniqueCountries = unique(countries)
uniqueCountries = 1×4 cell array
{'Belgium'} {'Japan'} {'UK'} {'USA'}
% Using strings.
countries2 = ["USA", "Belgium", "UK", "USA", "Japan"]
countries2 = 1×5 string array
"USA" "Belgium" "UK" "USA" "Japan"
uniqueCountries2 = unique(countries2)
uniqueCountries2 = 1×4 string array
"Belgium" "Japan" "UK" "USA"

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