Question

I often plug pre-configured lambdas into enumerable methods like ‘map’, ‘select’ etc. but the behavior of ‘inject’ seems to be different. e.g. with

mult4 = lambda {|item| item * 4 } 

then

(5..10).map &mult4 

gives me

[20, 24, 28, 32, 36, 40] 

However, if I make a 2-parameter lambda for use with an inject like so,

multL = lambda {|product, n| product * n } 

I want to be able to say

(5..10).inject(2) &multL 

since ‘inject’ has an optional single parameter for the initial value, but that gives me …

irb(main):027:0> (5..10).inject(2) &multL LocalJumpError: no block given         from (irb):27:in `inject'         from (irb):27 

However, if I stuff the ‘&multL’ into a second parameter to inject, then it works.

irb(main):028:0> (5..10).inject(2, &multL) => 302400 

My question is ‘why does that work and not the previous attempt?’

Answer 1

So the reason that

(5..10).map &mult4 

works and

(5..10).inject(2) &multL 

doesn’t is that ruby parens are implicit in the first case, so it really means

(5..10).map(&mult4) 

if you wanted, for the second case you could use

(5..10).inject 2, &multL 

The outside the parens trick only works for passing blocks to a method, not lambda objects.