[racket] First try with hashes in Racket

From: Joe Gilray (jgilray at gmail.com)
Date: Mon Apr 2 00:44:11 EDT 2012

Hi, On ProjectEuler #14 I tried to use hashes for the first time and
although I got it to work, I feel like my use of hashes needs help:

(define (euler14)
  (define ht (make-hash))
  (hash-set! ht 1 1)
  (define start-longest-chain 1)
  (define longest-chain 1)
  (for ([i (in-range 2 1000000)])
    (unless (hash-has-key? ht i)
      (let loop ([count 0] [cur i] [lst '()])
        (define col (collatz cur))
        (if (hash-has-key? ht col)
            (let ([colval (hash-ref ht col)])
              (when (< longest-chain (+ 1 colval (abs count)))
                  (set! longest-chain (+ 1 colval (abs count)))
                  (set! start-longest-chain i)))
              (for ([ps (cons (list i 1) lst)]) (hash-set! ht (car ps) (+
(abs count) (cadr ps) colval))))
            (loop (sub1 count) col (cons (list col count) lst)))))) (values
start-longest-chain longest-chain))


1) Why do I have to use two lines to set the (1 . 1) base case?  I really
tried to use the associations argument but got:

> (define ggg (make-hash (list '(1 4))))
> (hash-ref ggg 1)
> (hash-set! ggg 1 4)
> (hash-ref ggg 1)

I can't seem to make the optional argument to make-hash work to give me
simply a number.

2) My use of hash-has-key? feels clunky, Could I better use hash-ref?
especially here:

        (if (hash-has-key? ht col)
            (let ([colval (hash-ref ht col)]) ...

Thanks for any advice.
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