arrays - "ERROR: LoadError: BoundsError: attempt to access 0-element Array{Candidate,1} at index [1]"尝试填充复合类型的空数组时

Question

尝试使用用户输入填充空数组时出现错误。

const max = 9 # a maximal number of candidates

# Let us define a composite type for the candidates in our elections
mutable struct Candidate
    name::String
    votes::Int8
 end
 
 
candidates = Candidate[]

i = 1

while i < max
 println("Name of the candidate: ?")
 candidates[i].name = readline();
 println("Votes on him: ?")
 candidates[i].votes = parse(Int8, readline(), base=10);
 println("Thank you, let us move to the next candidate.")
 global i = i +1    
 end

在显示(“候选人姓名:？”)之后，我收到以下错误:

ERROR: LoadError: BoundsError: attempt to access 0-element Array{Candidate,1} at index [1]
Stacktrace:
 [1] getindex(::Array{Candidate,1}, ::Int64) at ./array.jl:731
 [2] top-level scope at /home/jerzy/ComputerScience/SoftwareDevelopment/MySoftware/MyJulia/plurality.jl:18 [inlined]
 [3] top-level scope at ./none:0
 [4] include at ./boot.jl:317 [inlined]
 [5] include_relative(::Module, ::String) at ./loading.jl:1044
 [6] include(::Module, ::String) at ./sysimg.jl:29
 [7] exec_options(::Base.JLOptions) at ./client.jl:266
 [8] _start() at ./client.jl:425
in expression starting at /home/jerzy/ComputerScience/SoftwareDevelopment/MySoftware/MyJulia/plurality.jl:16

或者，使用

candidates = Array{Candidate}(undef, 0)

代替

candidates = Candidate[]

结果是:

ERROR: LoadError: UndefRefError: access to undefined reference

很抱歉成为这样的新手。我靠 what I read in this wikibook .你能推荐我多读一些吗？

Answer 1

你几乎是正确的，问题是你的数组的长度是 0(你可以用 length(candidates) 验证它)，所以当你试图用 candidates[i] 设置数组的非零索引元素时，Julia 会提示.如果您事先不知道数组的长度，那么您应该使用 push!功能。

const max_candidates = 9 # a maximal number of candidates

while i < max_candidates
 println("Name of the candidate: ?")
 name = readline();
 println("Votes on him: ?")
 votes = parse(Int, readline());
 push!(candidates, Candidate(name, votes))
 println("Thank you, let us move to the next candidate.")
 global i = i + 1
end

我这里换了 max至 max_candidates因为 max干扰 Base max功能。
如果您知道候选人的数量，您可以使用 candidates = Vector{Candidate}(undef, max_candidates)初始化形式，注意 max_candidates而不是 0 ，因为您应该分配必要长度的向量。

candidates = Vector{Candidate}(undef, max_candidates)

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

请注意，我更改了 while至 for它可能对您的情况有用也可能没有用，但至少可以让您删除 global i = i + 1线。
如果最后一个版本适合您，那么您可能可以删除 mutable从结构定义来看，它通常对性能更好。