How to understand Ruby's .each and .map

Question

I am having trouble understanding the differences between map and each, and where and when to use them.

I read \"What does map do?\" and \

Answer 1

This is covered in Ruby's documentation in multiple places but the easiest to understand for your use is in the Array documentation for each:

each { |item| block } → ary
each → Enumerator

Calls the given block once for each element in self, passing that element as a parameter. Returns the array itself.

If no block is given, an Enumerator is returned.

a = [ "a", "b", "c" ]
a.each {|x| print x, " -- " }

produces:

a -- b -- c --

Note that it says "Returns the array itself."

Compare that to map:

map { |item| block } → new_ary
map → Enumerator

Invokes the given block once for each element of self.

Creates a new array containing the values returned by the block.

See also Enumerable#collect.

If no block is given, an Enumerator is returned instead.

a = [ "a", "b", "c", "d" ]
a.collect { |x| x + "!" }         #=> ["a!", "b!", "c!", "d!"]
a.map.with_index { |x, i| x * i } #=> ["", "b", "cc", "ffffd"]
a                                 #=> ["a", "b", "c", "d"]

Note that it says "Creates a new array containing the values returned by the block."

This example should help knowing the above:

foo = [1,2,3]
foo.each { |i| puts i + 1 } # => [1, 2, 3]
foo.map { |i| i + 1 } # => [2, 3, 4]

# >> 2
# >> 3
# >> 4

where # => is the return value of the block and # >> is the captured STDOUT from puts.

And, knowing all that, use each when you want to display elements in the array or extract and reuse those elements as parameters or to build things. Use map when you want to change the elements of the array into something else.