## DEV Community

Fran C. for Factorial

Posted on

# A trick with the Ruby documentation

I've never been able to remember the arguments for `Array#reduce`. Every time I think I've managed to do it I just get it wrong.

Luckily for me, Ruby has very good documentation on its standard library and, what's even better, it ships with a CLI to explore it so I don't even need to Google it ðŸ˜¬

``````ri "Array#reduce"
``````
``````= Array#reduce

(from ruby core)
=== Implementation from Enumerable
------------------------------------------------------------------------
enum.reduce(initial, sym) -> obj
enum.reduce(sym)          -> obj
enum.reduce(initial) { |memo, obj| block }  -> obj
enum.reduce          { |memo, obj| block }  -> obj

------------------------------------------------------------------------

Combines all elements of enum by applying a binary operation,
specified by a block or a symbol that names a method or operator.

The inject and reduce methods are aliases. There
is no performance benefit to either.

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.

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

# Sum some numbers
(5..10).reduce(:+)                             #=> 45
# Same using a block and inject
(5..10).inject { |sum, n| sum + n }            #=> 45
# Multiply some numbers
(5..10).reduce(1, :*)                          #=> 151200
# Same using a block
(5..10).inject(1) { |product, n| product * n } #=> 151200
# find the longest word
longest = %w{ cat sheep bear }.inject do |memo, word|
memo.length > word.length ? memo : word
end
longest                                        #=> "sheep"

``````

It has a very good interactive mode as well (`ri -i`) In which you can search whatever you want :)

Oh, and if you use Vim you can access it by using `K` over any word. Check this out: