How can I add a parentheses in a list? For example,
(multiply '(1 2) '(1 2 3))
It should output ((1 2 3) (2 4 6))
This is what I have now
(define multiply
(lambda (los1 los2)
(cond
((null? los1) '())
((null? los2) '())
(else (cons (* (car los1) (car los2))
(append (multiply(list (car los1)) (cdr los2))
(multiply(cdr los1) los2)))))))
The output I get is (1 2 3 2 4 6).
P/S: If this is a bad question or anything, don't hesitate to tell me or scold me. I am trying to learn to use stackoverflow.
Think about what the implications of the input and the output are. I imagine that you cannot do this with one single recursive procedure, but you can do it with two.
(list-multiply 5 '(1 2 3))
; ==> (5 10 15)
When you have that you iterate over the first list and use list-multiply for each element and of course the second argument is never iterated, it's just passed along.
(multiply '(1 2) '(1 2 3))
; ==> (cons (list-multiply 1 '(1 2 3))
; (cons (list-multiply 2 '(1 2 3))
; '()))
; ==> ((1 2 3) (2 4 6))
You can implement this simply by useinfg higher order functions:
(define (multiply lst1 lst2)
(map (lambda (e1)
(map (lambda (e2) (* e1 e2)) lst2))
lst1))
Notice the double map. One that iterates lst1 to make elements in the result and a second map that multiplies the one element with each element of the list. You might imagine that since these look similar the two recursive functions probably have a lot in common too.
PS: Learn to solve your problems without list and append. (list 1 2 3) is just a procedure that is equivalent to (cons 1 (cons 2 (cons 3 '()))) and (append '(1 2 3) '(4 5 6)) is equivalent to (cons 1 (cons 2 (cons 3 '(4 5 6)))). Try always building lists from end to beginning and in the event you need the opposite it might just need a reverse in the end.