if 关键字

if condition
  # code here, executed if condition evaluates to true.
end

if x > 100 then puts x end

if x > 100; puts x end

if condition
  # code executed if condition is true.
else
  # code executed if conditions is false.
end

if condition1
  # code executed if condition1 is true.
elsif condition2
  # code executed if condition2 is true.
elsif condition3
  # code executed if neither condition1
  # nor condition2 is true, but condition3 is true.
end

print "Enter a integer: "
n = gets.to_i
if n > 0
  puts "Your number is positive."
elsif n < 0
  puts "Your number is negative."
else
  puts "Your number is zero."
end

puts "Big numbe!" if x > 100

unless

if not (x == 1)
if !(x == 1)
unless x == 1

case 语句以一个表达式开始，然后处理列出得各种可能得匹配。每一个可能的匹配包含在一个 when 表达式中，该表达式由一个或多个可能的匹配和一段代码构成。

print "Exit the program? (yes or no): "
answer = gets.chomp
case answer
when "yes", "y"
  puts "Good-bye"
  exit
when "no", "n"
  puts "OK, we'll continue"
else
  puts "That's an unknow answer -- assuming you meant 'no'"
end

用 loop 方法实现无条件循环。loop 方法不带任何参数，但它可以带一个代码块。代码块包含想要循环执行的代码。代码块可以是大括号 {} 或者用关键字 do 和 end 。当然你也可以使用 break 来中断循环。或者使用 next 来跳到下一个循环。

n = 1
loop do
  n = n + 1
  if unless n == 7
  break if n > 9
end

条件循环可以根据 while 和 until 实现。

n = 1
while n < 11
  puts n
  n = n + 1
end
puts "Done!"

n = 1
begin
  puts n
  n = n + 1
end while n < 11
puts "Done!"

n = 1
until n > 10
  puts n
  n = n + 1
end

n = 1
n = n + 1 until n == 10
puts "We've reached 10!"

基于值列表的循环

celsius = [0, 10, 20, 30, 40, 50, 60, 70]
puts "Celsius\tFahrenheit"
for c in celsius
  puts "c\t#{Temperature.c2f(c)}"
end

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原文链接 : http://railser.cn/index.php/blog/ruby-for-rails-5

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