Cannot return value from a function: ERROR: LoadError: ArgumentError: `nothing` should not be printed; use `show`, `repr`, or

Question

我一直在用 Julia 解决哈佛 CS50 的问题集。这个脚本是我对 [plurality elections.]1

的解决方案

println("How many contenders do we have?")
const max_candidates = parse(Int, readline()) # a maximal number of candidates


# Let us define a composite type for the candidates in our elections
mutable struct Candidate
    name::String
    votes::Int64
    end
 
function vote(name)
for i in 1:max_candidates
        if candidates[i].name == name
        candidates[i].votes = candidates[i].votes + 1 
         end
       end
      end

function print_winner()
 max_votes = 0
 for i in 1:max_candidates
  if candidates[i].votes > max_votes
     max_votes = candidates[i].votes
     end
  end
  for i in 1:max_candidates
   if candidates[i].votes == max_votes
    candidates[i].name
    end
   end 
 end
  
candidates = Vector{Candidate}(undef, max_candidates)

for i in 1:max_candidates -1 
 println("Name of the candidate: ?")
 name = readline()
 votes = 0
 candidates[i] = Candidate(name, votes)
 println("Thank you, let us move to the next candidate.")
end

#The last candidate i registered outside of the loop because I do no want 
#the line println("Thank you, let us move to the next candidate.") to be executed after them.
println("Name of the last candidate: ?")
 name = readline()
 votes = 0
 candidates[max_candidates] = Candidate(name, votes)
 
println("How many voters do we have?")
voter_count = parse(Int, readline())

for i in 1:voter_count 
 println("Who are you voting for?")
 name = readline()
 vote(name)
 end 
 
winner = print_winner()

println(winner)

当我运行这个脚本时，我得到以下错误

ERROR: LoadError: ArgumentError: `nothing` should not be printed; use `show`, `repr`, or custom output instead.
Stacktrace:
 [1] print(::Base.TTY, ::Nothing) at ./show.jl:566
 [2] print(::Base.TTY, ::Nothing, ::Char) at ./strings/io.jl:42
 [3] println(::Base.TTY, ::Nothing) at ./strings/io.jl:69
 [4] println(::Nothing) at ./coreio.jl:4
 [5] top-level scope at none:0
 [6] include at ./boot.jl:317 [inlined]
 [7] include_relative(::Module, ::String) at ./loading.jl:1044
 [8] include(::Module, ::String) at ./sysimg.jl:29
 [9] exec_options(::Base.JLOptions) at ./client.jl:266
 [10] _start() at ./client.jl:425
in expression starting at /home/jerzy/C.../plurality.jl:65

错误消息中称为“表达式开始于 /home/jerzy/C.../plurality.jl:65”的表达式是脚本的姓氏。我不明白这是什么nothing？尽管如此，根据错误消息的建议，我修改了代码的最后一行，将其更改为：

println(winner)

至

show(winner)

并得到以下输出：

nothing

我做了一些 research here and there，但作为新手，我不明白为什么我不能 return 我的函数 的值print_winner。根据我的阅读，return 陈述不是强制性的。

在print_winner的定义中，当我代入

candidates[i].name

与

println(candidates[i].name)

然后当最后一行是

winner = print_winner()

然后我终于可以知道获胜者的名字了。但这不是我想要的方式。我想 return 一个值并将其分配给一个变量，然后用这个变量做一些事情。我可以在 PHP 或 Racket 中执行此操作，为什么我不能在 Julia 中执行此操作？

Answer 1

函数 print_winner 没有 return 任何东西，在这种情况下，对象 nothing 实际上是 returned。所以winner得到值nothing（从winner = print_winner()），println(winner)等同于println(nothing)，这就导致了错误

I want to return a value and assign it to a variable

然后就这样做：return 来自 print_winner 的值。 Julia 无法知道您希望此函数 return 做什么，因此您必须明确说明。默认情况下，Julia return 是函数表达式的值，在本例中是最后一个表达式的结果，这里是一个 for 循环。 for 循环的表达式值在 Julia 中是 nothing。

Cannot return value from a function: ERROR: LoadError: ArgumentError: `nothing` should not be printed; use `show`, `repr`, or

Cannot return value from a function: ERROR: LoadError: ArgumentError: `nothing` should not be printed; use `show`, `repr`, or

void

julia