Reverse words of a string in Ruby? - ruby

Question

I am trying to reverse the words of a string in Ruby, without using the reverse method. I want to implement the known algorithm of:
Reverse the whole string
Reverse each word in the reversed string.
Here is what I have come up with:
class String
def custom_reverse(start, limit)
i_start = start
i_end = limit - 1
while (i_start <= i_end)
tmp = self[i_start]
self[i_start] = self[i_end]
self[i_end] = tmp
i_start += 1
i_end -= 1
end
return self
end
def custom_reverse_words
self.custom_reverse(0, self.size)
i_start = 0
i_end = 0
while (i_end <= self.length)
if (i_end == self.length || self[i_end] == ' ')
self.custom_reverse(i_start, i_end)
i_start += 1
end
i_end += 1
end
end
end
test_str = "hello there how are you"
p test_str.custom_reverse_words
But the results are "yahthello ow ou er ereh"
What am I missing?

Answer 1

The gist of any reverse operation is to iterate over elements in the reverse order of what you'd normally do. That is, where you'd usually use the set (0..N-1) you'd instead go through (N-1..0) or more specifically N-1-i where i is 0..N-1:
class String
def reverse_words
split(/\s+/).map{|w|wl=w.length-1;(0..wl).map{|i|w[wl-i]}.join}.join(' ')
end
end
puts "this is reverse test".reverse_words.inspect
# => "siht si esrever tset"
The same principle can be applied to the words in a given string.
Interview questions of this sort are of highly dubious value. Being "clever" in production code is usually a Very Bad Idea.

Answer 2

Here's one way to reverse an array without using the built-in reverse:
class Array
def reverse
tmp_ary = self.dup
ret_ary = []
self.size.times do
ret_ary << tmp_ary.pop
end
ret_ary
end
end
%w[a b c].reverse # => ["c", "b", "a"]
tmp_ary.pop is the secret. pop removes elements from the end of the array.

Answer 3

The cleanest solution I could think of is:
class Array
def my_reverse
sort_by.with_index {|_, i| -i}
end
end
class String
def words
split(/\W+/)
end
def revert_words
words.my_reverse.join(' ')
end
def revert_each_word
words.map {|w| w.chars.my_reverse.join}.join(' ')
end
end

Answer 4

Once you define a simple and efficient array reverser:
def reverse_array(a)
(a.length / 2).times {|i| a[i],a[-(i+1)] = a[-(i+1)],a[i]}
a
end
You can reverse a sentence pretty straightforwardly:
def reverse_sentence(s)
reverse_array(s.split('')).join.split(" ").map{|w| reverse_array(w.split('')).join}.join(" ")
end
reverse_sentence "Howdy pardner" # => "pardner Howdy"

Answer 5

Here's another way:
class String
def reverse_words
split.inject([]){|str, word| str.unshift word}.join(' ')
end
def reverse_chars
each_char.inject([]){|str, char| str.unshift char}.join('')
end
end
Revised
Carey raises a good point, reverse_chars can be simplified, since string is already an Enumerable:
class String
def reverse_chars
each_char.inject(""){|str, char| str.insert(0, char) }
end
end