A few people, I believe primarily Alan Malloy and Anthony Grimes, have
created a Clojure library for what they call ordered sets and maps that do
exactly this. They are implemented not as you tried to do, but by
remembering a number for each element (for ordered sets) or key (for
ordered maps) that is the relative order that it was added in.
https://github.com/flatland/ordered
Andy
On Thu, Jun 6, 2013 at 7:56 AM, dennis zhuang <killme2...@gmail.com> wrote:
> Thanks,you are right.I want to creat a map which keeps elements in
> insertion order, but clojure doesn‘t have.
> 在 2013-6-6 下午10:02，"Andy Fingerhut" <andy.finger...@gmail.com>写道：
>
> Your comparator #(if (= %1 %2) 0 1) may happen to give the correct answers
>> for your example sorted-maps, but it is also a bad comparator that will
>> fail for larger examples:
>>
>> user=> (:a (sorted-map-by #(if (= %1 %2) 0 1) :z -26 :b 1 :a 2 :c 3 :m 13
>> :h 8))
>> nil
>> user=> (:z (sorted-map-by #(if (= %1 %2) 0 1) :z -26 :b 1 :a 2 :c 3 :m 13
>> :h 8))
>> nil
>>
>> That is because if two items are not =, by returning 1 you are telling
>> the caller "the first argument should come after the second argument".
>> Thus if at some time the comparator is called as (cmp :a :z), and later it
>> is called as (cmp :z :a), it returns the inconsistent results that :a
>> should come after :z, and later that :z should come after :a. No sorted
>> tree can hope to return correct results given such an inconsistent
>> comparator.
>>
>> More examples and discussion at the link below, if you are interested:
>>
>>
>> https://github.com/jafingerhut/thalia/blob/master/doc/other-topics/comparators.md
>>
>> Andy
>>
>>
>> On Thu, Jun 6, 2013 at 4:19 AM, dennis zhuang <killme2...@gmail.com>wrote:
>>
>>> Sorry, it's my mistake.
>>> Because treep map use the comparator to compare keys, and if the
>>> comparator returns 1 constantly,it can not find the item that equals the
>>> key.
>>>
>>> So i can modified the example,and it works:
>>>
>>> user=> (sorted-map-by #(if (= %1 %2) 0 1) :b 1 :a 2)
>>> {:b 1, :a 2}
>>> user=> (:a (sorted-map-by #(if (= %1 %2) 0 1) :b 1 :a 2))
>>> 2
>>> user=> (:b (sorted-map-by #(if (= %1 %2) 0 1) :b 1 :a 2))
>>> 1
>>>
>>>
>>> 2013/6/6 dennis zhuang <killme2...@gmail.com>
>>>
>>>> user=> (sorted-map-by (constantly 1) :b 1 :a 2)
>>>> {:b 1, :a 2}
>>>> user=> (:a (sorted-map-by (constantly 1) :b 1 :a 2))
>>>> nil
>>>> user=> (keys (sorted-map-by (constantly 1) :b 1 :a 2))
>>>> (:b :a)
>>>> user=> (count (sorted-map-by (constantly 1) :b 1 :a 2))
>>>> 2
>>>> user=> (:a (sorted-map-by (constantly 1) :b 1 :a 2))
>>>> nil
>>>>
>>>> It looks so strange.The result map has keys :a and :b,but i can't get
>>>> their values.
>>>> Why? I try to hack the code,but i can't find the reason.
>>>>
>>>>
>>>> --
>>>> 庄晓丹
>>>> Email: killme2...@gmail.com xzhu...@avos.com
>>>> Site: http://fnil.net
>>>> Twitter: @killme2008
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> 庄晓丹
>>> Email: killme2...@gmail.com xzhu...@avos.com
>>> Site: http://fnil.net
>>> Twitter: @killme2008
>>>
>>>
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