Ruby Strings - difference between << and +=

Question

Running the method fizzbuzz1 yields a 100 member list of numbers 1 to 100, where each multiple of 3 is replaced by "fizz", each multiple of 5 by "buzz", and each multiple of both 3 and 5 by "fizzbuzz":

def fizzbuzz1()
  result = Array.new(100, "")
  result.each_index do |index|
    value = index + 1
    result[index] += "fizz" if value % 3 == 0
    result[index] += "buzz" if value % 5 == 0
    result[index] = "#{value}" if result[index].size == 0
  end
end

2.0.0-p195 :055 > fizzbuzz1
 => ["1", "2", "fizz", "4", "buzz", "fizz", "7", "8", "fizz", ...and so on.

BUT, switching each += for a << yields something unexpected:

def fizzbuzz2()
  result = Array.new(100, "")
  result.each_index do |index|
    value = index + 1
    result[index] << "fizz" if value % 3 == 0
    result[index] << "buzz" if value % 5 == 0
    result[index] = "#{value}" if result[index].size == 0
  end
end

2.0.0-p195 :057 > entire_fizzbuzz2_result = fizzbuzz2
2.0.0-p195 :058 > entire_fizzbuzz2_result[5]
 => "fizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzz"

Most peculiarly, I also notice that if I take out the line: result[index] = "#{value}" if result[index].size == 0 to give:

def fizzbuzz3()
  result = Array.new(100, "")
  result.each_index do |index|
    value = index + 1
    result[index] << "fizz" if value % 3 == 0
    result[index] << "buzz" if value % 5 == 0
  end
end

2.0.0-p195 :062 > store_fizzbuzz3 = fizzbuzz3
2.0.0-p195 :063 > store_fizzbuzz3.reject { |each| store_fizzbuzz3[0] == each }
 => []

Which must mean that fizzbuzz3 returns a 100 member array where each element is the same, and has the characteristics:

2.0.0-p195 :066 > store_fizzbuzz3[1]
 => "fizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzzfizzfizzbuzzfizzbuzzfizzfizzbuzz" 
2.0.0-p195 :067 > store_fizzbuzz3[1].size
 => 212 
2.0.0-p195 :068 > store_fizzbuzz3[1].size / 4
 => 53

And 53 is a sort of interesting number in that it is the number of integers between 1 and 100 that are not divisible by 3 or 5...i.e. the number of "numbers" in the result of the fully functioning fizzbuzz1 up top.

What is going on here with the <<, could someone give me a little walkthrough?

Darshan Rivka Whittle · Accepted Answer · 2013-05-30T19:45:01.990

6

There are two interacting matters here:

The form of Array.new(), and
The style of string concatenation.

In the form you use, Array.new(100, "") creates an Array with 100 elements, each pointing to the same empty string. += returns a new string, so that doesn't matter. << appends to the existing string, so it does matter that they're all the same.

If you initialize result with this form of Array.new():

result = Array.new(100){""}

then fizzbuzz2 will work as expected, because you have 100 different empty strings.

edited May 30 '13 at 19:45

answered May 30 '13 at 19:39

Darshan Rivka Whittle

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didn't get the difference between `Array.new(100, "")` and `Array.new(100){""}` ? could you help more? – Arup Rakshit May 30 '13 at 20:41
@RubyLovely Sure. The first form creates an Array with `100` references to `""`, the empty string created when the interpreter reaches that line. The second form creates an Array with `100` elements, calling the block `{""}` for each element. Thus, `""` is interpreted 100 times, creating 100 unique empty string objects. See [Array::new](http://ruby-doc.org/core-2.0/Array.html#method-c-new) for the official documentation. – Darshan Rivka Whittle May 30 '13 at 20:56
@RubyLovely I edit all my text, code or otherwise, in `emacs`. I run Ruby with various versions installed by `rvm`. For Stack Overflow, I usually just use `irb`, as I did in this case. The editor, the version of Ruby, and how it's run are all irrelevant in this case. – Darshan Rivka Whittle May 30 '13 at 21:06
no, actually i was searching someone who can help me to install textmate2 editor,if anybody using it. As i am not getting a proper tutorial for the same. :) – Arup Rakshit May 30 '13 at 21:11

Ruby Strings - difference between << and +=

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