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)))
(begin
(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))
Specifically,
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)
'(4)
> (hash-set! ggg 1 4)
> (hash-ref ggg 1)
4
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.
-Joe
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