空的()在Matlab矩阵上有什么作用? [英] What does the empty `()` do on a Matlab matrix?

问题描述

在Matlab中，如果M是矩阵，表达式M()到底能做什么?

In Matlab, what exactly does the expression M() do where M is a matrix?

>> M = magic(3);
>> M() 

ans =

     8     1     6
     3     5     7
     4     9     2

在所有情况下表达式isequaln(M, M())是否都正确?是M()只是M的副本还是相同的表达式，或者在引用M()的意义上除了引用M以外还有其他含义吗?也许是在操作员超载的情况下?

Is the expression isequaln(M, M()) true under all circumstances? Is M() simply a copy of M, or an identical expression, or is there any context where referring to M() means something else than referring to M? Maybe in the case of operator overloading?

推荐答案

除了它会在某些功能(例如rand())和复活节彩蛋(例如imagesc()和spy()(这也可以在没有()的情况下使用，只要访问数组的维数小于5(似乎是@Rody Oldenhuis)，这似乎是访问整个数组的一种更有效的方法.例如:

Besides the fact the it would give the default operation on some function, such as rand(), and easter eggs such as imagesc() and spy() (this will work also without the ()) , it seems to be a more efficient way to access whole arrays as long as their dimensionality is below 5 (as @Rody Oldenhuis spotted) . For example:

a=rand(2^12);

tic
for j=1:1e5
a ;
end
toc

tic
for j=1:1e5
a(:)  ;
end
toc

tic
for j=1:1e5
a()   ; 
end
toc

产量:

Elapsed time is 0.047250 seconds.
Elapsed time is 0.022260 seconds.
Elapsed time is 0.011925 seconds.

但是，对于作业，a1=a与a1=a()之间几乎没有区别，后者的速度要慢1.5％...

However, for assignments there's very little difference between a1=a vs a1=a(), where the latter is slower by 1.5%...

也许该线程会回答您有关运算符重载的一些问题.

Perhaps this thread will answer some of your question regarding operator overloading.

这篇关于空的()在Matlab矩阵上有什么作用?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！