What does mean both the operators * and (*) in ruby?

Question

I just introduced myself to the Ruby splat oprator. And I played with it lot's of way. but the below experiment somehow made me think about it twice :)

langs = ["java", "csharp", "ruby", "haskell" ]
# => ["java", "csharp", "ruby", "haskell"]

 l1,*,l2 = *langs
# => ["java", "csharp", "ruby", "haskell"]
 l1
# => "java"
 l2
# => "haskell"

 l1,*,*,l2 = *langs
SyntaxError: (irb):27: syntax error, unexpected tSTAR
l1,*,*,l2 = *langs
      ^
    from /usr/bin/irb:12:in `<main>'

Yes, the error is obvious, as I used more then 1 *(splat) operators in the same argument list.

Now I tried to play with it.

l1,(*),(*),l2 = *langs
# => ["java", "csharp", "ruby", "haskell"]

Ahh! here it works. But couldn't understand why so?

 l1
# => "java"
 l2
# => "haskell"
 l1,(*),l2 = *langs
# => ["java", "csharp", "ruby", "haskell"]
 l1
# => "java"
 l2
# => "ruby"

From the above example it seems that it is doing skipping of array elements.

Questions are :

• (a) what the operator (*) is called?

• (b) when I used in splat(*) in the line l1,*,l2 = *langs it consumes all the elements - "csharp", "ruby". Is there any way to see what * consumes there technically? Obviously I am teling with the use if l1,*,l2 = *langs not by l1,l*,l2 = *langs.

Answer 1

This is due to how parentheses work with parallel assignment as explained by Matz.

For example:

a, b, c = *[1, 2, 3]
a => 1
b => 2
c => 3

Is different than:

a, (b, c) = *[1, 2, 3]
a => 1
b => 2
c => nil

Basically, the parenthesis say: assign the right hand element at this index to the variables in the parens. So 2 is assigned to b, with nothing left at index 1 to assign to c. Similarly, (*) will take only the element at the given index and distribute it.

# the * is interpreted to mean 'take all remaining elements'
a, * = 1, 2, 3, 4

# the * is interpreted to mean 'take all remaining elements except
# the last element'
a, *, c = 1, 2, 3, 4

# incorrect syntax, can't splat more than once on all remaining
# elements
a, *, *, c = 1, 2, 3, 4

# the * is interpreted to mean 'take all elements at index 1'
a, (*), c = 1, 2, 3, 4

# the *'s are interpreted to mean 'take all elements at index 1,
# then again at index 2'
a, (*), (*), c = 1, 2, 3, 4

Typically, the * operator is used in conjuction with a variable as *foo - but if not it will hold its place and take element assignments as if it were a variable (essentially discarding them)