property access of objects arrays

johhny boy さんによって質問されました 2012 年 2 月 15 日

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Create an object array and then try to access a public property of this array

C(1,1) = class1;
C(2,1) = class1;
C(1,2) = class1;
C(2,2) = class1;

%suppose class1 has a public property called 'a' , then :

C.a % returns C(1,1).a
[o1 o2 o3 o4] = C.a %returns o1 = C(1,1).a, o2 = C(2,1).a, o3 = C(1,2).a and o4 = C(2,2).a
[C.a] %returns [C(1,1).a C(2,1).a C(1,2).a C(2,2).a]

when you look at the above its looks like it acts as a method, where the 'a' property is called for each of the elements of the array. But I cant understand the last statement [C.a] .

When working with a normal function, putting square brackets around a function call does not make it return more than 1 output (if you call it without more than one output argument), but in this case it does. So this is not exactly a function call either.

Can anyone explain whats going on here

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回答者: Daniel Shub 2012 年 2 月 15 日

A potentially related question: http://www.mathworks.com/matlabcentral/answers/12116-cell-array-expansion

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回答者: Titus Edelhofer 2012 年 2 月 15 日

Hi Johhny,

the [] work here similar to accessing a field of a structure. There it is easy to explain using "comma seperated lists". Suppose you have the following structure:

x(1).a = 1;
x(2).a = 2;

Now if you access a via

x.a

this is treated by MATLAB as if you typed

x(1).a, x(2).a

Now if you combine using [], you get a vector:

y = [x(1).a x(2).a]
% or shorter
y = [x.a]

This is the same as for your class ...

Same by the way for strings and cells:

x(1).b = 'Hello';
x(2).b = 'MATLAB';
z = {x.b}

Titus

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johhny boy 2012 年 2 月 16 日

OK, fair enough. Does this mean that if it is a 2x2 structure

x(1,1).a = 1;

x(1,2).a = 2;

x(2,1).a = 3;

x(2,2).a = 4;

So should x.a mean

x(1).a, x(2).a, x(3).a, x(4).a

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