Modifying elements of fields in structures

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alphabetagamma 2022-8-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1778600-modifying-elements-of-fields-in-structures

评论： alphabetagamma 2022-8-16

This is similar to Select fields that start with specific letters in for loops - (mathworks.com), but I thought of making a separate post as I would like ask two questions.

I have a humongus structure known as

big_struct

with several fields. Each of these fields are doubles (30 x 1) dimension. So, they contain 30 numerical values.

Examples of these doubles are price_SH_RES_output_SH, price_AM_RES_output_SH etc.

I have two questions, but the second question is more important.

Q1: The following code becomes cumbersome when we add multiple fields, and I was wondering how to make it efficient and succint. If you could show the modifed code (perhaps by using a for loop?), then that would be very helpful.

interested_variable_vec = ["price", "output", "bond"]; 
interested_town_vec = ["South Hadley", "Amherst", "State College"];
town_code = ["SH", "AM", "SC"];
choose_variable1 = "price_SH";
choose_variable2 = "output_SH";
choose_variable3 = "bond_SH";
[~,town_idx] = ismember(town_of_interest, town_vec);
if town_idx == 0
    error('unknown town of interest: "%s"', town_of_interest);
end
if ~strcmp(interested_variable, 'bond') 
     fieldname1 = interested_variable + "_" + town_code(town_idx) +"_RES_" + choose_variable1;
     fieldname2 = interested_variable + "_" + town_code(town_idx) +"_RES_" + choose_variable2;
     fieldname3 = interested_variable + "_" + town_code(town_idx) +"_RES_" + choose_variable3;
end
if strcmp(interested_variable, 'bond') 
    if ~strcmp(town_code, 'SH')
        fieldname1 = interested_variable + "_" + town_code(town_idx) + "_RES_" + choose_variable1;
        fieldname2 = interested_variable + "_" + town_code(town_idx) + "_RES_" + choose_variable2;
        fieldname3 = interested_variable + "_" + town_code(town_idx) + "_RES_" + choose_variable2;
    end
end
if ~isfield(big_struct, fieldname1)
    error('town and variable combination does not exist: "%s"', fieldname1);
end
if ~isfield(big_struct, fieldname2)
    error('town and variable combination does not exist: "%s"', fieldname1);
end
impulse1 = big_struct.(fieldname1);
impulse2 = big_struct.(fieldname2);
impulse3 = big_struct.(fieldname3);

Q2. How do we modify the elements of a field? For instance, impulse1 is a (30 x 1). I would like the first 3 elements of impulse1, impulse2, impulse3 to take the first value in these respective fields

Example: if the elements of impulse1 are:

Then, I would like to the elements to be modified to:

21
21
21
01
95
85
...
...
01

I would like to do the same for the other fields and use them them below

% Add the projections of the variables with the forecast intervals
% create a  (23 x 1) dimension of empty array with 0s
% length(impulse) = 20 as the horizon is 20 quarters into the future
 difference = zeros(4 + length(impulse1) - 1, 1);
for ii = 1:4 
    % impulse prints the values from IRFs = (20 x 1) array
    difference(ii:horizon+ii-1) = difference(ii:horizon+ii-1) + 3*(impulse1 .* impulse2 .* impulse3);
end 

Thank you so much for your help in advance.

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dpb 2022-8-16

Yes, although you can do it without needing the temporary variable by adding the subscripting expression into the first. Since the array is a 1D vector, you can also do away with the second subscript.

"Student" receives "A-" :)

alphabetagamma 2022-8-16