How to dynamically create a local variable?

Question

I have a variable var = \"some_name\" and I would like to create a new object and assign it to some_name. How can I do it? E.g.

var         


        

Answer 1

Although, as others have pointed out, you cannot dynamically create local variables in Ruby, you can simulate this behavior to some degree using methods:

hash_of_variables = {var1: "Value 1", var2: "Value 2"}

hash_of_variables.each do |var, val|
  define_method(var) do
    instance_variable_get("@__#{var}")
  end
  instance_variable_set("@__#{var}", val)
end

puts var1
puts var2
var1 = var2.upcase
puts var1

Prints:

Value 1
Value 2
VALUE 2

Some libraries combine this technique with instance_exec to expose what appear to be local variables inside a block:

def with_vars(vars_hash, &block)
  scope = Object.new
  vars_hash.each do |var, val|
    scope.send(:define_singleton_method, var) do
      scope.instance_variable_get("@__#{var}")
    end
    scope.instance_variable_set("@__#{var}", val)
  end
  scope.instance_exec(&block)
end

with_vars(a: 1, b:2) do
  puts a + b
end

Prints: 3

Be aware though that the abstraction is by no means perfect:

with_vars(a: 1, b:2) do
  a = a + 1
  puts a
end

Results in: undefined method `+' for nil:NilClass. This is because a= defines an actual local variable, initialized to nil, which takes precedence over the method a. Then a.+(1) gets called, and nil doesn't have a + method, so an error is thrown.

So while this method is pretty useful for simulating read-only local variables, it doesn't always work well when you try to reassign the variable inside the block.