Changing the values of multiple objects at once?

I have an array of objects with certain properties.

Is it possible to change the values of a certain property for all objects in the array at once (without looping through them) ?

I'd also like to be able to change the properties by adding a fixed value to all of them (in other words, performing an operation on a single property for all objects at once). Is this also possible without a for loop?

Apologies if this is very simple.

e.g Below is a simplified version of what I'm trying to achieve

   classdef test < dynamicprops

   properties
      Value
   end

   methods

      function emission = is_greater(input,obj)
          a = rand;
          emission = a < input;
      end

   end

   end

   %an array of the object

   for i=1:1:10

      objarray(i)=test

   end

   %the below does not work, but shows what I'm trying to do
   objarray.Value = rand;

   objarray.Value = objarray.Value + 0.6;

[men8th]

Having faffed about with this for a bit now I can confirm that Guillaume's answer is the way to go. The functions deal() and num2cell() are what is needed.

What I have learnt since then though is that assignment using a for loop is actually significantly faster (at least for the cases I've tested using the performance testing framework) than converting to cell arrays and then assigning to cell arrays to the properties of the array of objects.

Finally, if performance is important, it is important to get the data structures right. Better to have a single object with array valued properties than an array of objects with scalar valued properties. In other words, if you are reading this page because you are looking for vectorisation type performance enhancements with object arrays, perhaps you should consider if a single object with array properties and vectorised methods might be better.

Answer 1

[Guillaume]

You do not need to create a special set function or override any operator to assign values to a property of an array of objects. The syntax is however slightly obscure and awkward as you need to use temporary cell arrays for the values to assign.

   %object creation
   for i=1:1:10
      objarray(i)=test
   end

   %assigning the same value to each object. Use deal:
   [objarray.Value] = deal(0.5);

   %assigning different values to each object. 
   %Convert array of values to cell array, then convert cell array to comma-separated list:
   values = num2cell(rand(1, numel(objarray)));
   [objarray.Value] = values{:};

   %add a constant to each property. 
   %convert list of values to array. add value. convert array to cell array. convert cell array to comma-separated list:
   values = num2cell([objarray.Value] + 0.6);
   [objarray.Value] = values{:};

Unfortunately, you can't do the conversion to cell array and the conversion of cell array to comma separated list on the same line due to matlab chaining rules, num2cell(x){:} is a syntax error. So in most cases, you need at least two lines. As I said, it's a bit awkward.

[Adam]

Ah yes, I always forget the usages of the deal function despite having had a sticky note on my desktop for about the last 4 years reminding me about the function! I also tend not to be too familiar with manipulation of comma-separated lists in 'non-standard' situations.

For the addition of a constant I would imagine going via cell arrays to not be especially performant though I haven't tested it. Nothing I've ever done involving cell arrays has been at all fast!

[Guillaume]

Considering that the use case for deal is extremely limited since the automatic conversion of cell arrays to c-s-l has been introduced, I would really like for it (or a new function) to have an option to automatically convert an array of numbers to a c-s-l. That way the syntax could be a one liner, and indeed avoid the costly conversion to cell array:

[objarray.Value] = betterdeal(rand(1, numel(objArray)));
[objarray.Value] = betterdeal([objarray.Value] + 0.6);

That obviously only works as long as you only want scalars in your list. I guess that's why it's not implemented.

[Art]

I think this technique is what I'm looking for... but I can't seem to find syntax that works for setting the 'Position' property of multiple text objects at once. Is there a way to do this? It seems that setting the 3-element vector is causing the problem somehow.

That being said, what I'm really trying to do is use a slider to move an MxN array of texts. The number of text objects can be fairly large (1000x50) so doing this in a loop is very sluggish. Maybe there's a better way than this? Thanks for any advice.

[Steven Lord]

If you're working with Handle Graphics object, you can change properties of a large number of objects at once using the set function.

t = gobjects(1, 10);
color = repmat({[0 0 0]}, 10, 1);
for whichText = 1:10
    t(whichText) = text(whichText, whichText, num2str(whichText));
    color{whichText} = rand(1, 3);
end
axis([0 11 0 11])

% set all text object Colors to 'r' (red)
set(t, 'Color', 'r')
% or set all text object Colors to different RGB values
set(t, {'Color'}, color)

[Art]

Thanks, but I think this works only for setting all objects properties to the same value? In my case I have read all current position values of the objects, adjusted them by some amount, and now am trying to put back in the new positions.

For some reason the following code does not work:

OrigPos = get(ObHandles,'Position');
set(ObHandles,'Position',OrigPos);

I get the error:

Error using Matlab.graphics.primitive.Text/set

Error setting property "Position" of class 'Text':

Value must be a 2 or 3 element vector of numeric or logical type

[Stephen23]

"but I think this works only for setting all objects properties to the same value?"

No, you missed the distinction that Steven Lord gave:

set(t, 'Color', 'r')         % <- all objects use the same value
set(t, {'Color'}, color)     % <- every object has its own value
%      ^       ^ The cell array is significant!

Change your code to follow the example Steven Lord gave (and as explained in the set documentation):

set(ObHandles,{'Position'},OrigPos);
%             ^          ^ you need these!

Answer 2

[Adam]

The default set function for properties works on scalar objects only, but you can write your own functions (and any other function you overload or write yourself) to support vectors (or even matrices) of objects which you then act on.

Then your code above should work fine. I don't tend to do this myself except in a few places because it is a lot of effort supporting vectors of objects in all functions, but it can be useful.

You can do this, though it just moves the for loop inside the class:

classdef MyClass < handle
  properties
    Value
  end
  methods
    function set.Value( objArray, value )
      validateattributes( objArray, { 'MyClass' }, { 'vector' } )
                        validateattributes( value, { 'numeric' }, { 'scalar' } )
      
      for i = 1:numel( objArray )
        objArray(i).Value = value;
      end
    end
  end
end

m(4) = MyClass;
m.Value = 7;

It quickly gets messy from a design perspective though because then you start asking yourself 'Should I also support a vector of values so that I can set the value of 10 objects to each value of an array of 10 numbers?'. And the code soon starts to get very complicated.

The plus operator could be similarly overloaded:

function objArray = plus( objArray, value )
   validateattributes( objArray, { 'MyClass' }, { 'vector' } )
   validateattributes( value, { 'numeric' }, { 'scalar' } )
      
   for i = 1:numel( objArray )
  objArray(i).Value = objArray(i).Value + value;
   end
end

or something similar. Again though you have the questions of what you support because the standard + operator supports more than just adding a scalar to an array (as well as being symmetric, but that is another matter entirely).

Section 17-45 of the Matlab OOP bible covers other ways to overload the plus operator with a double operator overload also, but the class needs to be basically a wrapper for one main value property for this to make much sense. Just adding 7 to an arbitrary class full of properties doesn't really have an obvious meaning.

Even the simple early example in 2-6 to 2-7 of that linked document shows how quickly design question can start to arise from relatively simple overloads. In that example a plus operator is overloaded that simply adds the 'Value' or one class to the 'Value' of another. That is fair enough, but the output is a double. Personally I would expect that if I added two of those 'BasicClass' objects together I would end up with a BasicClass object as my output, containing the summed values of the two inputs, rather than just a raw double.

[Stephen23]

Note that set and get for graphics objects supports multiple handles and/or multiple values in arrays. As Adam says the syntax gets a bit complex, but it can be done.

[Bryan Krause]

It doesn't appear this approach works, or at least it doesn't work in more recent versions; an error "Assigning to (n) elements using a simple assignment statement is not supported." is thrown before the set.(PropertyName) method is called.