Alan Malloy <a...@malloys.org> writes:
Hi Andy & Alan,
> This should be a function, not a macro. In fact it is just:
>
> (defn multicmp [& xs] (first (remove zero? xs)))
>
> But what you really wanted to begin with is a comparator function, so
> more like:
>
> (defn multicmp [& keys]
> (fn [a b]
> (or (first (remove zero? (map #(compare (% a) (% b))
> keys)))
> 0)))
>
> (sort-by (multicmp < =) coll)
Nice. I have a very similar function in my pocket, because I frequently
have to sort seqs of tuples (vectors). That's what I came up with:
--8<---------------cut here---------------start------------->8---
(defn seq-compare
"Returns a sequence comparator function that compares 2 sequences element by
element according to the given comparators `cmps', i.e., the first 2 elements
are compared with the first comparator, the second 2 elements with the second
comparator, and so on. Evaluates only as many comparators as are needed to
distinguish the sequences, i.e., evaluates the comparators until one returns
non-zero.
`cmps' must be comparator functions that get 2 elements and returns 0, if the
elements are equal, a negative integer, if the first is \"smaller\", or a
positive integer, if the second is \"smaller\".
If all comparators evaluate to 0, then the hash-codes are used as a last
resort.
Example: Sort a seq of 3-tuples of form [number number string] with
descending order of the first component, ascending order of second component,
and ascending orded of third components. Clearly, for numbers - is a valid
comparator, and for the strings we use compare which sorts lexicographically.
(sort (seq-compare #(- %2 %1) - compare)
[[1 10 \"b\"] [3 7 \"b\"] [1 2 \"b\"] [1 10 \"c\"] [3.0 17 \"a\"]])
;=> ([3 7 \"b\"] [3.0 17 \"a\"] [1 2 \"b\"] [1 10 \"b\"] [1 10 \"c\"])"
[& cmps]
(fn [a b]
(or (first (remove zero? (map #(%1 %2 %3) cmps a b)))
(- (hash a) (hash b)))))
--8<---------------cut here---------------end--------------->8---
Bye,
Tassilo
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