On Tue, Jun 23, 2009 at 8:02 PM, Stephen C. Gilardi<squee...@mac.com> wrote:
>
> On Jun 23, 2009, at 6:22 PM, pupsik wrote:
>
>> The following recursive function does not
>> terminate if I exexute it in my REPL.
>> What is wrong?
>> (This example is from the official Clojure-website).
>>
>> (defn my-zipmap [keys vals]
>> (loop [my-map {}
>> my-keys (seq keys)
>> my-vals (seq vals)]
>> (if (and my-keys my-vals)
>> (recur (assoc my-map (first my-keys) (first my-vals))
>> (rest my-keys)
>> (rest my-vals))
>> my-map)))
>>
>> (my-zipmap [:a :b :c] [1 2 3])
>
> The example needs to be updated for the lazier seqs that were implemented
> before the Clojure 1.0 release. As part of that change, we lost "nil
> punning". "rest" no longer returns nil if there are no more items. A new
> function "next" does though:
>
> user=> (source zipmap)
> (defn zipmap
> "Returns a map with the keys mapped to the corresponding vals."
> [keys vals]
> (loop [map {}
> ks (seq keys)
> vs (seq vals)]
> (if (and ks vs)
> (recur (assoc map (first ks) (first vs))
> (next ks)
> (next vs))
> map)))
> nil
Yes, this (Steve's) version, using next and testing directly with (and
ks vs) is the idiomatic way when you are not in turn producing a lazy
seq.
(if-not (or (empty? ks) (empty? vs))
is to be avoided. (No offense Cosmin :)
Rich
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