How do I pass a list as a list of arguments in racket?

Question

I have a statement like this:

 ((lambda (a b c) (+ a b c)) 1 2 3) ; Gives 6

And I would like to be able to also pass it a list as so:

Answer 1

That operation is called apply.

(apply + (list 1 2 3))   ; => 6

apply "expands" the last argument; any previous arguments are passed as is. So these are all the same:

(apply + 1 2 3 (list 4 5 6))
(apply + (list 1 2 3 4 5 6))
(+ 1 2 3 4 5 6)

Answer 2

Pay attention to the following definition

(define (a . b) (apply + b))
(a 1)
(a 1 2)
(a 1 2 3)

'.' gives you ability to pass any number of arguments to a function. You can still have required arguments

(define (f x . xs) (apply x xs)) ;; x is required
(f + 1 2 3) ;; x is +, xs is (1 2 3)