How to iterate over a Ruby array

A short memo about how to iterate over a Ruby array

The classic way to iterate over a Ruby array

Let's take a simple Ruby array :

a = [11, 'foo', 42]

You can use the .each method

a.each { |x| puts x }
# 11
# foo
# 42

If you are confused about the syntax, I wrote an article about Ruby blocks, procs and lambda, so it's equivalent to the following :

a.each do |x| 
  puts x 
end
# 11
# foo
# 42

The for loop

The for loop is available in Ruby, to be honest I don't see it very often, so I put it here as reference :

for e in a
  puts e
end
# 11
# foo
# 42

`each_with_index` for a Ruby array

If you need to know about the index, the following syntax is available :

a.each_with_index {|e, index| puts "#{e} => #{index}" }
# 11 => 0
# foo => 1
# 42 => 2

Again, this is equivalent to :

a.each_with_index do |e, index| 
  puts "#{e} => #{index}" 
end
# 11 => 0
# foo => 1
# 42 => 2

Do you want to create an array from an existing one?

You probably want the .map method here

a.map {|e| e.to_s + ' z' }
# ["11 z", "foo z", "42 z"]

You actually iterate on the array, in order to extract an array of the same size.

Note that array "a" is left unchanged.

Lastly, I wrote an article about how to map with index in Ruby.

Do you want to reject unwanted values?

Now it's .reject

a.reject {|e| e.is_a?(String) }
# [11, 42]
a.reject {|e| e.is_a?(Numeric) }
# ["foo"]

Conclusion

Iterate over a Ruby array is quite simple, given it's a big classic in programming. Just take time to feel at ease with the block notation, it's more or less like the anonymous JS functions. If you have a specific need, also have a look at the Enumerable module.