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

Question

Michael Sharp 2017-8-4

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https://ww2.mathworks.cn/matlabcentral/answers/351497-how-can-i-get-a-string-from-an-input-to-be-used-to-index-an-array

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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Answer 1

Patrick Eschbach 2017-8-4

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https://ww2.mathworks.cn/matlabcentral/answers/351497-how-can-i-get-a-string-from-an-input-to-be-used-to-index-an-array#answer_276798

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