From: Mike Stephens <rubfor@...>
Date: 2011-01-18T09:07:19+09:00
Subject: Calling by Reference - Two Questions

I know I'm not the first person to get stumped by how to get Ruby to
change an object in a method.

I want to do:

method a1 or a1.method

I can do

 a1 = method a1

but amongst other things that creates a new object.

a1 = [1,2]
puts " #{a1} # 12

def myswap1(a2)
  a3 = [0,0]
  a3[0] = a2[1]
  a3[1] = a2[0]
  a2 = a3
  end

myswap1 a1
puts " #{a1} " # 12

Ruby passes in a1, so at the start of the method a2 is the same
reference as a1. However when I assign a1 = a3, a1 becomes a different
reference and disappears when the method terminates.

So my first question is why won't Ruby let me do this:

a1 = [1,2]

public
def myswap5
  a3 = [0,0]
  a3[0] = self[1]
  a3[1] = self[0]
  self = a3
  end

a1.myswap5 # gives error message: Can't change the value of self

My second question is from this:

a1 = [1,2]

def myswap6(a2)
  a2.reverse!
  end

  myswap6 a1
  puts " #{a1}" #21

What magic is reverse! using, and how can I avail myself of it?

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