How do I delete a struct?

Hi, I am trying to make a predator-prey simulation and I've mapped structs in containers. One struct is one animal and the container represents either the predator or the prey. Now I want a prey to be eaten when it moves within a certain range of a predator. How do I delete a struct? Be easy on me, I'm a bit new to MATLAB. (;

Here's the code: the important part is below %ROOFDIEREN ---> %ETEN.

% VARIABELEN
numDays = 50;
geboortekansPrey = 0.00;
geboortekansPred = 0.00;
minleeftijdPrey = 10; 
minleeftijdPred = 10; 
axis = 100;
W = 0:1:axis;
r = 2;

% MAPPEN MET INITIËLE DIEREN
predMap = containers.Map('KeyType', 'double', 'ValueType', 'any');
pred = struct('x', 1, 'y', 1, 'age', 25);
predMap(1) = pred;

preyMap = containers.Map('KeyType', 'double', 'ValueType', 'any');
prey = struct('x', 2, 'y', 2, 'age', 15);
preyMap(1) = prey;
prey = struct('x', 3, 'y', 3, 'age', 20);
preyMap(2) = prey;

% DAGEN
for t = 1:numDays
    disp(['day ', num2str(t),':']);
        
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% PROOIDIEREN
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    
    disp('Prooidieren:');
    for p = 1:length(preyMap)
        newPrey = preyMap(p);

% COÖRDINATEN
        d = rand * r;
        rad = rand * 2 * pi;
        
        newpreyx = newPrey.x + cos(rad) * d;
        newpreyy = newPrey.y + sin(rad) * d;
        
        while newpreyx<0 || newpreyx>axis || newpreyy<0 || newpreyy>axis
            d = rand * r;
            rad = rand * 2 * pi;
            newpreyx = newPrey.x + cos(rad) * d;       
            newpreyy = newPrey.y + sin(rad) * d;
        end
                     
        newPrey.x = newpreyx;
        newPrey.y = newpreyy;
        
% LEEFTIJD
        newPrey.age = newPrey.age + 1;
        
        preyMap(p) = newPrey;
        
% WEERGAVE
        Leeftijd = sprintf('Leeftijd: %d', preyMap(p).age);
        disp(Leeftijd) 
        Plaats = sprintf('Coördinaten: (%d, %d)', preyMap(p).x, preyMap(p).y);
        disp(Plaats)
        
        
% GEBOORTE
        if preyMap(p).age > minleeftijdPrey
           out=rand;
            if out<geboortekansPrey
                prey = struct('x', W(randi([1,numel(W)])), 'y', W(randi([1,numel(W)])), 'age', 0);
                preyMap(length(preyMap) + 1) = prey;      
            end    
        end            
    end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% ROOFDIEREN
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    
    disp('Roofdieren:');
    for q = 1:length(predMap)
        newPred = predMap(q);
        
% COÖRDINATEN
        d = rand * r;
        rad = rand * 2 * pi;
        
        newpredx = newPred.x + cos(rad) * d;
        newpredy = newPred.y + sin(rad) * d;
        
        while newpredx<0 || newpredx>axis || newpredy<0 || newpredy>axis
            d = rand * r;
            rad = rand * 2 * pi;
            newpredx = newPred.x + cos(rad) * d;       
            newpredy = newPred.y + sin(rad) * d;
        end
                     
        newPred.x = newpredx;
        newPred.y = newpredy;
        
        
% LEEFTIJD
        newPred.age = newPred.age + 1;
        
% ETEN
        if abs(newPred.x - newPrey.x)<v && abs(newPred.y - newPrey.y)<v
            delete??????
        end
       
        predMap(q) = newPred;
        
% WEERGAVE
        Leeftijd = sprintf('Leeftijd: %d', predMap(q).age);
        disp(Leeftijd) 
        Plaats = sprintf('Coördinaten: (%d, %d)', predMap(q).x, predMap(q).y);
        disp(Plaats)
        
% GEBOORTE
        if predMap(q).age > minleeftijdPred
           out=rand;
            if out<geboortekansPred
                pred = struct('x', W(randi([1,numel(W)])), 'y', W(randi([1,numel(W)])), 'age', 0);
                predMap(length(predMap) + 1) = prey;      
            end    
        end            
    end
end

[Rik]

You probably want the Euclidean distance instead:

%change this
abs(newPred.x - newPrey.x)<v && abs(newPred.y - newPrey.y)<v
%to this
hypot(newPred.x - newPrey.x,newPred.y - newPrey.y)<v

Otherwise the prey will get eaten in a square are, not a circle.

It looks like your actual goal is to remove the prey animal from the database, can you confirm this? In that case you should loop through all the preys to see if any of them is close to the predator.

[Nathan Bhoedjang]

Yes, wow I can't believe I missed that. That is indeed my goal.

Answer 1

[Rik]

If my guesses are correct, the code below should be close to what you need.

