为什么||或者在rails中表现不同？ [复制]

Question

可能的重复：
i = true and false in Ruby is true?
What is the difference between Perl's ( or, and ) and ( ||, && ) short-circuit operators?
Ruby: difference between || and 'or'

|| 是否与 Rails 中的 or 相同？

案例A：

 @year = params[:year] || Time.now.year
 Events.all(:conditions => ['year = ?', @year])

将在script/console 中生成以下SQL：

 SELECT * FROM `events` WHERE (year = 2000)

案例 B：

 @year = params[:year] or Time.now.year
 Events.all(:conditions => ['year = ?', @year])

将在script/console中产生以下SQL：

 SELECT * FROM `events` WHERE (year = NULL)

Answer 1

原因|| and or 行为不同是因为运算符优先级。

两者||和 && 的优先级高于赋值运算符，赋值运算符 (=) 的优先级高于和/或

因此，您的表达式实际上将按如下方式进行评估：-

@year = params[:year] || Time.now.year

被评估为

@year = ( params[:year] || Time.now.year )

和

@year = params[:year] or Time.now.year

被评估为

( @year = params[:year] ) or Time.now.year

如果对优先规则有疑问，请使用括号来明确您的意思。

Answer 2

引用http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Operators:

二元“或”运算符将返回其两个操作数的逻辑析取。与“||”相同但优先级较低。

a = nil
b = "foo"
c = a || b  # c is set to "foo" its the same as saying c = (a || b)
c = a or b  # c is set to nil   its the same as saying (c = a) || b which is not what you want.

所以你or 的工作方式是：

(@year = params[:year]) or Time.now.year

所以params[:year] 分配给@year，表达式的第二部分没有分配给任何东西。如果你想使用 or，你应该使用显式括号：

@year = (params[:year] or Time.now.year)

这就是区别。