R 中的“retracemem”有什么作用？

发布于 2024-12-03 09:57:40 字数 803 浏览 3 评论 0原文

回答关于 tracemem 最近，我了解了 retracemem。 ?retracemem 的帮助和其中的示例让我一无所知。

retracemem 实际上做了什么？为什么？它没有做我想象的那样，即将一个对象指向另一个对象的内存位置，至少据我所知。

我尝试了像 .Internal(inspect()) 这样的神奇调用，它对我没有任何作用：

> a = 1:10
> b = a[-1]
> .Internal(inspect(a))
@00000000087AE578 13 INTSXP g0c4 [NAM(2)] (len=10, tl=23336) 1,2,3,4,5,...
> .Internal(inspect(b))
@00000000087AE8E8 13 INTSXP g0c4 [NAM(2)] (len=9, tl=7208) 2,3,4,5,6,...
> retracemem(b,retracemem(a))
> .Internal(inspect(b))
@00000000087AE8E8 13 INTSXP g0c4 [NAM(2)] (len=9, tl=7208) 2,3,4,5,6,...

原文

After answering about tracemem recently, I learned about retracemem. The help for ?retracemem and the example therein leave me unenlightened.

What does retracemem actually do and why? It doesn't do what I thought it would do, i.e. point one object to the memory location of another, at least as far as I can tell.

I tried a wizardly invocation like .Internal(inspect()), which performs no magic for me:

> a = 1:10
> b = a[-1]
> .Internal(inspect(a))
@00000000087AE578 13 INTSXP g0c4 [NAM(2)] (len=10, tl=23336) 1,2,3,4,5,...
> .Internal(inspect(b))
@00000000087AE8E8 13 INTSXP g0c4 [NAM(2)] (len=9, tl=7208) 2,3,4,5,6,...
> retracemem(b,retracemem(a))
> .Internal(inspect(b))
@00000000087AE8E8 13 INTSXP g0c4 [NAM(2)] (len=9, tl=7208) 2,3,4,5,6,...

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小糖芽 2024-12-10 09:57:40

我认为 retracemem() 只能让您标记一个变量副本，这不会产生 Tracemem 语句（例如上面的 b 它实际上只是 的副本>a 没有第一个元素），因为是从同一源变量派生的，因此您可以继续观察副本/部分副本传播并查看它们是否从同一源派生。

例如，A 的内存如何被复制/传播：

> A <- 1:10
> tracemem(A)
[1] "<0x100a2a978>"
> B <- A                                # Assignment to B doesn't make copy
> C <- A + 1                            # Assignment to C makes copy, alters it
tracemem[0x100a2a978 -> 0x1020ebbf0]: 
> D <- C + 1                            # Assignment to D makes copy, alters it
tracemem[0x1020ebbf0 -> 0x1020ebc98]: 
> E <- B + 1                            # Assignment to E makes copy, alters it
tracemem[0x100a2a978 -> 0x1020a4208]: 
> F <- A[-1]                            # Assignment to F doesn't make copy?
> G <- F + 1                            # Even after altering it?
> retracemem(F, retracemem(A))          # Hint to R that F is really A derived
tracemem[<0x100a2a978> -> 0x1009c5910]: 
> G <- F + 1                            # Reassignment to G makes copy, alters it
tracemem[0x1009c5910 -> 0x1020a4748]:

但话又说回来，我可能完全错了......

I think retracemem() just lets you tag a variable copy, which wouldn't produce a tracemem statement (such as a b above which is really just a copy of a sans the first element), as being derived from the same source variable, so you can continue to watch the copies/partial copies propagate and see that they derive from the same source.

For example, how does A's memory get copied/propagated:

> A <- 1:10
> tracemem(A)
[1] "<0x100a2a978>"
> B <- A                                # Assignment to B doesn't make copy
> C <- A + 1                            # Assignment to C makes copy, alters it
tracemem[0x100a2a978 -> 0x1020ebbf0]: 
> D <- C + 1                            # Assignment to D makes copy, alters it
tracemem[0x1020ebbf0 -> 0x1020ebc98]: 
> E <- B + 1                            # Assignment to E makes copy, alters it
tracemem[0x100a2a978 -> 0x1020a4208]: 
> F <- A[-1]                            # Assignment to F doesn't make copy?
> G <- F + 1                            # Even after altering it?
> retracemem(F, retracemem(A))          # Hint to R that F is really A derived
tracemem[<0x100a2a978> -> 0x1009c5910]: 
> G <- F + 1                            # Reassignment to G makes copy, alters it
tracemem[0x1009c5910 -> 0x1020a4748]:

But then again, I could be entirely wrong...

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