How to change local variable value by using method?

user_6947378 0
Viewed 257

I wrote some code:

a = 4
b = 7
def swaps(g, f)
  g ^= f; f ^= g; g ^= f
end

swaps(a, b)
p a, b

I thought I wanted result: a value is 7, b value is 4, but I got result:

4, 7
binding ruby variables
2 Answers

You do not have access to those local variables inside your method.

You can play with it by passing binding but this shouldn't be used in real life:

Objects of class Binding encapsulate the execution context at some particular place in the code and retain this context for future use.

def change(local_variable_name, binding, new_value)
  binding.local_variable_set(local_variable_name, v)
end

Test:

a = 1

a                      #=> 1
change(:a, binding, 2) #=> 2
a                      #=> 2

Your case:

def swap(first, second, binding)
  old_first  = binding.local_variable_get(first)
  old_second = binding.local_variable_get(second)

  binding.local_variable_set(first, old_second)
  binding.local_variable_set(second, old_first)
end

a = 1
b = 2
swap(:a, :b, binding)
a #=> 2
b #=> 1

Reference:

user_17728300

Better I read comments before! Credits to Jordan Running and Johan Wentholt

I leave this post just to point out these solutions.

Maybe parallel assignment can do it, instead of defining a method:

a = 4
b = 7

a, b = b, a

p a #=> 7
p b #=> 4

Or, if it sounds clearer:

def swap(a, b)
  return b, a
end

a, b = swap(a, b)
user_52390300
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