Dot indexing is not supported for variables of this type. For an array in an object

Hi, i'm trying to make a method in a object which receives an array and the object itself as input arguments and uses a property as indices for the array that is used as input. But I get the prompt that dot indexing is not supported for variables of this type. I have tried this outside of an object so this function should work but it doesn't, it probably has something to do with the self.currenGridCell syntax but I do not understand how I can fix this. I am new to OOP so all or any help is welcome.

classdef Herd

    properties
        name;
        nCows{mustBePositive, mustBeInteger}; 
        currentGridCell(1,2);
        
    end

    methods
    
        function newHerd = Herd(name, nCows, currentGridCell)
                if nargin == 3 % if all three arguments (r, p, a) are given to the Constructor
                    newHerd.name = name;
                    newHerd.nCows = nCows;
                    newHerd.currentGridCell = currentGridCell;
                end
        end

        function grasslands = eat(grasslands, self)
            grasslands(self.currentGridCell(1,1), self.currentGridCell(1,2)) = grasslands(self.currentGridCell(1,1), self.currentGridCell(1,2)) - (self.nCows*30);


        end
    end
end

[Cris LaPierre]

Have you gone through the Object-Oriented Programming Onramp? If you are new to OOP, this is a quick, interactive way to learn how to do this in MATLAB.

[Matt J]

@Aron your post doesn't demonstrate how to produce the error nor the error messages you receive.

[Walter Roberson]

How are you calling the function eat() ?

[Aron]

I make an array of the class of Herd and use the ".eat" on this array with input argument the "grasslands" array on which these "Herds" would interact (this is an agent-based modelling assignment). But I get the error already when I assign self.currentGridCell to another variable before using it as an index for the grasslands so the "self." should be the problem. I call the function in the main file as such:

clc
clearvars
close
tic 
%clearing console and variables

numberOfLocations = 3;
gridDimensions = 20;
weeksSeason = 13; 
timeSim = 1; %temporal resolution is a week and this for a 100 
% years with each year consisting of 52 weeks, type the amount of years
% here
grasslands = zeros(gridDimensions, gridDimensions);
grasslands = grasslands + 10000;
matGrMod = [1, 1.5, 1, 0.5]; %matrix with grass modifiers for one year
matGrMod = repmat(matGrMod, [1, timeSim]); %matrix with grass modifiers for a timeSim years

names = ["Jacob","Michael","Joshua","Matthew","Andrew","Christopher","Daniel","Joseph", ...
"Ethan","Nicholas","William","Anthony","David","Ryan","Tyler"];


locations = startingLocations(numberOfLocations, gridDimensions);


herdArray = Herd.empty(3,0);
familyArray = Family.empty(3,0);


for i = 1:numberOfLocations
    herdArray(i) = Herd(names(i), 200, [locations(i,1), locations(i,2)]);
    familyArray(i) = Family(names(i), 200, [locations(i,1), locations(i,2)]);
end


for i = matGrMod % a for loop going through 13 weeks for each grass modifier
    for j = 1:weeksSeason
    grasslands = grasslands + (100*i);
    grasslands(grasslands>40000) = 40000;
    grasslands(grasslands<0) = 0;
    herdArray.eat(grasslands) %!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    
    end
end

toc

[Aron]

The startingLocations file looks like this and the Family used so should not be a problem.

function [locations] = startingLocations(numberOfLocations, gridDimensions)

if numberOfLocations > gridDimensions*gridDimensions
    throw(MException('startingLocations:tooManySamples', 'Trying to sample %d different positions from %d locations', numberOfLocations, gridDimensions*gridDimensions))
end

rng(804694) % replace 'shuffle' with your r-number here

[X,Y] = meshgrid(1:gridDimensions, 1:gridDimensions);
allLocations = horzcat(reshape(X,[],1), reshape(Y,[],1));
locationIdxs = randsample(size(allLocations, 1), numberOfLocations);
locations = allLocations(locationIdxs,:);

end

Answer 1

[Walter Roberson]

When you use an object method using dot syntax on an object, then the object is always passed as the first parameter to the method. Your code is expecting it to be passed as the second parameter to the method.

[Aron]

Super helpful, thanks!!