Call fields of struct more efficiently in a loop

I want to do the following more efficiently.

vec.time = [1000,5000];
vec.nest = {'center','corner'};
vec.grid = [50,100,1000];

for i.time = 1:length(vec.time)
    p.time = vec.time(i.time);
    for i.nest = 1:length(vec.nest)
        p.nest = vec.nest(i.nest);
        for i.grid = 1:length(vec.grid)
            p.grid = vec.grid(i.grid);
            
            %Code

        end
    end
end

Essentially, the fields of the structs vec, i, p will be the same. The vec struct contains all the values that I want to run in a loop, the i struct acts as a counter for them, and the p struct runs the code with the particular value selected. Is there a way such that I can call the fields by their order number rather than their name? Though, I do want the p struct with all the field names in the end, since the rest of the code depends on it. Thank you!

Edit: I've been looking into similar problems, and I think it might be done using struct2cell and cell2struct. For this particular example, if I have an array as

order = [1 1 1 ; 1 1 2 ; 1 1 3 ; 1 2 1 ; 1 2 2 ; 1 2 3 ;
         2 1 1 ; 2 1 2 ; 2 1 3 ; 2 2 1 ; 2 2 2 ; 2 2 3];     

then what I can do is

vec_cell = struct2cell(vec);
fields = fieldnames(vec);
p_cell = cell(length(vec_cell),1);
for i_order = 1:length(order)
    for i_cell = 1:length(vec_cell)
        p_cell{i_cell} = vec_cell{i_cell}(order(i_order,i_cell));
    end
    p = cell2struct(p_cell,fields);
    
    %Code

end

While this may not be as efficient, it certainly seems more robust, since I only need to add information to the vec struct, and it would calculate the p struct for every iteration. I do need help with constructing the order array, given the number of fields in the vec struct, and the length of each field. Thank you!

Edit2: I realised that values in vec.nest cannot be assigned this way, since they are in a cell, while others are in an array. But I can resolve this by assigning values to them (1 is 'center', 2 is 'corner').

[Walter Roberson]

It is not permitted to use a structure field as a for loop control variable. Only simple unindexed variables can be used.

Answer 1

[Walter Roberson]

Is there a way such that I can call the fields by their order number rather than their name?

fn = fieldnames(appropriate_structure);
for K = 1 : length(fn)
    appropriate_structure.(fn{K})     %effectively call by number
end

Though, I do want the p struct with all the field names in the end

Consider using structfun()

Note that structfun() is not necessarily more efficient than a for loop: it is more compact than a for loop, but structfun does a for loop internally.

[Tejas]

Please see edit. Thank you.