julia变量的范围:在开放表达式中的循环内重新分配 [英] scope of julia variables: re-assigning within a loop in open expression

问题描述

我正在努力在Julia中的循环中重新分配变量.我有一个以下示例:

I am struggling with re-assigning a variable in a loop in Julia. I have a following example:

infile = "test.txt"

feature = "----"

for ln in 1:3
    println(feature)
    feature =  "+"
end



open(infile) do f
#if true
    # println(feature)
    # feature = "----"
    println(feature)
    for ln in 1:5 #eachline(f)
        println("feature")
        #fails here
        println(feature)
        # because of this line:
        feature =  "+"
    end
end

如果我在循环中重新分配，它将失败.我看到了变量作用域的问题，因为涉及嵌套作用域. 引用表示，循环引入了软' 范围.我无法从手册中找到范围open表达式所属的内容，但是看起来它搞砸了，就好像我将open替换为if true一样，事情运行也很顺利.

It fails if I do reassignment within the loop. I see that the issue with the variable scope, and because nested scopes are involved. The reference says that the loops introduce 'soft' scope. I cannot find out from the manual what scope open expression belongs to, but seemingly it screws things up, as if I replace open with if true, things run smoothly.

我是否正确理解open引入了硬"作用域，这就是为什么重新分配追溯地使变量不确定的原因?

Do I understand correctly that open introduces 'hard' scope and that it is the reason why re-assignment retroactively makes the variable undefined?

推荐答案

您应该想到的

open("file") do f
    ...
end

为

open(function (f)
    ...
end, "file")

也就是说，do引入了与function或->相同的硬作用域.

that is, do introduces the same kind of hard scope as a function or -> would.

因此要能够从该函数写入feature，您需要做

So to be able to write to feature from the function, you need to do

open(infile) do f
    global feature  # this means: use the global `feature`
    println(feature)
    for ln in 1:5
        println("feature")
        println(feature)
        feature =  "+"
    end
end

请注意，只有在顶级(模块)范围内才是这种情况；一旦进入函数，就没有硬作用域.

Note that this is only the case in top-level (module) scope; once inside a function, there are no hard scopes.

(在这种情况下，for循环是一条红色鲱鱼；无论循环的软作用域如何，对feature的访问都将受到do引入的匿名函数的硬作用域的限制.)

(The for loop in this case is a red herring; regardless of the soft scope of the loop, the access to feature will be limited by the hard scope of the anonymous function introduced by do.)

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