如何在ruby中使用哈希值获取默认值

Question

我想在ruby中使用哈希时尝试获取默认值.查找文档,您使用了一个fetch方法.因此,如果未输入哈希,则默认为值.这是我的代码.

def input_students
  puts "Please enter the names and hobbies of the students plus country of    birth"
  puts "To finish, just hit return three times"

  #create the empty array
  students = []
  hobbies = []
  country = []
  cohort = []

  # Get the first name
  name = gets.chomp
  hobbies = gets.chomp
  country = gets.chomp
  cohort = gets.chomp

  while !name.empty? && !hobbies.empty? && !country.empty? && cohort.fetch(:cohort, january) do #This is to do with entering twice
    students << {name: name, hobbies: hobbies, country: country, cohort: cohort} #import part of the code.
    puts "Now we have #{students.count} students"

    # get another name from the user
    name = gets.chomp
    hobbies = gets.chomp
    country = gets.chomp
    cohort = gets.chomp
  end
  students
end

Answer 1

你有几个选择.@dinjas提到了一个,可能是你想要使用的那个.假设您的哈希是

h = { :a=>1 }

然后

h[:a] #=> 1
h[:b] #=> nil

假设默认值是4.然后就像dinjas所说,你可以写

h.fetch(:a, 4) #=> 1
h.fetch(:b, 4) #=> 4

但其他选择是

h.fetch(:a) rescue 4 #=> 1
h.fetch(:b) rescue 4 #=> 4

要么

h[:a] || 4 #=> 1
h[:b] || 4 #=> 4

您还可以使用Hash#default =将默认值构建到哈希本身:

h.default = 4
h[:a] #=> 1
h[:b] #=> 4

或者像这样定义哈希:

g = Hash.new(4).merge(h)
g[:a] #=> 1
g[:b] #=> 4

见Hash :: new.

Answer 2

你只需要给fetch它一个可以处理的默认值.january由于您没有声明具有该名称的任何变量,因此它不知道该怎么做.如果要将默认值设置为字符串"january",则只需要像这样引用它:

cohort.fetch(:cohort, "january") 

文档中fetch有一些不错的例子.

另外,cohort是不是Hash,这是一个String自gets.chomp回报String.fetch用于从a中"获取"值Hash.你使用它的方式应该抛出类似于的错误:undefined method 'fetch' for "whatever text you entered":String.

最后,由于您在条件中使用它,因此您的调用结果将fetch被评估其真实性.如果您设置了默认值,则始终将其评估为true.

如果你只是想设置一个默认值,cohort如果它是空的,你可以这样做:

cohort = gets.chomp
cohort = "january" if cohort.empty?
while !name.empty? && !hobbies.empty? && !country.empty?
  students << {
    name: name,
    hobbies: hobbies,
    country: country,
    cohort: cohort
  }
  ... # do more stuff

希望这很有帮助.