How to make the array of the alphabet to rotate from "z" to "a" (ruby) - ruby

Question

I am doing a Caesar cipher. I thought that the unless statement will work but it doesn't with or without then. Then I changed the unless with if and put ; in the place of then and it reads : undefined method `>' for nil:NilClass.
def caesar_cipher(input, key)
input.each do |x|
numbers = x.ord + key.to_i unless (numbers > 122) then numbers = x.ord + key - 26
letters = numbers.chr
print letters
end
end
puts "Write the words you want to be ciphered: "
input = gets.chomp.split(//)
puts "Write the key (1 - 26): "
key = gets.chomp
caesar_cipher(input,key)

Answer 1

Here are a couple of Ruby-like ways to write that:
#1
def caesar_cipher(input, key)
letters = ('a'..'z').to_a
input.each_char.map { |c| letters.include?(c) ?
letters[(letters.index(c)+key) % 26] : c }.join
end
caesar_cipher("this is your brown dog", 2)
#=> "vjku ku aqwt dtqyp fqi"
#2
def caesar_cipher(input, key)
letters = ('a'..'z').to_a
h = letters.zip(letters.rotate(key)).to_h
h.default_proc = ->(_,k) { k }
input.gsub(/./,h)
end
caesar_cipher("this is your brown dog", 2)
#=> "vjku ku aqwt dtqyp fqi"
The hash h constructed in #2 equals:
h = letters.zip(letters.rotate(key)).to_h
#=> {"a"=>"c", "b"=>"d", "c"=>"e", "d"=>"f", "e"=>"g", "f"=>"h",
# ...
# "u"=>"w", "v"=>"x", "w"=>"y", "x"=>"z", "y"=>"a", "z"=>"b"}
h.default_proc = ->(_,k) { k } causes
h[c] #=> c
if c is not a lowercase letter (e.g., a space, capital letter, number, punctuation, etc.)

Answer 2

If you write a branch with condition (if or unless) at the end of a line, after an initial statement, there are two things that apply and affect you:
The condition is assessed before the statement on its left. In your case that means numbers has not been assigned yet so it is nil.
The branch decision is whether or not to run the initial statement, you do not branch to the statement after the then.
You can solve this simply by converting your condition to an if and moving it to a separate line:
def caesar_cipher(input, key)
input.each do |x|
numbers = x.ord + key.to_i
if (numbers > 122)
numbers = x.ord + key - 26
end
letters = numbers.chr
print letters
end
end
There are arguably better ways of coding this cipher in Ruby, but this should solve your immediate problem.

Answer 3

There is a more elegant way to loop repeating sequences in ruby. Meet Enumerable#cycle.
('a'..'z').cycle.take(50)
# => ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j",
# "k", "l", "m", "n", "o", "p", "q", "r", "s", "t",
# "u", "v", "w", "x", "y", "z", "a", "b", "c", "d",
# "e", "f", "g", "h", "i", "j", "k", "l", "m", "n",
# "o", "p", "q", "r", "s", "t", "u", "v", "w", "x"]
Therefore, translating a single letter given a key can be written as:
('a'..'z').cycle.take(letter.ord + key.to_i - 'a'.ord.pred).last
And the entire method can look prettier:
def caesar_cipher(phrase, key)
phrase.each_char.map do |letter|
('a'..'z').cycle.take(letter.ord + key.to_i - 'a'.ord.pred).last
end.join
end
puts caesar_cipher('abcxyz', 3) # => defabc
Note that this is slower than the alternative, but it also has the benefit that it's easier to read and the key can be any number.