Question

You must write each of the following scheme functions. You must use only basic scheme functions do not use third-party libraries to support any of your work. Do not use any function with side effects.

This problem need to use DrRacket software. Racket Language.

Write a function named (first-n L1 N) that returns the first N elements of L1. If N is negative, return the empty list. If N exceeds the length of L1 return all elements of L1.

(first-n '(a b c d e f) 3) ---> (a b c)
(first-n '(a b c d e f) -3) ---> ()
(first-n '(a b c d e f) 33) ---> (a b c d e f)
(first-n '() 0) ---> ()

Answer 1

Code:

(define count 1) ;;Initialized count = 1 to keep track our traversal of list

(define (first-n L1 N) ;;MAIN_FUNCTION
(first L1 N count)) ;;calls sub-function Which returns first N-number of elements of list

(define (first L1 N count) ;;SUB_FUNCTION
(cond ((null? L1) L1) ;;if list is empty returns List
((< N 0) '()) ;;if (N < 0) RETURNS NULL LIST
((= N 0) '()) ;;if (N == 0) reurns NULL list
((= N 1) (car L1)) ;;if (N == 1) Returns 1st element
((= N count) (cons (car L1) '())) ;;if (N == count) returns list which contains elelemnt at index N

(else (cons (car L1) (first (cdr L1) N (+ 1 count)))))) ;;else construct list with first element and continue

Snapshot of Code and Output:

File Edit View Language Racket Insert Scripts Tabs Help Untitled (define...) #lang scheme (define count 1) ;; Initialized count = 1 to keep track our traversal of list (define (first-n L1 N) ;;MAIN FUNCTION (first L1 N count)) ;; calls sub-function which returns first N-number of elements of list (define (first L1 N count) ;; SUB FUNCTION (cond ((null? L1) 11) ;if list is empty returns List ((NO) '0) ;;if (N < 0) RETURNS NULL LIST ((= N O 'O) ;;if (N == 0) reurns NULL list ((= N 1) (car L1)) ;;if (N == 1) Returns 1st element ((= N count) (cons (car Ll) '()) ;;if (N == count) returns list which contains elelemnt at index N (else (cons (car L1) (first (cdr L1) N (+ 1 count)))))) ;;else construct list with first element and contin Welcome to DrRacket, version 7.4 [3m). Language: scheme, with debugging; memory limit: 128 MB. > (first-n '(a b c d e f) 3) (a bc) > (first-n '(a b c d e f) -3) 0 > (first-n '(a b c d e f) 33) (a b c d e f) > (first-n'( 0) (