setting object properties value for object array in matlab

Question

I have created an array of objects and I would like assign a property value in a vector operation without using a for loop. Unfortunately I get an error.

A simplified example of the problem.

classdef clsMyClass < handle
   properties 
      dblMyProperty1
   end 
   methods
        function obj = clsMyClass()
        end      
   end
end

And when running

vecMyArray = clsMyClass.empty(100,0);
vecMyArray(100) = clsMyClass;    
vecMyArray.dblMyProperty1 = 1:100;

We get the following error:

??? Incorrect number of right hand side elements in dot name assignment. Missing [] around left hand side is a likely cause.

Any help would be appreciated.

My two cents: for loop is probably your fastest solution. Also the first line of code (.empty(...)) doesn't do anything and its effect is immediately replaced by the second line of code. — Bee
– Bee, Commented Jun 14, 2013 at 16:06
Thanks to MATLAB's vectorization, you're doing this "wrong". Consider turning the class "inside-out" - rather than have an array of objects, have a single object whose properties are themselves arrays. See this treatise on MATLAB and OOP. — Dang Khoa
– Dang Khoa, Commented Jun 14, 2013 at 16:07
@DangKhoa, I plan to have only 20 objects which hold multiple large arrays. I would like to call methods of these 20 objects at once and change their properties. — Ruut
– Ruut, Commented Jun 14, 2013 at 16:17
@Bee, thanks you are right. I added this line, because of reasons explained here: stackoverflow.com/a/7879881/903186 — Ruut
– Ruut, Commented Jun 14, 2013 at 16:18
Are you initializing the objects? Why not just set the property with a default inside the class definition? — Dang Khoa
– Dang Khoa, Commented Jun 14, 2013 at 16:25

Dang Khoa · Accepted Answer · 2013-06-14 21:16:18Z

1

I see what you're trying to do now. Use disperse from the MATLAB File Exchange:

>> [vecMyArray.dblMyProperty1] = disperse(1:100);
>> vecMyArray(1).dblMyProperty1
ans = 
    1
>> vecMyArray(10).dblMyProperty1
ans = 
    10

answered Jun 14, 2013 at 21:16

Dang Khoa

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2 Comments

Ruut Over a year ago

I think this will solve my problem. Unfortunately I am using MATLAB Version 7.9.1.705 (R2009b) SP1 and this gives the error: ??? Undefined function or method 'disperse' for input arguments of type 'double'. Therefore I can not validate your solution

Dang Khoa Over a year ago

This is not a built in function, it's from the MATLAB file exchange. Hit the link to download it.

chessbot · Accepted Answer · 2013-06-14 17:46:08Z

1

You can use the deal function for exactly this purpose:

[vecMyArray.dblMyProperty1] = deal(1:100);

See: http://www.mathworks.com/company/newsletters/articles/whats-the-big-deal.html

Edit: No you can't, actually; that'll set them to all be the vector 1:100.

answered Jun 14, 2013 at 17:46

chessbot

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Comments

macduff · Accepted Answer · 2013-06-14 17:22:25Z

0

I think you'll find your answer here in "Struct array errors." Even though this is a class, similar rules apply.

Unfortunately missing [] is not the cause, since adding them causes more errors. The cause is that you cannot assign the same value to all fields of the same name at once, you must do it one at a time, as in the following code:

So you'll need:

for ii=1:100
  vecMyArray(ii).dblMyProperty1 = ii;
end

I know it's not satisfying, but I think it at least helps us to definitively understand this error.

answered Jun 14, 2013 at 17:22

macduff

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