The difference between :+ and &:+

Question

I have code like this

 list << num if num.to_s.split("").map(&:to_i).map(&:factorial).inject(:+) == num

It works, and I was wondering how inject works without the & (ampersand) in front of the :+. I am asking for someone to explain what the differences are between :+ and &:+.

one of them is wearing a hat? ...sorry, couldn't resist – musefan Jul 01 '14 at 08:52 — musefan, Jul 01 '14 at 08:52
@musefan No, one of them has hairs. – sawa Jul 01 '14 at 08:59 — sawa, Jul 01 '14 at 08:59

score 6 · Accepted Answer · answered Jul 01 '14 at 08:55

&:+ is translated to a proc, while :+ is a Symbol. inject supports receiving symbols, which is translated internally to a proc:

If you specify a block, then for each element in enum the block is passed an accumulator value (memo) and the element. 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.

The difference between :+ and &:+

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