Error in sample ruby program to swap element in an array - ruby

Question

I created a sample ruby program to swap elements in an array
class Swap
def swp(a,b)
self[a],self[b] = self[b],self[a]
self
end
array = [1,2,3]
array.swp(1,2)
puts array
end
I am getting the following error
NoMethodError: private method `swp' called for [1, 2, 3]:Array
from (irb):77:in `<class:Swap>'
from (irb):71
from /home/rahulv/.rvm/gems/ruby-2.0.0-p481/gems/railties-3.2.16/lib/rails/commands/console.rb:47:in `start'
from /home/rahulv/.rvm/gems/ruby-2.0.0-p481/gems/railties-3.2.16/lib/rails/commands/console.rb:8:in `start'
from /home/rahulv/.rvm/gems/ruby-2.0.0-p481/gems/railties-3.2.16/lib/rails/commands.rb:41:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'
Please help to fix it.
UPDATE
I tried the same program with dynamic array input like this:
class Swap
def swp(a,b)
self[a],self[b] = self[b],self[a]
self
end
end
puts "enter the array elements "
input = gets.chomp
ary = []
while input != 'fin'
ary << input.to_i
input = gets.chomp
end
puts ary
puts "enter the elements to be swapped"
a=gets
b=gets
s=Swap.new
ary = ary.s.swp(a,b)
puts ary
Now the output error is like this:
enter the array elements
1
2
3
4
5
fin
enter the positions to be swapped
1
2
NoMethodError: private method `s' called for [0, 1, 2, 3, 4, 5]:Array
from (irb):17:in `<class:Swap>'
from (irb):1
from /home/rahulv/.rvm/gems/ruby-2.0.0-p481/gems/railties-3.2.16/lib/rails/commands/console.rb:47:in `start'

Answer 1

array.class = Array, you should add swp method for Array class.
class Array
def swp(a,b)
self[a],self[b] = self[b],self[a]
self
end
end
array = [1,2,3]
array.swp(0,1) #The array index start from 0, not 1
puts array.inspect
=> [2, 1, 3]
Update:
class Array
def swp(a,b)
self[a],self[b] = self[b],self[a]
self
end
end
puts "enter the array elements "
ary = []
input = 0
while 1
input = gets.chomp
if input != 'fin'
ary << input.to_i
else
break
end
end
puts "enter the positions to be swapped"
a=gets.to_i
b=gets.to_i
ary.swp(a,b)
puts ary.inspect

Answer 2

I don't like editing classes like that, for example if there's another class in the ancestor tree that implements the actual swp method (for any reason) and you implemented yours, and you try to use the actual documented one, you'll find unexplained behaviour, and it might be uneasy to figure out why it's behaving like that, beacuse your method is hiding, it's more maintainable if you add your methods to the ancestry tree using a module and then including it to the class.
module Swappable
def swp(a, b)
# method goes here
end
end
Then in another file you would do
class Array
include Swappable
end
This way when you call Array.ancestors you'd see your module
Array.ancestors
=> [Array, Swappable, ....]
And when you try something like
Array.instance_method(:swp).owner
you'll get Swappable, while an open class editing would return Array
This is just my oppinion on this matter, you can pick whatever works for you.

Answer 3

You can try the following code blog for take input an array and swap with two positions.
Here I initialize Swap class with an input array and then pass two swap position.
Main Class
class Swap
def initialize(array)
#array = array#[1,2,3,4,5]
end
def swp(a,b)
#array[a],#array[b] = #array[b],#array[a]
#array
end
end
Take Input for array
puts "ArrayInput :: Enter numbers separated by spaces, press 'Enter' to finish:"
array = gets
array = array.split(" ")
Take Input for swap positions
puts "SwapInput:: For FirstPosition, press 'Enter' to finish:"
first_position = gets.to_i
puts "SwapInput:: For SecondPosition, press 'Enter' to finish:"
second_position = gets.to_i
Initialize Swap Class
swap = Swap.new(array)
array = swap.swp(first_position, second_position)
puts "Array after swap:"
puts array