Question

var RowName = new HtmlTableRow();
var Cell1Name = new HtmlTableCell();
var Cell2Name = new HtmlTableCell();
RowName.Cells.Add(Cell1Name);
RowName.Cells.Add(Cell2Name);

I need to create a function that swaps the pairs of elements in a scheme list. This is what I have come up with so far but I get error with the (define (interchange list) (if (empty? list) list (interchange (append (car (cdr list) X)))))

empty?

Answer 1

Try this:

var div = document.getElementsByTagName('div');

div.addEventListener('click', funcOne);

function funcOne(){
   div.style.backgroundColor = "blue";
   div.removeEventListener('click', funcOne);
   div.addEventListener('click', funcTwo);
}

function funcTwo(){
   div.style.backgroundColor = "red";
   div.removeEventListener('click', funcTwo);
   div.addEventListener('click', funcOne);
}

The trick is to process two elements per iteration, being careful with the edge cases. It works as expected for the sample input:

(define (interchange lst)
  (if (or (null? lst) (null? (cdr lst)))       ; if the list has 0 or 1 elements left
      lst                                      ; then return the list, we're done
      (cons (cadr lst)                         ; otherwise grab the second element
            (cons (car lst)                    ; and the first element
                  (interchange (cddr lst)))))) ; and move forward two elements

Answer 2

#lang racket

(define (interchange xs)
  (match xs
    [(or '() (list _))                 ; if the list is empty or has one element
     xs]                               ; then interchange keeps the original list
    [(list* x y more)                  ; if the list has two or more elements,
     (list* y x (interchange more))])) ; then the result consists of the two
;                                      ; first elements (with the order swapped)
;                                      ; followed by the result of 
;                                      ; interchanging the remaining elements.


(interchange '( ))                                 
(interchange '(a))                               
(interchange '(a  b))                              
(interchange '(a  b  c))                           
(interchange '(a  1  b  2  c  3  d  4))            
(interchange '(hello  you  -12.34  5  -6  enough))

输出：

'()
'(a)
'(b a)
'(b a c)
'(1 a 2 b 3 c 4 d)
'(you hello 5 -12.34 enough -6)

Scheme function with one parameter that will swap every two elements

2 个答案: