Assign Array values to object Array in Matlab

Question

I have the following problem:

[obj(:).Radius] = arrayOfRadius;

I have an Array with radius for every single object. The upper code doesn't work though the object has the same size as the right hand arrayOfRadius.

I could solve this with a for loop, sure:

for i = 1:length(obj)
   obj(i).Radius = arrayOfRadius(i);
end

That's not the way I'd like to solve it. I already found the "deal" function. But the deal function copies the whole arrayOfRadius() in every object.

Can someone help me out?

Thank you very much.

sco1 · Accepted Answer · 2016-02-01 19:37:53Z

5

This is one of the more obnoxious areas of MATLAB's indexing that I really wish they would fix, especially with the move of graphics handles to objects from doubles making it non-intuitive how to set properties of multiple objects at the same time.

There exists a workaround for deal, though it requires the intermediate use of num2cell:

% Initialize a structure
obj(6).radius = 6;

radii = [1 2 3 4 5 6];
C = num2cell(radii);
[obj(:).radius] = deal(C{:});

Which returns:

>> [obj.radius]

ans =

     1     2     3     4     5     6

As desired.

answered Feb 1, 2016 at 19:37

sco1

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1 Comment

rayryeng Over a year ago

Hehehe 20 seconds away from each other. Nicely done.

rayryeng · Accepted Answer · 2016-02-01 19:44:18Z

4

A very hacky solution would be to take the array and turn it into a cell array, then use a comma-separated list to finally do the assignment. Assuming that obj is already declared, do something like this:

A = num2cell(arrayOfRadius);
[obj.Radius] = A{:};

num2cell converts the array into a cell array of individual elements and doing A{:} converts the cell array into a comma-separated list and we'll deal out each element into its respective slot in the structure. Doing obj.Radius unpacks each Radius field in your structure into a comma-separated list. Therefore the above code is equivalent to doing:

[obj(1).Radius, obj(2).Radius, ..., obj(N).radius] = A(1), A(2), ..., A(N)

N is the total number of elements in arrayOfRadius.

Reproducible example

>> clear
>> obj(4).Radius = 0;
>> disp([obj.Radius])
     0

>> arrayOfRadius = [1 2 3 4];
>> A = num2cell(arrayOfRadius);
>> [obj.Radius] = A{:};
>> disp([obj.Radius])
     1     2     3     4

My Two Cents...

FWIW, using the for loop approach is actually more readable. Doing this workaround really makes the code obfuscated... especially if you're only copying elements from an array to a structure. How many times is this copying going to be performed? If it's only once or a few times, stick with the for loop for readability.

edited Feb 1, 2016 at 19:44

answered Feb 1, 2016 at 19:38

rayryeng

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2 Comments

Diluvian Over a year ago

Thank you very much, too! The procedure repeats very often with a lot of elements. The for loop is significantly slower in that case, so I want to do the "fast" version with that approach.

rayryeng Over a year ago

@Diluvian ah I see. In that case, use num2cell and comma-separated lists. Good luck!

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