How to find all longest words in a string? - ruby

Question

If I have a string with no spaces in it, just a concatenation like "hellocarworld", I want to get back an array of the largest dictionary words. so I would get ['hello','car','world']. I would not get back words such as 'a' because that belongs in 'car'.
The dictionary words can come from anywhere such as the dictionary on unix:
words = File.readlines("/usr/share/dict/words").collect{|x| x.strip}
string= "thishasmanywords"
How would you go about doing this?

Answer 1

I would suggest the following.
Code
For a given string and dictionary, dict:
string_arr = string.chars
string_arr.size.downto(1).with_object([]) { |n,arr|
string_arr.each_cons(n) { |a|
word = a.join
arr << word if (dict.include?(word) && !arr.any? {|w| w.include?(word) })}}
Examples
dict = File.readlines("/usr/share/dict/words").collect{|x| x.strip}
string = "hellocarworld"
#=> ["hello", "world", "loca", "car"]
string= "thishasmanywords"
#=> ["this", "hish", "many", "word", "sha", "sma", "as"]
"loca" is the plural of "locus". I'd never heard of "hish", "sha" or "sma". They all appear to be slang words, as I could only find them in something called the "Urban Dictonary".
Explanation
string_arr = "hellocarworld".chars
#=> ["h", "e", "l", "l", "o", "c", "a", "r", "w", "o", "r", "l", "d"]
string_arr.size
#=> 13
so for this string we have:
13.downto(1).with_object([]) { |n,arr|...
where arr is an initially-empty array that will be computed and returned. For n => 13,
enum = string_arr.each_cons(13)
#<Enumerator: ["h","e","l","l","o","c","a","r","w","o","r","l","d"]:each_cons(13)>
which enumerates over an array consisting of the single array string_arr:
enum.size #=> 1
enum.first == string_arr #=> true
That single array is assigned to the block variable a, so we obtain:
word = enum.first.join
#=> "hellocarworld"
We find
dict.include?(word) #=> false
so this word is not added to the array arr. It is was in the dictionary we would check to make sure it was not a substring of any word already in arr, which are all of the same size or larger (longer words).
Next we compute:
enum = string_arr.each_cons(12)
#<Enumerator: ["h","e","l","l","o","c","a","r","w","o","r","l","d"]:each_cons(12)>
which we can see enumerates two arrays:
enum = string_arr.each_cons(12).to_a
#=> [["h", "e", "l", "l", "o", "c", "a", "r", "w", "o", "r", "l"],
# ["e", "l", "l", "o", "c", "a", "r", "w", "o", "r", "l", "d"]]
corresponding to the words:
enum.first.join #=> "hellocarworl"
enum.last.join #=> "ellocarworld"
neither of which are in the dictionary. We continue in this fashion, until we reach n => 1:
string_arr.each_cons(1).to_a
#=> [["h"], ["e"], ["l"], ["l"], ["o"], ["c"],
# ["a"], ["r"], ["w"], ["o"], ["r"], ["l"], ["d"]]
We find only "a" in the dictionary, but as it is a substring of "loca" or "car", which are already elements of the array arr, we do not add it.

Answer 2

This can be a bit tricky if you're not familiar with the technique. I often lean heavily on regular expressions for this:
words = File.readlines("/usr/share/dict/words").collect(&:strip).reject(&:empty?)
regexp = Regexp.new(words.sort_by(&:length).reverse.join('|'))
phrase = "hellocarworld"
equiv = [ ]
while (m = phrase.match(regexp)) do
phrase.gsub!(m[0]) do
equiv << m[0]
'*'
end
end
equiv
# => ["hello", "car", "world"]
Update: Strip out blank strings which would cause the while loop to run forever.

Answer 3

Starting at the beginning of the input string, find the longest word in the dictionary. Chop that word off the beginning of the input string and repeat.
Once the input string is empty, you are done. If the string is not empty but no word was found, remove the first character and continue the process.