Ruby own reverse string method

Question

I try to implement my own reverse string method:

def string_reverse(string)
  half = string.length / 2
  half.times do |i|
    string[i] = string[-i-1]
    string[-i-1] = string[i]
  end
end

But it only reverses the first half of the string. What am i missing here?

For problems like this, it helps to go step-by-step, with paper, through an imaginary use case. Eg, "the string is 'hi there', length 8, so half is 4. The first time through the loop, i is 0, so I set string[0] to be string[-0-1], which is -1...." Go all the way through for one example string and see if each step makes sense and what the string would be after that step. Some very simple test cases would help you, too: fail "couldn't reverse foo, got #{string_reverse('foo')}" unless string_reverse("foo") == "oof" — Nathan Long
– Nathan Long, Commented Mar 5, 2015 at 10:54

Arup Rakshit · Accepted Answer · 2015-03-05 11:52:04Z

6

You need a third variable to swap the characters, or use parallel assignment:

def string_reverse(string)
  half = string.length / 2
  half.times do |i|
    string[i], string[-i-1] = string[-i-1], string[i]
  end
end

edited Mar 5, 2015 at 11:52

Arup Rakshit

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answered Mar 5, 2015 at 10:46

Baldrick

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Comments

user3118220 · Accepted Answer · 2015-03-05 11:08:53Z

1

def string_reverse(string)
  half = string.length / 2
  half.times do |i|
    x = string[i]
    string[i] = string[-i-1]
    string[-i-1] = x
  end
end

answered Mar 5, 2015 at 11:08

user3118220

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1 Comment

user3118220 Over a year ago

Baldrick answer is perfect :)

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