Select fields that start with specific letters in for loops

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 am trying to do the following, but it is very inefficient and time consuming.

interested_variable_vec = ['price', 'output', 'bond'] 
interested_town_vec = ['South Hadley', 'Amherst', 'State College']
% interested_variable takes any one of the variable from
% interested_variable_variable_vec. Eg: interested_variable = 'price'

if strcmp(interested_variable, 'price') && strcmp(interested_town, 'South Hadley')
            innovation = big_struct.price_SH_RES_output_SH;

elseif  strcmp(interested_variable, 'price') && strcmp(interested_town, 'Amherst')
            innovation = big_struct.price_AM_RES_output_SH;

elseif  strcmp(interested_variable, 'price') && strcmp(interested_town, 'State College')
            innovation = big_struct.price_EU_RES_output_SH;
            
elseif strcmp(interested_variable, 'output') && strcmp(interested_town, 'South Hadley')
            innovation = big_struct.output_SH_RES_output_SH;

elseif  strcmp(interested_variable, 'output') && strcmp(interested_town, 'Amherst')
            innovation = big_struct.output_AM_RES_output_SH;
            
elseif strcmp(interested_variable, 'bond') && strcmp(interested_town, 'South Hadley')
            innovation = big_struct.bond_SH_RES_output_SH;
            
elseif  strcmp(interested_variable, 'bond') && strcmp(interested_town, 'Amherst')
            innovation = big_struct.bond_AM_RES_output_SH;

elseif  strcmp(interested_variable, 'bond') && strcmp(interested_town, 'State College')
            innovation = big_struct.bond_EU_RES_output_SH;

end

I thought of using a for loop, but I am not good at for loops. Here's what I think the for loop might look like, but I am not sure how to implement it. I appreciate if you could help fix it.

shock = output_SH;

for i = length(city_vec)
    for j = 1:length(interested_variable_vec)
        
        if strcmp(interested_variable, interested_variable_vec(j,:)) && strcmp(interested_town, interested_town_vec(i,:))
            innovation = big_struct.j_i_RES_shock;
    end
end

Answer 1

[Walter Roberson]

%do not use apostrophes, use double-quote for this work
interested_variable_vec = ["price", "output", "bond"]; 
interested_town_vec = ["South Hadley", "Amherst", "State College"];
town_code = ["SH", "AM", "EU"];

if ~ismember(interested_variable, interested_variable_vec)
    error('unknown interested variable: "%s"', interested_variable);
end

[~,town_idx] = ismember(intereted_town, interested_town_vec);
if town_idx == 0
    error('unknown interested town: "%s"', interested_town);
end


fieldname = interested_variable + "_" + town_code(town_idx) + "_RES_output_SH";
if ~isfield(big_struct, fieldname)
    error('variable and town combination does not exist: "%s"', fieldname);
end

innovation = big_struct.(fieldname);

[alphabetagamma]

This is great. Thanks a lot. However, town_code(town_idx) seems to output only the first letter.

e.g.:

town_code(town_idx)

output is S, instead of SH probably because the index is 1.

Consequently, the fieldname price_S_RES_output_SH doesn't exist and yields an error. Is there any way to fix that?

[Walter Roberson]

Read the very first line of the code, the comment.