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

Question

Michael Cole 2022-11-25

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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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Answer 1

Walter Roberson 2022-11-26

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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 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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Answer 2

Image Analyst 2022-11-26

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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"