用= []与重新分配矩阵删除矩阵元素 [英] Deleting matrix elements by = [] vs reassigning matrix

问题描述

这两种在Matlab中删除元素的方法之间是否有区别:

Is there any difference between these two methods for deleting elements in Matlab:

ElementsToDelete = [0 0 1 0 1 0 0 1 1 0]

A = 1:10
A(ElementsToDelete) = []

%Versus

A = 1:10
A = A(~ElementsToDelete)

有没有一种方法比另一种更合适?效率上有区别吗?还是它们完全可以互换?

Are there times when one method is more appropriate than the other? Is there a difference in efficiency? Or are they completely interchangeable?

推荐答案

尝试一下:

A = rand(1e3, 1);
b = A<0.5;

tic; 
for ii = 1:1e5
    a = A;       
    a(b) = [];
end
toc

tic; 
for ii = 1:1e5
    a = A;        
    a = a(~b);
end
toc

结果:

Elapsed time is 1.654146 seconds
Elapsed time is 1.126325 seconds

因此，差异是速度因数1.5，有利于重新分配.但是，这种情况更糟:

So the difference is a speed factor of 1.5 in favour of re-assigning. This however, is worse:

A = rand(1e4, 1);

stop = 0;    
for jj = 1:10
    a = A;
    start = tic;
    for ii = 1:1e5
        a(a < rand) = [];
    end
    stop = stop + toc(start);
end
avg1 = stop/10


stop = 0;    
for jj = 1:10
    a = A;
    start = tic;
    for ii = 1:1e5
        a = a(a > rand);
    end
    stop = stop + toc(start);
end
avg2 = stop/10

avg1/avg2

结果:

avg1 = 1.1740235 seconds
avg2 = 0.1850463 seconds

avg1/avg2 = 6.344485136963019

因此，该因子增加到了6以上.

So, the factor's increased to well over 6.

我的猜测是删除(即用[]赋值)通过逻辑索引在内部循环中每次出现true时都会重写整个数组.这是无可救药的，效率低下，就像这样测试时很明显.另一方面，重新分配可以事先确定新数组的大小，并相应地对其进行初始化.无需重写.

My guess is that deletion (i.e., assigning with []) re-writes the entire array on each and every occurrence of a true in the internal loop through the logical indices. This is hopelessly inefficient, as becomes apparent when testing it like this. Re-assigning on the other hand can determine the size of the new array beforehand and initialize it accordingly; no re-writes needed.

为什么JIT不能将其中一个编译为另一个对我来说是个谜，因为删除是一种更加直观的表示法恕我直言.但是，如您所见，与替代方法相比，它效率低下，因此应谨慎使用. 切勿在循环内使用它！

Why the JIT does not compile the one into the other is a mystery to me, because deletion is a far more intuitive notation IMHO. But, as you see, it is inefficient compared to alternatives, and should thus be used sparingly. Never use it inside loops!

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