What does the "map" method do in Ruby?

ruby map range enumeration
bigpotato picture bigpotato · Aug 23, 2012 · Viewed 322.1k times · Source

I'm new to programming. Can someone explain what .map would do in:

params = (0...param_count).map

Answer

Danil Speransky picture Danil Speransky · Aug 23, 2012

The map method takes an enumerable object and a block, and runs the block for each element, outputting each returned value from the block (the original object is unchanged unless you use map!):

[1, 2, 3].map { |n| n * n } #=> [1, 4, 9]

Array and Range are enumerable types. map with a block returns an Array. map! mutates the original array.

Where is this helpful, and what is the difference between map! and each? Here is an example:

names = ['danil', 'edmund']

# here we map one array to another, convert each element by some rule
names.map! {|name| name.capitalize } # now names contains ['Danil', 'Edmund']

names.each { |name| puts name + ' is a programmer' } # here we just do something with each element

The output:

Danil is a programmer
Edmund is a programmer

Related questions

Difference between map and collect in Ruby?

I have Googled this and got patchy / contradictory opinions - is there actually any difference between doing a map and doing a collect on an array in Ruby/Rails? The docs don't seem to suggest any, but are there perhaps …

Read More
Map an array modifying only elements matching a certain condition

In Ruby, what is the most expressive way to map an array in such a way that certain elements are modified and the others left untouched? This is a straight-forward way to do it: old_a = ["a", "b", "c"] # ["a", "…

Read More
Other Ruby Map Shorthand Notation

I am aware of the shorthand for map that looks like: [1, 2, 3, 4].map(&:to_s) > ["1", "2", "3", "4"] I was told this is shorthand for: [1, 2, 3, 4].map{|i| i.to_s} This makes perfect sense. My question is this: It seems there should …

Read More