Question

以下两个代码段应该相同，但是它们返回不同的输出

movies = {
  StarWars: 4.8, 
  Divergent: 4.7
  }

puts "What would you like to do? "

choice = gets.chomp

case choice
when "add"
  puts "What movie would you like to add? "
  title = gets.chomp
  if movies[title.to_sym] = nil
    puts "What rating does the movie have? "
    rating = gets.chomp
    movies[title.to_sym] = rating.to_i
    puts "#{title} has been added"
  else
    puts "That movie already exists! Its rating is #{movies[title.to_sym]}."
  end

movies = {
  StarWars: 4.8, 
  Divergent: 4.7
  }

puts "What would you like to do? "

choice = gets.chomp

case choice
when "add"
  puts "What movie would you like to add? "
  title = gets.chomp
  if movies[title.to_sym].nil?
    puts "What rating does the movie have? "
    rating = gets.chomp
    movies[title.to_sym] = rating.to_i
    puts "#{title} has been added"
  else
    puts "That movie already exists! Its rating is #{movies[title.to_sym]}."
  end

输出1： 你想干什么？加您要添加什么电影？星球大战那部电影已经存在！评分是。

输出2： 你想干什么？加您要添加什么电影？星球大战那部电影已经存在！评分是4.8。

PS：我没有粘贴其余的代码，因为这是不相关的

Answer 1

文件之间的差异归结为这两行：

if movies[title.to_sym] = nil

和

if movies[title.to_sym].nil?

第一个是赋值语句，第二个是测试从movies哈希中获取的值是否为nil。

问题在于赋值语句的结果被有条件地评估为错误布尔值：moves[title.to_sym]的值设置为nil，然后条件量为if nil。

流行测验。以下程序的输出是什么？为什么？

h = {a: 42}

p "one" if h[:a] == 42
p "two" if h[:a] == nil
p "three" if h[:a] = 42
p "four" if h[:a] = nil

答案：

“一个”“三个”（第一个条件是成功的典型比较；第二个是失败的典型比较；第三个是返回真实值42的赋值；第四个是返回a的真实值）假值，无）

尽管存在争议，但Yoda conditions解决了这个偶然的分配错误：

p "yoda assignment" if nil = 42

输出：

(repl):1: Can't assign to nil
p "yoda assignment" if nil = 42

以下两个代码段之间有什么区别？

1 个答案: