Julia:数组中的赋值 [英] Julia: Assignment in Arrays

问题描述

当为数组索引多个级别时，它可以正常工作.但是当我用它来赋值时，它没有.有谁知道为什么 A 下面没有改变?

When indexing more than one level for an array, it works fine. But when I used it to assign values, it did not. Does anyone know why A does not change below?

In  [4]: A = rand(6)

Out [4]: 6-element Array{Float64,1}:
 0.111552
 0.155126
 0.78485 
 0.147477
 0.362078
 0.959022

In  [5]: A[3:5][[true,false,true]]

Out [5]: 2-element Array{Float64,1}:
 0.78485 
 0.362078

In  [6]: A[3:5][[true,false,true]] = [99, 999]

Out [6]: 2-element Array{Int64,1}:
  99
 999

In  [7]: A

Out [7]: 6-element Array{Float64,1}:
 0.111552
 0.155126
 0.78485 
 0.147477
 0.362078
 0.959022

推荐答案

这是因为按范围和向量索引数组会返回一个带有输出的新数组(而不是原始数组的视图).您的声明相当于以下内容:

This is because indexing arrays by ranges and vectors returns a new array with the output (instead of a view into the original array). Your statement is equivalent to the following:

julia> A = rand(6)
6-element Array{Float64,1}:
 0.806919
 0.445286
 0.882625
 0.556251
 0.719156
 0.276755

julia> B = A[3:5]
3-element Array{Float64,1}:
 0.882625
 0.556251
 0.719156

julia> B[[true,false,true]] = [99, 999]
2-element Array{Int64,1}:
  99
 999

julia> A'
1x6 Array{Float64,2}:
 0.806919  0.445286  0.882625  0.556251  0.719156  0.276755

julia> B'
1x3 Array{Float64,2}:
 99.0  0.556251  999.0

您实际上可以通过一些表达式实用程序看到 Julia 正在做的事情.注意显式括号——它调用了 setindex！关于索引的结果，它已经做了一个副本.(GenSym() 是一种指定临时变量的内部方式):

You can actually see that this is what Julia is doing through some of its expression utilities. Note the explicit parentheses — it's calling setindex! on the result of indexing, which has made a copy. (GenSym() is an internal way of specifying a temporary variable):

julia> :(A[3:5][[true,false,true]] = [99, 999])
:((A[3:5])[[true,false,true]] = [99,999])

julia> expand(:(A[3:5][[true,false,true]] = [99, 999]))
:(begin
        GenSym(0) = (top(vect))(99,999)
        setindex!(getindex(A,colon(3,5)),GenSym(0),(top(vect))(true,false,true))
        return GenSym(0)
    end)

目标是最终让所有数组索引返回视图而不是副本，但这仍然是一项正在进行的工作.

The goal is to eventually have all array indexing return views instead of copies, but that's still a work-in-progress.

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