Reverse a String in Ruby by Reading Backwards - ruby-on-rails

Question

I'm just practicing and learning while loops and conditionals (not yet into arrays). I'm trying to reverse any string by concatenating letters, starting from the last letter of the word to the first. eg., for cat, start at t, then a, then c to get tac.
I don't get what's wrong in the code. I'm wondering why the 6th line ( reverse += letter) gives the error message:
6: in `+': no implicit conversion of nil into String (TypeError)
What's being nil'ed?
def is_reversed(word)
i = word.length
reverse = ""
while i > word.length || i != -1
letter = word[i]
reverse += letter
i = i - 1
end
return reverse
end
puts is_reversed("cat")

Answer 1

There are a few issue here but the bottom line is that you're looping wrong. You can either count up to a number or down from a number and it looks like you want to count down by starting at word.length. That's fine, but let's look and see what you're actually doing.
With while i > word.length || i != -1 you're checking each iteration that i is...greater than the length of the word? How would it get that way (you're not adding to i anywhere) and why would you want to check that?
Since you chose to count down, we want to stop when there are no letters remaining. So change your condition to while i > 0. Now we will loop only while there are letters left to go through.
There's another problem though - because indices start at 0, trying to get word[i] when i == 3 will get you nil! So you actually want to move the i = i - 1 to be the first line within your loop.
After these changes, you should have:
def is_reversed(word)
i = word.length
reverse = ""
while i > 0
i = i - 1
letter = word[i]
reverse += letter
end
return reverse
end
puts is_reversed("cat")

Answer 2

def is_reversed(word)
i = word.length
reverse = ""
while i > word.length || i != -1
i = i - 1
letter = word[i]
reverse += letter
end
return reverse
end
puts is_reversed("cat")
// will return 'tact'
Very first time in the loop above you're trying to find the letter you're putting the index as word.length which actually doesn't exist & hence returns nil which threw the error.
To get the last letter of a string you'll have to do i = i - 1 before you do anything else inside the loop.
Second, I think your condition is flawed. If you try to find the element at -1 in an array or string in ruby it will give you the last element.
And the first condition i > word.length will never satisfy as i's value is word.length.
So you can do something like this
def is_reversed(word)
i = word.length
reverse = ""
while i > 0
i = i - 1
letter = word[i]
reverse += letter
end
return reverse
end
puts is_reversed("cat")
//returns 'tac'

Answer 3

I realize this has been answered, but how about something more like:
def is_reversed(w)
w.split("").each_with_object("").with_index{ |(l,a), i| a << w[(w.length-1-i)] }
end
In console:
is_reversed('this is reversed')
=> "desrever si siht"
is_reversed('and so is this')
=> "siht si os dna"
is_reversed('antidisestablishmentarianism')
=> "msinairatnemhsilbatsesiditna"
Notes:
105 characters instead of 152
3 lines instead of 10
Uses << instead of += (which is faster)
Avoids the while i > 0 bit which is, IMO, not very idiomatic Ruby
Avoids unnecessary variable assignments (i = word.length, reverse = "", i = i - 1, and letter = word[i])