%replace this
if abs(newPred.x - newPrey.x)<v && abs(newPred.y - newPrey.y)<v
    delete??????
end
%with this
for p=numel(preyMap):-1:1%loop backwards
    newPrey=preyMap(p);
    if hypot(newPred.x - newPrey.x,newPred.y - newPrey.y)<v
        preyMap(p)=[];%remove entry
    end
end

[Nathan Bhoedjang]

I do get an error with dot indexing now and the simulation stops on day two.

[Nathan Bhoedjang]

It doesn't always stop on day two because of all of the random variables, but it always does at some point in the run. It seems to get stuck at newPrey.x whenever a prey gets eaten.

[Rik]

I have never worked with maps before, so I (wrongly) assumed you were using a struct array. The documentation describes you need to use the remove function to remove entries from a map, so probably something like this:

remove(preyMap,p)

[Nathan Bhoedjang]

It kind of works now, thanks! However, the coordinations are only integers now.

[Rik]

Because you are using a container you need to keep track of the keys you have removed. If you change to a struct array you wouldn't need to do that. Luckily, we can use the keys function.

v=2;%you forgot to define this predation range

% VARIABELEN
numDays = 50;
geboortekansPrey = 0.00;
geboortekansPred = 0.00;
minleeftijdPrey = 10;
minleeftijdPred = 10;
axis = 100;
W = 0:1:axis;
r = 2;

% MAPPEN MET INITIËLE DIEREN
predMap = containers.Map('KeyType', 'double', 'ValueType', 'any');
pred = struct('x', 1, 'y', 1, 'age', 25);
predMap(1) = pred;

preyMap = containers.Map('KeyType', 'double', 'ValueType', 'any');
prey = struct('x', 2, 'y', 2, 'age', 15);
preyMap(1) = prey;
prey = struct('x', 3, 'y', 3, 'age', 20);
preyMap(2) = prey;

% DAGEN
for t = 1:numDays
    fprintf('\nday %d:\n',t);
    
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    % PROOIDIEREN
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    
    fprintf('Prooidieren:\n');
    for p = cell2mat(keys(preyMap))
        newPrey = preyMap(p);
        
        % COÖRDINATEN
        d = rand * r;
        rad = rand * 2 * pi;
        
        newpreyx = newPrey.x + cos(rad) * d;
        newpreyy = newPrey.y + sin(rad) * d;
        
        while newpreyx<0 || newpreyx>axis || newpreyy<0 || newpreyy>axis
            d = rand * r;
            rad = rand * 2 * pi;
            newpreyx = newPrey.x + cos(rad) * d;
            newpreyy = newPrey.y + sin(rad) * d;
        end
        
        newPrey.x = newpreyx;
        newPrey.y = newpreyy;
        
        % LEEFTIJD
        newPrey.age = newPrey.age + 1;
        
        preyMap(p) = newPrey;
        
        % WEERGAVE
        fprintf('Leeftijd: %d\n', preyMap(p).age);
        fprintf('Coördinaten: (%.2f, %.2f)\n', preyMap(p).x, preyMap(p).y);
        
        % GEBOORTE
        if preyMap(p).age > minleeftijdPrey
            out=rand;
            if out<geboortekansPrey
                prey = struct('x', W(randi([1,numel(W)])), 'y', W(randi([1,numel(W)])), 'age', 0);
                preyMap(length(preyMap) + 1) = prey;
            end
        end
    end
    
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    % ROOFDIEREN
    %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    
    fprintf('Roofdieren:\n');
    for q = cell2mat(keys(predMap))
        newPred = predMap(q);
        
        % COÖRDINATEN
        d = rand * r;
        rad = rand * 2 * pi;
        
        newpredx = newPred.x + cos(rad) * d;
        newpredy = newPred.y + sin(rad) * d;
        
        while newpredx<0 || newpredx>axis || newpredy<0 || newpredy>axis
            d = rand * r;
            rad = rand * 2 * pi;
            newpredx = newPred.x + cos(rad) * d;
            newpredy = newPred.y + sin(rad) * d;
        end
        
        newPred.x = newpredx;
        newPred.y = newpredy;
        
        
        % LEEFTIJD
        newPred.age = newPred.age + 1;
        
        % ETEN
        for p = cell2mat(keys(preyMap))
            newPrey=preyMap(p);
            if hypot(newPred.x - newPrey.x,newPred.y - newPrey.y)<v
                remove(preyMap,p);%remove entry
            end
        end
        
        predMap(q) = newPred;
        
        % WEERGAVE
        fprintf('Leeftijd: %d\n', predMap(q).age);
        fprintf('Coördinaten: (%.2f, %.2f)\n', predMap(q).x, predMap(q).y);
        
        % GEBOORTE
        if predMap(q).age > minleeftijdPred
            out=rand;
            if out<geboortekansPred
                pred = struct('x', W(randi([1,numel(W)])), 'y', W(randi([1,numel(W)])), 'age', 0);
                predMap(length(predMap) + 1) = prey;
            end
        end
    end
end