How can I get a string from an input to be used to index an array?

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Michael Sharp
Michael Sharp 2017-8-4
评论： Patrick Eschbach 2017-8-14
So I'm trying to write this code that I want to act as a music scale key for various different keys. The code asks the user to input the key and mode they want to practice in (i.e E major) and it'll spit out the correct scale for that key and mode, so for E major it will print E F# G# A B C# D#.
major = [1 3 5 6 8 10 12]; % an array to index chromatic array for major scale notes
chromatic = ["C", "C#", "D", "D#", "E", "F", "F#", "G", ...
"G#", "A", "A#", "B"];
key = input('Try again, what key do you practice in?\n', 's');
key_split = strsplit(key);
S = char(key_split);
if contains(S(1,:), 'b')
chromatic = chromatic_flats;
mode = S(2,:);
else
chromatic = chromatic_sharps;
mode = S(2,:);
end
k = find(strcmp(key_split(1), chromatic));
% Strcmp passes T/F to the chromatic array and find finds the corresponding array index
chromatic = circshift(chromatic, 13 - k);
% Cicshift shifts the chromatic array to start at the key choosen from
fprintf('\nPracticing in the key of %s %s\n', char(key_split(1)), char(mode))
practice_key = chromatic(mode)**
So I only really need help with the last line of code that I double starred, everything above that is the code to change the chromatic scale to start with they key the user selected. I know the input from the user is a string and I can't index an array with a string, so I can't find the right solution to that. I've looked up structures and cell arrays but not sure how to implement them if they are the correct solution.

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回答（1 个）

Patrick Eschbach
Patrick Eschbach 2017-8-4
Hi Michael,
It looks like you want to return a subset of your array of notes, i.e., a portion of the chromatic data structure, depending on the key and mode the user inputs. If you just want to return the portion of the chromatic structure starting at a particular note, you could use strfind to find the starting index of a string inside chromatic and then index with that number.
See info about strfind: https://www.mathworks.com/help/matlab/ref/strfind.html
Example: practice_key = chromatic(strfind(chromatic, mode), :);
If you really want to index with a string, you should look into mapping string keys to some other sort of value using the map class. See info about the map class: https://www.mathworks.com/help/matlab/ref/containers.map-class.html
I do not think it would be necessary to use cell arrays or structures for this particular problem.
-Pat

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