Codewars: "Return or rotate": why isn't my attempted solution working? - ruby

Question

These were the instructions given on Codewars (https://www.codewars.com/kata/56b5afb4ed1f6d5fb0000991/train/ruby):
The input is a string str of digits. Cut the string into chunks (a chunk here is a substring of the initial string) of size sz (ignore the last chunk if its size is less than sz).
If a chunk represents an integer such as the sum of the cubes of its digits is divisible by 2, reverse that chunk; otherwise rotate it to the left by one position. Put together these modified chunks and return the result as a string.
If
sz is <= 0 or if str is empty return ""
sz is greater (>) than the length of str it is impossible to take a chunk of size sz hence return "".
Examples:
revrot("123456987654", 6) --> "234561876549"
revrot("123456987653", 6) --> "234561356789"
revrot("66443875", 4) --> "44668753"
revrot("66443875", 8) --> "64438756"
revrot("664438769", 8) --> "67834466"
revrot("123456779", 8) --> "23456771"
revrot("", 8) --> ""
revrot("123456779", 0) --> ""
revrot("563000655734469485", 4) --> "0365065073456944"
This was my code (in Ruby):
def revrot(str, sz)
# your code
if sz > str.length || str.empty? || sz <= 0
""
else
arr = []
while str.length >= sz
arr << str.slice!(0,sz)
end
arr.map! do |chunk|
if chunk.to_i.digits.reduce(0) {|s, n| s + n**3} % 2 == 0
chunk.reverse
else
chunk.chars.rotate.join
end
end
arr.join
end
end
It passed 13/14 test and the error I got back was as follows:
STDERR/runner/frameworks/ruby/cw-2.rb:38:in `expect': Expected: "", instead got: "095131824330999134303813797692546166281332005837243199648332767146500044" (Test::Error)
from /runner/frameworks/ruby/cw-2.rb:115:in `assert_equals'
from main.rb:26:in `testing'
from main.rb:84:in `random_tests'
from main.rb:89:in `<main>'
I'm not sure what I did wrong, I have been trying to find what it could be for over an hour. Could you help me?

Answer 1

I will let someone else identify the problem with you code. I merely wish to show how a solution can be speeded up. (I will not include code to deal with edge cases, such as the string being empty.)
You can make use of two observations:
the cube of an integer is odd if and only if the integer is odd; and
the sum of collection of integers is odd if and only if the number of odd integers is odd.
We therefore can write
def sum_of_cube_odd?(str)
str.each_char.count { |c| c.to_i.odd? }.odd?
end
Consider groups of 4 digits in the last example, "563000655734469485".
sum_of_cube_odd? "5630" #=> false (so reverse -> "0365")
sum_of_cube_odd? "0065" #=> true (so rotate -> "0650")
sum_of_cube_odd? "5734" #=> true (so rotate -> "7345")
sum_of_cube_odd? "4694" #=> true (so rotate -> "6944")
so we are to return "0365065073456944".
Let's create another helper.
def rotate_chars_left(str)
str[1..-1] << s[0]
end
rotate_chars_left "0065" #=> "0650"
rotate_chars_left "5734" #=> "7345"
rotate_chars_left "4694" #=> "6944"
We can now write the main method.
def revrot(str, sz)
str.gsub(/.{,#{sz}}/) do |s|
if s.size < sz
''
elsif sum_of_cube_odd?(s)
rotate_chars_left(s)
else
s.reverse
end
end
end
revrot("123456987654", 6) #=> "234561876549"
revrot("123456987653", 6) #=> "234561356789"
revrot("66443875", 4) #=> "44668753"
revrot("66443875", 8) #=> "64438756"
revrot("664438769", 8) #=> "67834466"
revrot("123456779", 8) #=> "23456771"
revrot("563000655734469485", 4) #=> "0365065073456944"
It might be slightly faster to write
require 'set'
ODD_DIGITS = ['1', '3', '5', '7', '9'].to_set
#=> #<Set: {"1", "3", "5", "7", "9"}>
def sum_of_cube_odd?(str)
str.each_char.count { |c| ODD_DIGITS.include?(c) }.odd?
end