Ruby longest palindrome. Nested while loops - arrays

Question

I am writing some code to find the longest palindrome in a string. I want to start at index 0 and then push the increasing length of the substring to an array:
ex:
string = "ababa"
[["a", "b"], ["a", "b", "a"], ["a", "b", "a", "b"], ["a", "b", "a", "b", "a"]]
It should then start on index 1 and do the same:
ex:
string = "ababa"
[["b","a"],["b","a","b"],["b","a","b","a"]
This should continue until the index is length -1 .However, the following code stops after it has gone through all iterations beginning with the first index and only returns:
[["a", "b"], ["a", "b", "a"], ["a", "b", "a", "b"], ["a", "b", "a", "b", "a"]]
What is the flaw in my logic? Code below
def longest_palindrome(s)
array = s.chars
start = 0
place = 1
output = []
while start < s.length - 1
while place < s.length
output << array[start..place]
place += 1
end
start += 1
end
return output
end

Answer 1

I believe this is what you are after:
def longest_palindrome(s)
arr = s.chars
output = []
(0...s.length).each do |start|
(start + 1...s.length).each do |place|
output << arr[start..place]
end
end
output
end
longest_palindrome("ababa")
=> [["a", "b"], ["a", "b", "a"], ["a", "b", "a", "b"], ["a", "b", "a", "b", "a"], ["b", "a"], ["b", "a", "b"], ["b", "a", "b", "a"], ["a", "b"], ["a", "b", "a"], ["b", "a"]]
Using iterators really simplifies things. Here is a more concise version:
def longest_palindrome(s)
output = (0...s.length).flat_map do |start|
(start + 1...s.length).map do |place|
s[start..place]
end
end
end
longest_palindrome("ababa")
=> ["ab", "aba", "abab", "ababa", "ba", "bab", "baba", "ab", "aba", "ba"]

Answer 2

A working, non-optimized, written on-the-fly 30 minutes, who knows how awfully slow longest palindrome finder:
def longest_palindrome(string, min_size = 2)
string = string.downcase # we will not be taking
return string if string == string.reverse # skip all calculaions of the passed string itself is a palindrome in original order
letters = string.chars # Convert string to array of bytes
combinations = [] # Initialize all letter combinations
(min_size..letters.size).each do |n| # min_size is the shortest length a palindrome is allowed to be, default 2
combinations.concat(letters.combination(n).to_a) # concat all combinations for n amount of characters in the string
end
palindromes = [] # Initialize array for all palindromes
combinations.each do |combo| # interate every combo
combo.size.times do # for every letter ordering is done via size of this letter combo
palindromes << combo.dup if combo == combo.reverse # add to list of palindromes if the combinations is the same backwards
combo.rotate! # rotate the letters for next order checking
end
end
palindromes.sort {|a, b| a.size <=> b.size }.last.join # sort the palidromes by length, take the biggest one, and return it as a full string
end
p longest_palindrome("racecar") #=> racecar
p longest_palindrome("applesauce") #=> pecep
p longest_palindrome("madam im adam") #=> mada m adam
p longest_palindrome("madamimadam") #=> madamimadam
but it works, sort of.
As you can see if sort of gets odd when spaces/puncuation are added to the mix.
You can always clean the string of it in the first line if need be.
Viva la algorithmless coding!