For loop to access fields from structure and save them in an array

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_many_shocks_SH, PIE_AM_many_shocks_SH, etc

In total, there are about 1000 fields in big_struct, but I only have to use a few them. I'd like store those in arrays, in this case (30 x 1) doubles, manipulate them, then append each of the selected field in a (30 x 3) double. Then, I will make plots.

impulse_len = 5;
town_vec = ["South Hadley", "Amherst", "State College"];
town_code = ["SH", "AM", "SC"];



f_SH_pie = big_struct.PIE_SH_many_shocks_SH;  % (30 x 1)
f_SH_pie =[repmat(f_SH_pie(1),impulse_len,1); f_SH_pie]; % (40 x 1)
f_SH_pie = f_SH_pie(1:end-impulse_len,:); % (30 x 1)


f_AM_pie = big_struct.PIE_AM_many_shocks_SH;  
f_AM_pie =[repmat(f_AM_pie(1),impulse_len,1); f_AM_pie];
f_AM_pie = f_AM_pie(1:end-impulse_len,:);

f_SC_pie = big_struct.PIE_SC_many_shocks_SH;  
f_SC_pie =[repmat(f_SC_pie(1),impulse_len,1); f_SC_pie];
f_SC_pie = f_SC_pie(1:end-impulse_len,:);

pie_mat = [f_SH_pie, f_AM_pie, f_SC_pie];

Here's what I tried, but it's obviously wrong

for i = 1:length(town_code)
    "f_"+ (i) + "_pie" = big_struct.PIE_ + (i) +" _many_shocks_SH"; 
    "f_"+ (i) + "_pie" = [repmat(f_(i)_pie(1),impulse_len,1); f_(i)_pie];
    "f_"+ (i) + "_pie" = f_i()_pie(1:end-impulse_len,:);

end

If you could show me how to fix it, I would really appreciate it. Thanks a lot.

[Stephen23]

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Answer 1

[Walter Roberson]

town_codes = ["SH", "AM", "SC"];
fields = "PIE_" + town_codes + "_many_shocks_SH";
f_pie_cell = arrayfun( @(field) big_struct.(field)([ones(1,impulse_len), 1:end-impulse_len]), fields, 'uniform', 0);
pie_mat = [f_pie_cell{:}];
new_fields = "f_" + town_codes + "pie";
f_pie = cell2struct(f_pie_cell, new_fields, 2);

The result, f_pie, will be a struct with fields f_SH_pie, f_AM_pie, f_SC_pie