On Mon, Feb 13, 2012 at 5:16 PM, James <jegathi...@gmail.com> wrote:
> Hi,
>
> I am new to clojure and I am very excited about learning the
> language. I have a question about replacing an element in a list. I
> have elements that look like this :
>
> {:id G__781, :value 1}
>
> The list looks like this:
>
> ({:id G__821, :value 1} {:id G__820, :value 1} {:id G__819, :value 1}
> {:id G__818, :value 1} {:id G__817, :value 1})
>
> I want to update an element in the list by conjoining a new list with
> an updated element. For example, I'll update the head of the list:
> ({:id G__821, :value 10} {:id G__820, :value 1} {:id G__819, :value 1}
> {:id G__818, :value 1} {:id G__817, :value 1})
>
> My update code (it really builds a new list with and replaces the
> updated element) looks like this:
>
> (defn update-id-list [id-list new-id new-id-list]
> (doseq[old-id id-list
> new-id-list (conj new-id-list (choose-id old-id new-id) )]
> (println "New List " new-id-list)) new-id-list)
>
> (defn choose-id[old-id new-id]
> (if (= (:id old-id) (:id new-id)) new-id old-id))
>
> This doesn't work. It never conjoin's elements and ultimately returns
> an empty list. I've tried using various other methods but I keep
> going in circles.
>
> Any ideas why this wouldn't work?
Functions like conj create and return a new list, rather than modify
the original in-place. You'd need to change your doseq to something
like:
(for [old-id id-list] (choose-id old-id new-id))
I'd also use better names, perhaps
(for [old-entry entry-list] (choose-entry old-entry new-entry))
But for this sort of job you might be better off using a larger map:
{G__821 {:id G__821 :value 1} G__820 {:id G__820 :value 1} ... }
(assoc entry-map old-id new-entry)
e.g. (assoc entry-map G__821 {:id G__821 :value 10})
or even (update-in entry-map [id :value] (constantly new-value))
e.g. (update-in entry-map [G__821 :value] (constantly 10))
If you actually transform the old value with a function to get the new
one, so much the better:
(update-in entry-map [G__821 :value] #(* 10 %))
etc.
Incidentally, are all of these symbols G__xxx defined somewhere? You
might want keywords or strings instead, to avoid having to quote them
to avoid errors.
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