Parallel Assignment operator in Ruby

Question

Hi,

I was going through an example from Programming in Ruby book. This is that example

def fib_up_to(max)
  i1, i2 = 1, 1 # parallel assignment (i1 = 1 and i2 = 1)
  while i1 <= max
    yield i1
    i1, i2 = i2, i1+i2
  end
end
fib_up_to(100) {|f| print f, " " }

The above program simply prints the fibonacci numbers upto 100. Thats fine. My question here is when i replace the parallel assignment with something like this,

i1 = i2
i2 = i1+i2

I am not getting the desired output. My question here is, is it advisable to use parallel assignments? (I come from Java background and it feels really wierd to see this type of assignment)

One more doubt is : Is parallel assignment an operator??

Thanks

Answer 1

The problem with doing the assignment in 2 separate statements is that i2 = i1 + i2 will then use the new value of i1 rather than the previous value required to correctly generate the Fibonacci sequence.

When you use parallel assignment all of the expressions on the right hand side are evaluated first and then assigned to the receiving variables on the left hand side.

Parallel assignment isn't a special operator. Essentially what is on the right hand side acts as an Array where if we list multiple variables on the left hand side then the array is unpacked and assigned into the respective variables.

Here are some more examples:

irb(main):020:0> a = 1, 2 # assign to a single variable
=> [1, 2]
irb(main):021:0> a
=> [1, 2]
irb(main):022:0> a, b = 1, 2 # unpack into separate variables
=> [1, 2]
irb(main):023:0> a
=> 1
irb(main):024:0> b
=> 2
irb(main):025:0> a, b = [1, 2, 3] # 3 is 'lost' as no receiving variable
=> [1, 2, 3]
irb(main):026:0> a
=> 1
irb(main):027:0> b
=> 2
irb(main):028:0> first, *rest = [1, 2, 3] # *rest consumes the remaining elements
=> [1, 2, 3]
irb(main):029:0> first
=> 1
irb(main):030:0> rest
=> [2, 3]

It is a useful feature of ruby, for example it facilitates having methods that return multiple values e.g.

def sum_and_difference(a, b)
  a + b, a - b
end

sum, difference = sum_and_difference 5, 3

In Java the closest thing would be to have a method that returned int[] but if we wanted to return a string and a number we'd need to create a little POJO to act as struct for the return value or return Object[] and clutter the code up with casts. See this other question that I answered recently for a more practical example.