价值或“默认”

Question

  

可能重复：
  Assigning a substitute value in case of nil

在Lua我使用

x = value or "default_if_value_is_nil"

（作为if value ~= nil then x = value end的快捷方式）

我可以在Ruby中做类似的事情吗？

Answer 1

x = value || "default_if_value_is_nil_or_false"

请注意这里的“或假”，尽管Lua也可以这样做。

Answer 2

你可以在ruby中实际做同样的事情

x = nil_value || "default"

请注意，这也适用于任何其他“假”值

x = false_value || "default"

Answer 3

x = value or "default_if_value_is_nil"

是完全合法的Ruby语句，但要注意or具有运算符优先级中最低优先级之一。还要知道在Ruby布尔操作中除了false和nil之外一切都是正确的，因此即使value不是nil而且是false，这个Ruby语句也会回答默认值。

puts '>>> assignment = has a higher priority than or <<<'
value = 'y'
x = value or "default_if_value_is_nil"
print 'value=', value.inspect, ', x=', x.inspect, "\n"

value = false
x = value or "default_if_value_is_nil"
print 'value=', value.inspect, ', x=', x.inspect, "\n"

value = nil
x = value or "default_if_value_is_nil"
print 'value=', value.inspect, ', x=', x.inspect, "\n"

puts '>>> put parenthesis around or expression to have a higher priority than = <<<'
value = 'y'
x = (value or "default_if_value_is_nil")
print 'value=', value.inspect, ', x=', x.inspect, "\n"

value = false
x = (value or "default_if_value_is_nil")
print 'value=', value.inspect, ', x=', x.inspect, "\n"

value = nil
x = (value or "default_if_value_is_nil")
print 'value=', value.inspect, ', x=', x.inspect, "\n"

puts '>>> || has a higher priority than = <<<'
value = 'y'
x = value || "default_if_value_is_nil"
print 'value=', value.inspect, ', x=', x.inspect, "\n"

value = false
x = value || "default_if_value_is_nil"
print 'value=', value.inspect, ', x=', x.inspect, "\n"

value = nil
x = value || "default_if_value_is_nil"
print 'value=', value.inspect, ', x=', x.inspect, "\n"

输出：

>>> assignment = has a higher priority than or <<<
value="y", x="y"
value=false, x=false
value=nil, x=nil
>>> put parenthesis around or expression to have a higher priority than = <<<
value="y", x="y"
value=false, x="default_if_value_is_nil"
value=nil, x="default_if_value_is_nil"
>>> || has a higher priority than = <<<
value="y", x="y"
value=false, x="default_if_value_is_nil"
value=nil, x="default_if_value_is_nil"