Note: to repeatedly assoc-in over a seq of indices you'd want
(reduce #(assoc-in %1 [%2 :c] 50) input-vec index-seq)
Replace the key :c and value 50 as needed. If the 50 would actually be
computed from the index, an expression using %2 would work. If the 50 would
actually be computed from the existing value associated with the :c (or
whatever) key, use update-in instead of assoc-in and pass in the function
that transforms the existing value to the appropriate new value as the last
argument to update-in.
On Tue, Jul 30, 2013 at 6:27 PM, Cedric Greevey <cgree...@gmail.com> wrote:
> That's basically the same as the first solution, in terms of how it works
> under the hood.
>
> If the desire is for the output to share structure with the input, not
> just the maps but the vector as well, then you need to do something a bit
> more complicated. First you need to find the map you want, and return its
> index in the vector (assuming it will be unique if present). Then you need
> to use assoc-in with the index, like (assoc-in v [idx :c] 50) or whatever.
>
> If there may be more than one map that should be adjusted, you could
> return a seq of indices and repeatedly assoc-in, but I doubt this would
> give superior efficiency unless the vector was typically huge and the
> number of maps needing changes typically much smaller. With million-element
> vectors and two or three maps needing updating it would likely be
> preferable to the (vec (for ...)) approach. With dozen-element vectors with
> half a dozen maps needing updating I'd go with (vec (for ...)).
>
>
>
> On Tue, Jul 30, 2013 at 4:58 PM, Jim - FooBar(); <jimpil1...@gmail.com>wrote:
>
>> Hi,
>>
>> how about ....
>>
>> (vec
>> (for [{:keys [a b c] :as m} aa]
>> (if (= a 50) (assoc m :c 3) m)))
>>
>> :)
>> Jim
>>
>>
>>
>> On 30/07/13 18:53, cej38 wrote:
>>
>> Suppose I have a vector of maps (this could also be a vector of records)
>>
>> (def aa [{:a 0 :b 0 :c 0} {:a 50 :b 0 :c 0} {:a 100 :b 0 :c 0}])
>>
>> and I want to go in an change the value of associated with :c when :a
>> has a value of 50, so that the final col of maps looks like
>>
>> [{:a 0 :b 0 :c 0} {:a 50 :b 0 :c 3} {:a 100 :b 0 :c 0}].
>>
>>
>>
>> I came up with a function that does this:
>>
>> (defn find-assoc-in
>> [k v mv]
>> (let [f #(if (= ((first k) %) (first v))
>> (assoc % (second k) (second v))
>> %)]
>> (map f mv)))
>>
>>
>> (find-assoc-in [:a :c] [50 3] aa)
>>
>>
>> The problem with this function is that it potentially does a lot
>> of unnecessary copying of elements that aren't changed, and thus has a lot
>> of garbage collection. I would like help coming up with something that
>> does much less garbage collection.
>>
>>
>> Will someone please point out a better way of doing this?
>>
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