これも再帰。ちょっと日を置いて書いたらすんなり書けた

test_chop.rb

#!/usr/bin/env ruby
# http://codekata.com/kata/kata02-karate-chop/

require 'test/unit'

def chop(target, values)
  middle = values.length / 2

  return -1     if values[middle].nil?
  return middle if target == values[middle]
  return -1     if middle == 0

  if target < values[middle]
    index = chop(target, values[0..middle-1])
    return index if index != -1
  else
    index = chop(target, values[middle+1..values.length-1])
    return middle + 1 + index if index != -1
  end

  -1
end

class TestChop < Test::Unit::TestCase
  def test_chop
    assert_equal(-1, chop(3, []))
    assert_equal(-1, chop(3, [1]))
    assert_equal(0,  chop(1, [1]))
    #
    assert_equal(0,  chop(1, [1, 3, 5]))
    assert_equal(1,  chop(3, [1, 3, 5]))
    assert_equal(2,  chop(5, [1, 3, 5]))
    assert_equal(-1, chop(0, [1, 3, 5]))
    assert_equal(-1, chop(2, [1, 3, 5]))
    assert_equal(-1, chop(4, [1, 3, 5]))
    assert_equal(-1, chop(6, [1, 3, 5]))
    #
    assert_equal(0,  chop(1, [1, 3, 5, 7]))
    assert_equal(1,  chop(3, [1, 3, 5, 7]))
    assert_equal(2,  chop(5, [1, 3, 5, 7]))
    assert_equal(3,  chop(7, [1, 3, 5, 7]))
    assert_equal(-1, chop(0, [1, 3, 5, 7]))
    assert_equal(-1, chop(2, [1, 3, 5, 7]))
    assert_equal(-1, chop(4, [1, 3, 5, 7]))
    assert_equal(-1, chop(6, [1, 3, 5, 7]))
    assert_equal(-1, chop(8, [1, 3, 5, 7]))
  end
end

ループと再帰の他にどんな書き方があるか思いつかないなー