Does anyone see the error in this simple Ruby function? - ruby

Question

This function is supposed to take a string and return the characters in reverse order.
def reverse(string)
reversedString = "";
i = string.length - 1
while i >= 0
reversedString = reversedString + string[i]
i -= 1
end
puts reversedString
end
however all the tests return false:
puts(
'reverse("abc") == "cba": ' + (reverse("abc") == "cba").to_s
)
puts(
'reverse("a") == "a": ' + (reverse("a") == "a").to_s
)
puts(
'reverse("") == "": ' + (reverse("") == "").to_s
)
Does anyone see what the problem is?

Answer 1

Try to use the default String class reverse method like this:
"Hello World".reverse
"Hello World".reverse!
Check Ruby's String class API at https://ruby-doc.org/core-2.4.0/String.html
If you want to make your custom method, you could use a map like this:
string = String.new
"Hello World".chars.each { | c | string.prepend c }

Answer 2

The problem is your function isn't returning its result, it's printing it. It needs to return reversedString.
As a rule of thumb, functions should return their result. Another function should format and print it.
def reverse(string)
reversedString = "";
i = string.length - 1
while i >= 0
reversedString = reversedString + string[i]
i -= 1
end
return reversedString
end
Note: This was probably an exercise, but Ruby already has String#reverse.
It's good that you're writing tests, but the way you're writing them it's hard to tell what went wrong. Look into a Ruby testing framework like MiniTest.
require "minitest/autorun"
class TestReverse < Minitest::Test
def test_reverse
assert_equal "cba", reverse("abc")
assert_equal "a", reverse("a")
assert_equal "", reverse("")
end
end
That would have told you that your function is returning nil.
1) Failure:
TestReverse#test_reverse [test.rb:16]:
Expected: "cba"
Actual: nil

Answer 3

To make this more Ruby-like yet avoid using the built-in String#reverse method you'd do this:
def reverse(string)
string.chars.reverse.join('')
end
Remember that in Ruby the result of the last operation is automatically the return value of the method. In your case the last operation is puts which always returns nil, eating your value. You want to pass it through.
Try to design methods with a simple mandate, that is, this function should focus on doing one job and one job only: reversing a string. Displaying it is beyond that mandate, so that's a job for another method, like perhaps the caller.
To avoid calling any sort of reverse method at all:
def reverse(string)
result = ''
length = string.length
length.times do |i|
result << string[length - 1 - i]
end
result
end
You can often avoid for almost completely and while frequently if you use things like times or ranges (0..n) to iterate over.

Answer 4

puts prints and returns nil, so the whole method returns nil. If, for debugging reasons , you want to inspect what your method is returning, use p which returns it's argument (reversedString in this case).
def reverse(string)
reversedString = ""
i = string.length - 1
while i >= 0
reversedString = reversedString + string[i]
i -= 1
end
p reversedString # !!!
end
And all 3 tests return true

Answer 5

If I was going to do this, I'd probably take advantage of an array:
ary = 'foo bar baz'.chars
reversed_ary = []
ary.size.times do
reversed_ary << ary.pop
end
reversed_ary.join # => "zab rab oof"
pop removes the last character from the array and returns it, so basically it's walking backwards through ary, nibbling at the end and pushing each character onto the end of reversed_ary, effectively reversing the array.
Alternately it could be done using a string:
ary = 'foo bar baz'.chars
reversed_str = ''
ary.size.times do
reversed_str << ary.pop
end
reversed_str # => "zab rab oof"
or:
reversed_str += ary.pop
I just saw that #tadman did a similar thing with the string. His would run more quickly but this is more readable, at least to my eyes.