Outputting years with no repeated digits

I'm working through a course now, and was tasked with the following:

Write a function, no_repeats(year_start, year_end), which takes a range of years and outputs those years which do not have any repeated digits.

You should probably write a helper function, no_repeat?(year) which returns true/false if a single year doesn't have a repeat.

I did not know about each_char yet, so I wrote the following:

def no_repeats(year_start, year_end)
(year_start..year_end).to_a.keep_if {|year| no_repeat?(year)}
end

def no_repeat?(year)
year_array = year.to_s.split(//)
year_array == year_array.uniq
end


I'm wondering if:

1. there is an easier way to make an array from a string with no spaces
2. there is a way to convert each character in a number to an item in a single array without converting the number to a string first
3. there is a better way to do this problem

This is the model solution for a beginner:

def no_repeats(year_start, year_end)
no_repeats = []
(year_start..year_end).each do |yr|
no_repeats << yr if no_repeat?(yr)
end

no_repeats
end

def no_repeat?(year)
chars_seen = []
year.to_s.each_char do |char|
return false if chars_seen.include?(char)
chars_seen << char
end

return true
end


I'm also wondering about the advantages and disadvantages of my code compared to the model solution. I kind of get that shorter code is better, and readability matters. I don't know how to test code for time yet.

1 Answer

The model solution they propose is very unidiomatic, your code is far better. I'd just made some minor changes:

• Use String#chars instead of String#split(//).
• Use Enumerable#select? instead of Array#keep_if to avoid modifying data in-place. It's better to work with immutable streams of data (with methods from Enumerable).

I'd write:

def no_repeats(year_start, year_end)
(year_start..year_end).select { |year| no_repeat?(year) }
end

def no_repeat?(year)
year_digits = year.to_s.chars
year_digits == year_digits.uniq
end

• thank you for the help. I see exactly why you changed the variable name from "array" to "digits" and why chars is a clearer method than split. I'm curious why you'd take select over keep_if. Please explain. Commented Oct 19, 2016 at 11:33
• Array#keep_if changes the original array in-place. In this particular code that's not a problem, but as a general rule, it's not a good idea, I guess you've read about Functional Programming. In Ruby you usually work with a functional (immutable) streams of data, using Enumerable methods. github.com/tokland/tokland/wiki/RubyFunctionalProgramming Commented Oct 19, 2016 at 11:43