Question

I recently ran into this issue:
I’ve always used inject like so (i knew that (0) part is optional and can be omitted)

array = [13,23,13]
#=> [13, 23, 13]
array.inject(0) { |sum,i| sum+i }
#=> 49

By chance i found out that you can use:

array.inject(:+)
#=> 49
array.inject(:-)
#=> -23
array.inject(:*)
#=> 3887
array.inject(:/)
#=> 0

Googling on the issue i found a nice article on inject, but no mentioning there about what i’ve tried….
Can anyone explain to me or give some info about these inject commands that I’ve just used?

Answer 1

From the doc on Enumerable#inject:

… If you specify a symbol instead, then each element in the collection will be passed to the named method of memo. In either case, the result becomes the new value for memo. At the end of the iteration, the final value of memo is the return value for the method.

If you do not explicitly specify an initial value for memo, then uses the first element of collection is used as the initial value of memo.

So, if you specify a symbol, it treats it as a method name and invokes this method on every element of the enumerable, replacing memo as stated above. Now, the math operators (+-*/) are just methods, nothing else. These lines produce identical result:

13 + 23 # => 36
13.+(23) # => 36
13.send(:+, 23) # => 36

When you pass a symbol to inject or reduce it uses the third form to dynamically apply that operator to elements:

[1,2,3].inject(:+) # => 6

This shorthand can be used with methods other than operators as well:

[{"a"=>1}, {"b"=>2}].inject(:merge) # => {"a"=>1, "b"=>2}