What does this mean in Ruby language?
Run the following code,
a = [1, 2, 3, 4, 5] head, *tail = a p head p tail
You will get the result
1 [2, 3, 4, 5]
Who can help me to explain the statement head,*tail = a, Thanks!
head, *tail = a means to assign the first element of the array a to head, and assign the rest of the elements to tail.
*, sometimes called the "splat operator," does a number of things with arrays. When it's on the left side of an assignment operator (=), as in your example, it just means "take everything left over."
If you omitted the splat in that code, it would do this instead:
head, tail = [1, 2, 3, 4, 5] p head # => 1 p tail # => 2
But when you add the splat to tail it means "Everything that didn't get assigned to the previous variables (head), assign to tail."
First, it is a parallel assignment. In ruby you can write
a,b = 1,2
and a will be 1 and b will be 2. You can also use
a,b = b,a
to swap values (without the typical temp-variable needed in other languages).
The star * is the pack/unpack operator. Writing
a,b = [1,2,3]
would assign 1 to a and 2 to b. By using the star, the values 2,3 are packed into an array and assigned to b:
a,*b = [1,2,3]
I don't know Ruby at all, but my guess is that the statement is splitting the list a into a head (first element) and the rest (another list), assigning the new values to the variables head and tail.
This mechanism is usually referred (at least in Erlang) as pattern matching.