describe "#binary_search" do
      it "searches AddressBook for a non-existent entry" do
        book.import_from_csv("entries.csv")
        entry = book.binary_search("Dan")
        expect(entry).to be_nil
      end
    end

    it "searches AddressBook for Bill" do
      book.import_from_csv("entries.csv")
      entry = book.binary_search("Bill")
      expect(entry).to be_a Entry
      check_entry(entry, "Bill", "555-555-3454", "bill@blocmail.com")
    end

    it "searches AddressBook for Bob" do
      book.import_from_csv("entries.csv")
      entry = book.binary_search("Bob")
      expect(entry).to be_a Entry
      check_entry(entry, "Bob", "555-555-5564", "bob@blocmail.com")
    end

    it "searches AddressBook for Joe" do
      book.import_from_csv("entries.csv")
      entry = book.binary_search("Joe")
      expect(entry).to be_a Entry
      check_entry(entry, "Joe", "555-555-5532", "joe@blocmail.com")
    end

    it "searches AddressBook for Sally" do
      book.import_from_csv("entries.csv")
      entry = book.binary_search("Sally")
      expect(entry).to be_a Entry
      check_entry(entry, "Sally", "555-675-5555", "sally@blocmail.com")
    end

    it "searches AddressBook for Sussie" do
      book.import_from_csv("entries.csv")
      entry = book.binary_search("Sussie")
      expect(entry).to be_a Entry
      check_entry(entry, "Sussie", "555-555-2036", "sussie@blocmail.com")
    end

    it "searches AddressBook for Billy" do
      book.import_from_csv("entries.csv")
      entry = book.binary_search("Billy")
      expect(entry).to be_nil
    end
 end


 //address_book.rb

 def binary_search(name)

      lower = 0
      upper = entries.length - 1

      # #2
      while lower <= upper
        # #3
        mid = (lower + upper) / 2
        mid_name = entries[mid].name

        # #4
        if name == mid_name
          return entries[mid]
        elsif name < mid_name
          upper = mid - 1
        elsif name > mid_name
          lower = mid + 1
        end
      end

      # #5

      return nil
    end