how to change nil into " " in array - ruby

Question

#ask user for string input
puts "Enter Text"
# text user has entered
text = "Test text"
#put text into a array - "split" by just words and spaces
words = text.downcase.split('')
#array of letters a to z in array
a_z = ('a'..'z').to_a
#idealy - take each letter in text string and match them to a_z's index from 0-25
#space values in text are replaced in array with " " instead of nil
words.map { |x| a_z.index(x) }
I am trying to take text, split text, turn text into a number based on a to z index ... shift that number and recombine and turn back into text...
the problem I am encountering now is that spaces in my text array show up as nil since their is no numerical value for nil in the a-z index.
how can I replace nil with " " so that my array can for example look like this:
#=> [1,2," "]
instead of:
#=> [1,2,nil]

Answer 1

Code
def shift_and_remove(str, shift)
h = make_hash 'a'..'z', shift
h.default_proc = ->(h,k) {' '}
str.gsub(/./,h)
end
def make_hash(r, shift)
base = r.first.ord
r.each_with_object({}) { |c,h| h[c] = (base + (c.ord-base+shift) % 26).chr }
end
Examples
str = "Now is the time for 007 to appear!"
shift_and_remove(str, 0) #=> " ow is the time for to appear "
shift_and_remove(str, 1) #=> " px jt uif ujnf gps up bqqfbs "
shift_and_remove(str, 4) #=> " sa mw xli xmqi jsv xs ettiev "
shift_and_remove(str, -1) #=> " nv hr sgd shld enq sn zoodzq "
shift_and_remove(str, -2) #=> " mu gq rfc rgkc dmp rm ynncyp "
Notice that in all cases "N" has been converted to a space, "w" of "Now" has been shifted to "a" in the third example and "a" of "appear" has been shifted to "z" and "y", respectively, in the last two examples.
Explanation
When shift #=> 2, the hash is as follows:
h[c] = ('a'.ord + (c.ord-'a'.ord+shift) % 26).chr }
#=> {"a"=>"c", "b"=>"d", "c"=>"e", "d"=>"f", "e"=>"g", "f"=>"h", "g"=>"i",
# "h"=>"j", "i"=>"k", "j"=>"l", "k"=>"m", "l"=>"n", "m"=>"o", "n"=>"p",
# "o"=>"q", "p"=>"r", "q"=>"s", "r"=>"t", "s"=>"u", "t"=>"v", "u"=>"w",
# "v"=>"x", "w"=>"y", "x"=>"z", "y"=>"a", "z"=>"b"}
All letters "a" to "x" are mapped to the next letter, whereas "y" and "z" are mapped to "a" and "b" respectively. Similarly, when shift #=> -1, "a" is mapped to "z" and all other letters are mapped to the preceding letter.
This uses the form of String#gsub that employs a hash to determine the replacement of each character in the string (/./). The method Hash#default_proc= creates a proc that gives the value of h[k] when the hash h does not have a key k, namely a space.
Capital letters
If any capital letters present are to be shifted to capital letters (rather than converted to spaces), only a slight modification is required.
def shift_and_remove(str, shift)
h = make_hash('a'..'z', shift).merge make_hash('A'..'Z', shift)
h.default_proc = ->(h,k) {' '}
str.gsub(/./,h)
end
shift_and_remove(str, 0) #=> "Now is the time for to appear "
shift_and_remove(str, 1) #=> "Opx jt uif ujnf gps up bqqfbs "
shift_and_remove(str, 4) #=> "Rsa mw xli xmqi jsv xs ettiev "
shift_and_remove(str, -1) #=> "Mnv hr sgd shld enq sn zoodzq "
shift_and_remove(str, -2) #=> "Lmu gq rfc rgkc dmp rm ynncyp "