在球拍中用Lambda进行迭代？

Question

This is probably a simple question, but I cannot seem to figure it out. In the example below, I want to create a function list-of-obj that recursively creates a list of items.

Example:

> (list-of-obj 'a 5)
'(a a a a a)

The code:

#lang racket
(define (list-of-obj obj n)
  (letrec ((helper
           (lambda (obj n result)
             (if (> n 0)
               (helper obj (- n 1) (cons obj result))
               result))))
      helper obj n 0))

This, however results in output of 0 . Any pointers? I'm new to racket/scheme.

Answer 1

The most idiomatic solution in Racket would be:

(make-list 5 'a)
=> '(a a a a a)

That is, you should not reinvent the wheel if a built-in function does what you want. But if you need to write it from scratch, this should do the trick:

(define (list-of-obj obj n)
  (letrec ((helper
            (lambda (obj n result)
              (if (> n 0)
                  (helper obj (- n 1) (cons obj result))
                  result))))
    (helper obj n '())))

There were two problems with your code:

• The call to helper must be surrounded by () , because that's how you call a procedure in Scheme.
• The initial value for the accumulator must be an empty list given that we're building a list as the output, not a 0 as you had it.