Basic RSpec issue - method runs but tests fail - ruby-on-rails

Question

Given all the hype over TDD, I decided it was time to dig in and add that to the list of things to study. I'm running into an issue, and I'm 100% certain it's just a function of something being wrong with my tests in RSpec. I'm still brand new to RSpec, so I'm having trouble figuring it out... my method works just fine, but the test for the method does not.
Method Code (I know I can refactor this A LOT. This is one of the first Ruby programs I wrote awhile back, which explains the ugliness)
def caesar_cipher(string,offset)
string=string.to_s
offset=offset.to_i
cipher=[]
string.each_byte do |i|
#capital letters
if (i>64 && i<91)
if (i+offset)>90
cipher << (i+offset-26).chr
else
cipher << (i+offset).chr
end
elsif (i>96 && i<123)
if (i+offset)>122
cipher << (i+offset-26).chr
else
cipher << (i+offset).chr
end
else
cipher << i.chr
end
end
cipher=cipher.join('')
puts "The encrypted string is: #{cipher}"
end
puts "Enter the string you'd like to encrypt"
string=gets.chomp
puts "Enter the offset you'd like to use"
offset=gets.chomp
caesar_cipher(string,offset)
Test Code (Just for one generic case, all lower case input)
require './caesarCipher.rb'
describe "caesar_cipher" do
it 'should handle all lower case input' do
caesar_cipher("abcdefg", 3).should == "defghij"
end
end
Method output:
$ ruby caesarCipher.rb
Enter the string you'd like to encrypt
abcdefg
Enter the offset you'd like to use
3
The encrypted string is: defghij
Test Output:
$ rspec spec/caesar_cipher_spec.rb
Enter the string you'd like to encrypt
Enter the offset you'd like to use
The encrypted string is: require './caesarCipher.rb'
The encrypted string is: defghij
F
Failures:
1) caesar_cipher should handle all lower case input
Failure/Error: caesar_cipher("abcdefg", 3).should == "defghij"
expected: "defghij"
got: nil (using ==)
# ./spec/caesar_cipher_spec.rb:5:in `block (2 levels) in <top (required)>'
Finished in 0.00542 seconds (files took 0.14863 seconds to load)
1 example, 1 failure
Failed examples:
rspec ./spec/caesar_cipher_spec.rb:4 # caesar_cipher should handle all lower case input
Any help on why the tests are failing? Judging by the output it looks like it's running it twice or something in the tests.

Answer 1

Add cipher or return cipher after this line
puts "The encrypted string is: #{cipher}"
And it should work
To explain the fix given, the last expression in a method is the return value. You've passed the value to STDOUT but not as the return value of the method, so RSpec was failing.