As part of my assignment i have been given the task to:

Write a function that takes two lists as input, a list L1 containing characters and a list L2 containing numbers. For each of the number n contained in L2, the function will display the first n character of the list L1. For example: L1 '("h" "e" "l" "l" "o" "w" "o" "r" "l" "d") L2 '(2 3 1 5 10) Output: h e
h e l
h
h e l l o
h e l l o w o r l d.

i know the function takes two arguments but i don't know how to apply the second the list which is like a selector,it reads each number in the second list and extracts the character in that position in the first list.

( define two-sequences
( lambda ( l1 l2 )
(for/list ([ i l1 ] [ j l2 ])

which function should i use to do this.

CodePudding user response：

You need to print the first n elements of a list. Here's how you do that:

To print the first n elements of a list, lst:

• if n is zero print a newline;
• if n is 1 print the first element of lst and a newline;
• if n is more than 1 print the first element of the lst, a space, and then print n - 1 elements of the rest of lst (hint: you can use a function you are currently writing to do this);
• if n is anything else then this is an error.

Write this as a proceducre: call it print-n for instance. Then the procedure which answers the question is:

(define (print-ns l ns)
  (for ([n (in-list ns)])
    (print-n l n)))

And

> (print-ns '("h" "e" "l" "l" "o" "w" "o" "r" "l" "d") '(2 3 1 5 10))
h e
h e l
h
h e l l o
h e l l o w o r l d

CodePudding user response：

In Common Lisp or emacs lisp this function you want is called elt which gives the nth element of a list. - In Racket, you have to define it yourself:

(define (elt l n)
  (cond ((empty? l) '())
        ((zero? n) (car l))
        (else (elt (cdr l) (- n 1)))))
    
(elt '(a b c d e f) 3) ;;=> 'd

In every-day racket, however, one wouldn't use lists - which one has to traverse like this - but vectors - whose access via an index is direct and thus much more faster.

(vector-ref #("a" "b" "c" "d" "e" "f") 3) ;;=> "d"

#(...) is equal to (vector ...) and creates a vector out of the sequence.