为什么|| 和/或在rails中表现不同?
可能重复:
在Ruby中,i = true和false是真的吗?
Perl(或,和)和(||,&&)短路运营商之间有什么区别?
Ruby:||之间的区别 和’或’
是||
or
Rails相同?
案例A:
@year = params[:year] || Time.now.year Events.all(:conditions => ['year = ?', @year])
将在script/console
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
生成以下SQL:
SELECT * FROM `events` WHERE (year = NULL)
||的原因 和/或表现不同是因为运算符优先级。
两个|| 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
所以你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,请使用括号使您的意思清楚。
引自http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Operators :
二进制“或”运算符将返回其两个操作数的逻辑析取。 它与“||”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
作为:
(@year = params[:year]) or Time.now.year
所以params[:year]
被分配给@year
,而表达式的第二部分没有分配给任何东西。 如果要使用或,则应使用显式括号:
@year = (params[:year] or Time.now.year)
这就是区别。