无法从函数返回值:错误:加载错误:参数错误:"不应打印任何内容";请使用"show&quo

Question

我一直在和Julia一起解决哈佛CS50的问题集。这个脚本是我[多数选举]的解决方案。1

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)

当我运行此脚本时，出现以下错误

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

错误消息中的表达式称为"表达式从/home/jerzy/C 开始.../plurality.jl：65"是脚本的姓氏。 我不明白这是什么nothing？ 尽管如此，按照错误消息的建议，我修改了代码的最后一行，将其从：

println(winner)

自

show(winner)

并得到以下输出：

无

我在这里和那里做了一些研究，但作为一个新手，我不明白为什么我不能从我的函数print_winner返回一个值。从我读到的内容来看，返回声明不是强制性的。

在print_winner的定义中，当我替换

candidates[i].name

跟

println(candidates[i].name)

然后当最后一行是

winner = print_winner()

然后我终于能够得到获胜者的名字。但这不是我想要的方式。我想返回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中做到这一点，为什么我不能在Julia中做到这一点？

Answer 1

函数print_winner不返回任何内容，在这种情况下，对象nothing实际上被返回。所以winner得到值nothing(来自winner = print_winner()(，println(winner)等效于println(nothing)，这会导致错误。

我想返回一个值并将其分配给一个变量

然后这样做：从print_winner返回一个值。Julia 无法知道您希望此函数返回什么，因此您必须明确说明它。默认情况下，Julia 返回函数表达式的值，在本例中是最后一个表达式的结果，此处为for循环。for循环的表达式值在 Julia 中nothing。