Updating structure data within a containers.map.

Hi, I just got started on a pred-prey simulation. I started out with structures two preys and I put them in a map. Now I want to make their age (starting with the prey's) increase over time. When I run my code I get the following error in the second for loop: only one level of indexing is supported by a containers.Map. Even when I try to change the age outside of the loop I get the same error. Can anyone help me out? I'm new to MATLAB...

numDays = 100;

preyMap = containers.Map('KeyType', 'double', 'ValueType', 'any');
prey1 = struct('x', 2, 'y', 2, 'age', 365, 'lastbirth', 100);
preyMap(1) = prey1;
prey2 = struct('x', 3, 'y', 3, 'age', 365, 'lastbirth', 100);
preyMap(2) = prey2;


for p = 1:length(preyMap)
    disp(preyMap(p).age)
    for t = [1:numDays]
        preyMap(p).age = preyMap(p).age + t
        disp(preyMap(p).age)
    end
end

Answer 1

[Walter Roberson]

for p = 1:length(preyMap)
    disp(preyMap(p).age)
    for t = [1:numDays]
        tp = preyMap(p);
        tp.age = tp.age + t;
        preyMap(p) = tp;
        disp(preyMap(p).age)
    end
end

I would suggest, however, that putting a structure inside a map is not as efficient as just creating a nonscalar structure and indexing into it.

[Nathan Bhoedjang]

Great thanks! I will try your other suggestion as well.

[teeeeee]

@Walter can you show a very basic example of what you said: "creating a nonscalar structure and indexing into it", (or alternatively just point me to an example of this somewhere in the docs?).

I am trying to have some varying types of data stored as a map such that I can access attributes nicely by a label (character vector), without having to keep track of integer indexing. Something like the following:

keys = {'Jan','Feb','Mar'};
data_map = containers.Map('KeyType','char','ValueType','any');

% loop over keys and put some data in the map
for k = 1:length(keys)
    
    data_struct.prices = rand(1,5);   % create some dummy numbers
    data_struct.description = 'HIGH'; % create a dummy label
    
    % place the structure in the map at the relevant key
    data_map(keys{k}) = data_struct; 
    
    % delete ready for next iteration of the loop
    clear data_struct
end

% a nice way to access the data (no integer indexing required, only the key)
disp( data_map('Feb').description )

However, sometime later, when I come to add some extra data, such as the following I get an error:

% data_map('Feb').temperature = rand(1,8); % gives Error

Instead, I find I have to do something like the following:

% do it instead with a temporary structure
temp_struct = data_map('Feb');
temp_struct.temperature = rand(1,8);
data_map('Feb') = temp_struct;
clear temp_struct
disp( data_map('Feb').temperature )

Is this really the best way to do this, or were you hinting in your previous answer that there is a nicer way?

Thank you!