How to create destructive functions

Question

I want to create a destructive function. Let's consider the following example:

def not_really_destructive_fun(arr)
  arr = [1,1,1]
  puts "arr changed to: " + arr.to_s
end
def destructive_fun(arr)
  p "Destructive function run."
  arr.map!{|x| x+= 1}
end

Output of calling such methods is the following:

a = [1,2,3]
not_really_destructive_fun a # => arr changed to: [1, 1, 1]
a # => [1, 2, 3]
destructive_fun a            # => "Destructive function run."
a # => [2, 3, 4]

map! still changed the original value. However, = didn't. How can I write a destructive function?

Your example is illustrating scope more than mutation (i.e. destruction). In the first method you're array argument is being assigned to `arr` then right away you're re-assigning a new array to `arr`. This is why it not not changing the original array. — Nathan, Sep 02 '15 at 11:12
Read this article - http://robertheaton.com/2014/07/22/is-ruby-pass-by-reference-or-pass-by-value/, and just earlier someone else had asked same question - http://stackoverflow.com/questions/32338232/variable-scope-and-modification — Wand Maker, Sep 02 '15 at 11:17

Aleksei Matiushkin · Accepted Answer · 2015-09-02T12:25:31.400

3

All the parameters in Ruby are passed to methods using call-by-sharing. That said, you might change the content of a variable passed, but can not reassign it (the latter will just reassign a value, leaving the old value intact.)

So, one should modify content, e.g.:

def not_really_destructive_fun(arr)
    # that won’t work
    # arr = [1,1,1]
    # that will
    arr.replace [1,1,1]
    # and that will
    arr.clear # array is now empty
    arr << 1 << 1 << 1
end

edited Sep 02 '15 at 12:25

answered Sep 02 '15 at 11:14

Aleksei Matiushkin

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Pass / call by reference usually means the opposite, i.e. that you can assign to the variable being passed. – Stefan Sep 02 '15 at 11:39
@Stefan Frankly, this part of terminology was always tricky to me. I treat it like one might change anything save for the reference itself. What would be the right term here? – Aleksei Matiushkin Sep 02 '15 at 11:48
Wikipedia mentions a third term: [call-by-sharing](https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_sharing) – Stefan Sep 02 '15 at 11:52

score 2 · Answer 2 · answered Sep 02 '15 at 11:14

2

You could use replace to replace the an array with another:

def really_destructive_fun(arr)
  arr.replace([1,1,1])
  puts "arr changed to: #{arr}"
end

a = [1,2,3]
really_destructive_fun(a)
#=> arr changed to: [1, 1, 1]

a
#=> [1, 1, 1]

answered Sep 02 '15 at 11:14

spickermann

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How to create destructive functions

2 Answers2