# Modifying elements of fields in structures

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alphabetagamma il 12 Ago 2022
Commentato: alphabetagamma il 16 Ago 2022
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:
1.21
1.01
0.95
0.85
0.78
0.66
...
0.01
Then, I would like to the elements to be modified to:
1.21
1.21
1.21
1.01
0.95
0.85
...
...
0.01
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.
##### 10 CommentiMostra 8 commenti meno recentiNascondi 8 commenti meno recenti
dpb il 16 Ago 2022
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 il 16 Ago 2022
Haha thanks for the grade, and for helping me understand solve this problem.

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