问题
So, I've just learned that instead of writing things like:
[1,2,3,4,5].inject {|x,y| x + y} => 15
I could write
[1,2,3,4,5].inject(:+) => 15
I also learned that instead of writing
[1,2,3,4,5].select {|x| x.even?} => [2,4]
I could write
[1,2,3,4,5].select(&:even?) => [2,4]
My question is why one (select) uses the & and the other one (inject) doesn't. I'm sure that the : are because even? and + are treated at symbols, but I'd love clarification behind why the & is used in one and why the : are used.
Also, I'm aware that I could do these notations on more than just inject and select.
Many thanks!
回答1:
& operator in this case converts a symbol to a proc. So the code does this under the hood:
[1,2,3,4,5].select {|elem| elem.send :even? } # => [2, 4]
Implementation of inject method recognizes the need for these shortcut method specification and adds a special handling for when symbol is passes as a parameter. So, in this case, it basically does what & operator does.
why one (select) uses the & and the other one (inject) doesn't
Because nobody implemented select this way. Ruby is open-source, they may even accept your patch. Go and fix this :)
P.S.: But if it were up to me, I would instead remove inject's special handling. It feels a little bit redundant and confusing in presence of Symbol#to_proc (that's what & operator uses).
来源：https://stackoverflow.com/questions/16605597/ruby-trying-to-grasp-a-new-notation-inject-vs-selecteven-why-one-has