On 11.01.2009, at 08:56, Tzach wrote:
> Following your good advice, I also update the next-cell function to
> work in a lazy way instead of sorting the values of the entire board.
Good idea!
> (defn next-cell [board]
> "return the next potential cell to set, and the valid alternatives"
> (first (for [n (range 1 10)]
> (filter
> #(= n (count (second %)))
> (map-board board valid-values)))))
Your implementation has the disadvantage of recomputing (map-
board ...) nine times. You can avoid this with a simple modification:
(defn next-cell [board]
"return the next potential cell to set, and the valid alternatives"
(let [vv (map-board board valid-values)]
(first (for [n (range 1 10)]
(filter
#(= n (count (second %)))
vv)))))
Since the values of the lazy sequence vv are cached, nothing will
ever be recomputed.
However, I think there is also a bug in your function: (first
(for ..)) will return the result of the first iteration of the for,
even if it is emtpy. What you want is the first element of the first
non-empty element of the for sequence. You can get this with (first
(apply concat (for [n ...))). Concat again creates a lazy sequence,
so nothing is computed unless necessary.
Konrad.
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