Map and reduce methods in Ruby

There are two methods in Ruby's Enumerable module that often people coming
from an object oriented background find confusing but are indeed very
useful: .map and .reduce.

They both are constructions taken from functional programming so they
both take a block as a parameter.

There are also .collect and .inject aliases for them. They are there
only to use whatever one the domain you are working thinks is best.

.map / .collect

Map takes a function and returns the array that has in each position
the result of applying that function to the corresponding element of the
original array.


This is a highly practical method. Imagine that, for example, you have
an array of people and you want to get an array of the names. It is
as easy as:

.reduce / .inject

Reduce is similar to map but instead of receiving a function that takes
only one parameter it receives a function that receives an accumulator and
the value. Instead of returning an array it just returns the accumulator.
We can say that this method reduces the array to a single value.


Imagine now that with our array of people we want to get the total number
of letters that all the names have. We can do:

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