With a = 1:4 and b = [], why does a(1) = [] remove an element while a(1) = b returns an error?

Some example code:

a = 1:4;
a(1) = [];
assert(isequal(a,2:4))

b = [];
a(1) = b; % Returns an error.

What is going on here?

I am using R2017b.

Answer 1

[Stephen23]

Basically this is a "special" syntax:

X(...) = []

https://www.mathworks.com/help/matlab/matlab_prog/matlab-operators-and-special-characters.html#bvg3oy_-5

Do not think of the RHS as being a variable. Think of it as being just that exact syntax.

If MATLAB was consistent (which it isn't), then both cases would be interpreted as attempts to allocate an empty array to a non-empty part of an array (which of course is not possible, hence the error). However because this allocation would always be an error and the syntax is really quite convenient, apparently the designers of MATLAB took this compact syntax (that is otherwise entirely wasted) and changed it by definition to remove rows/columns/... from arrays. So the answer to your question is "by definition".

Note that it really does not matter how you try to define the RHS, you will get an error:

>> A = 1:4;
>> B = [];
>> A(1) = B
???  In an assignment  A(I) = B, the number of elements in B and
 I must be the same.

>> A(1) = zeros(0)
???  In an assignment  A(I) = B, the number of elements in B and
 I must be the same.

>> B = 5:9;
>> A(1) = B(2:1)
??? Improper assignment with rectangular empty matrix.

>> A(1) = 1:0
??? Improper assignment with rectangular empty matrix.

The only way is to use that exact syntax, because that is its purpose by design:

>> A = 1:4;
>> A(1) = []
A =
     2     3     4

[Skyx]

Is there a named method for this special case that itself could be overloaded to give a different behavior for certain classes?

[Matt J]

There's no method handling just that particular case only, but you can overload subsasgn() to control it.

For an example, you can look at my zero-based indexable array class:

>> a=ZeroBased(0:3), b=[];

a =

       0     1     2     3

>> a(2)=b

a =

       0     1     3

[Image Analyst]

Skyx, what different behavior do you want it to do? Do you want to delete/remove elements? Set elements to null but not remove them (I believe requires a cell array)? Something else???

[Skyx]

I was thinking that if I have an array of my own class (say, MyClass), it might be nice to be able to remove elements not only with the ... = [] syntax, but also with something akin to ... = MyClass.empty. However, since, the ... = [] syntax is really a special case, I guess I shouldn't be trying for this.