Question

I was going through an example from the Programming in Ruby book:

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, " " }

This simply prints the Fibonacci numbers up to 100. That’s fine. When I replace the parallel assignment with:

i1 = i2
i2 = i1+i2

I am not getting the desired output.

Is it advisable to use parallel assignments? I come from a Java background and it feels really weird to see this type of assignment.

One more question is: Is parallel assignment an operator?

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.