朱莉娅可变范围 [英] Julia Variable scope

问题描述

我正在尝试在while循环中使用一些全局变量(m，n，r)，但是julia 1.0.0告诉我这些变量未定义.该代码适用于julia 0.7.0，并带有一些警告. 这是我正在使用的代码(是的，写得不好，我希望这不是问题.为了简单起见，我删除了println语句):

I'm trying to use some global variables (m, n, r) in a while loop, but julia 1.0.0 is telling me that those variables aren't defined. The code works with julia 0.7.0, with some warnings. This is the code I'm using (yes, it's poorely written, I hope this is not the problem. I removed a println statement for the sake of simplicity):

m = readline()
n = readline()
m = parse(Int, m)
n = parse(Int, n)
r = m % n
while (r > 0)
        println( "m: $(m) n: $(n) r: $(r)" )
        r = m % n
        m = n
        n = r
end

这是茱莉亚1.0.0的结果:

This is the results with julia 1.0.0:

ERROR: LoadError: UndefVarError: m not defined
Stacktrace:
 [1] top-level scope at euclide.jl:11 [inlined]
 [2] top-level scope at ./none:0
 [3] include at ./boot.jl:317 [inlined]
 [4] include_relative(::Module, ::String) at ./loading.jl:1038
 [5] include(::Module, ::String) at ./sysimg.jl:29
 [6] exec_options(::Base.JLOptions) at ./client.jl:229
 [7] _start() at ./client.jl:421
in expression starting at euclide.jl:10

朱莉娅0.7.0:

┌ Warning: Deprecated syntax `implicit assignment to global variable `r``.
│ Use `global r` instead.
└ @ none:0
┌ Warning: Deprecated syntax `implicit assignment to global variable `m``.
│ Use `global m` instead.
└ @ none:0
┌ Warning: Deprecated syntax `implicit assignment to global variable `n``.
│ Use `global n` instead.
└ @ none:0

该代码适用于julia 0.7.0，但为什么不适用于1.0.0版?

The code works with julia 0.7.0, but why doesn't it work with version 1.0.0 ?

推荐答案

您必须像这样在循环内声明在全局范围中定义的变量，并在循环局部范围中将分配给声明为global :

You have to declare variables you define in global scope and assign to in loop local scope as global inside the loop like this:

m = readline()
n = readline()
m = parse(Int, m)
n = parse(Int, n)
r = m % n
while (r > 0)
        println( "m: $(m) n: $(n) r: $(r)" )
        global r = m % n
        global m = n
        global n = r
end

您必须执行此操作的原因是while循环引入了新的局部范围，因此，没有像m = n这样的global关键字分配就告诉Julia m是while循环内的局部变量，因此然后在println( "m: $(m) n: $(n) r: $(r)" )行中，Julia决定m尚未定义.

The reason you have to do it is that while loops introduce a new local scope, so without global keyword assignment like m = n tells Julia that m is a local variable inside a while loop so then in the line println( "m: $(m) n: $(n) r: $(r)" ) Julia decides that m is yet undefined.

另请参见 https://docs.julialang.org/zh_/latest/manual/variables-and-scoping/和

See also https://docs.julialang.org/en/latest/manual/variables-and-scoping/ and Why does this assignment inside a loop fail in Julia 0.7 and 1.0? for further explanation of the scoping rules in Julia.

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