Problem Creating Structure Field with For Loop

Question

Allen Hammack 2022-5-7

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https://ww2.mathworks.cn/matlabcentral/answers/1713875-problem-creating-structure-field-with-for-loop

评论： Allen Hammack 2022-5-7

I'm having trouble creating and populating a structure field using a for loop. What I've attempted is

for j = 1:6
    structure{1}.file_name_string(j) = strcat('file_name_',num2str(j));
end 

which yields

Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.

When I try

for j = 1:length(velocity_raw{1}.velocity_files)
    strcat('file_name_',num2str(j))
end  

I get the entries that I want, but they aren't assigned to the structure field.

I don't understand what the error message means, and I don't understand how to fix the problem. I expect that my problem is basic, but I just haven't been able to figure it out. Can someone please help?

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Answer 1

Jonas 2022-5-7

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编辑：Jonas 2022-5-7

在 MATLAB Online 中打开

struct are indexed with round brackets, after that use {} if the structure field file_name_string shall be a cell

for j = 1:6
structure(1).file_name_string{j} = strcat('file_name_',num2str(j));
end

this gives you a 1x1 struct with a field which is a cell array of length 6

if you want a 1x6 struct with a field which is ancharacter array, use

for j = 1:6
    structure(j).file_name_string = strcat('file_name_',num2str(j));
end

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Jonas 2022-5-7

i dont know, maybe you created a weird variable in your workspace. please type

clear

and try the code again, i have no problems running the code.

Allen Hammack 2022-5-7

Thank you! I cleared my variables, and everything works correctly.

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Answer 2

Steven Lord 2022-5-7

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https://ww2.mathworks.cn/matlabcentral/answers/1713875-problem-creating-structure-field-with-for-loop#answer_959255

在 MATLAB Online 中打开

Do you want the names of the fields in the struct array to be file_name_ followed by a number (file_name_1, file_name_2, etc.)?

s = struct;
for k = 1:5
    fn = "file_name_" + k;
    s.(fn) = k.^2;
end
s
s = struct with fields:
    file_name_1: 1
    file_name_2: 4
    file_name_3: 9
    file_name_4: 16
    file_name_5: 25

Or do you want to use actual file names as the field names (something like myImageFile1, myTextFile2, etc.?)

t = struct;
files = {'census.mat', 'eight.tif'};
for k = 1:length(files)
    fn = matlab.lang.makeValidName(files{k}); % since field names can't contain periods
    t.(fn) = k.^2;
end
t
t = struct with fields:
    census_mat: 1
     eight_tif: 4

Or do you want to do something else? If so please provide a small concrete example of what you want to do (don't worry about how to create the struct using code, just explain in words.)