results from sort-by are not sorted

[sheng.p...@gmail.com]

Hey there, 

in my lein repl, 

(sort-by #{:age} [{:age 3,:name "luo"},{:age 1,:name "sheng"}]) 

returns 

({:age 3, :name "luo"} {:age 1, :name "sheng"}) 

rather than 

({:age 1, :name "sheng"}, {:age 3, :name "luo"}). 

Why?

[Bill Xie]

clojure.core/sort-by

([keyfn coll] [keyfn comp coll])

  Returns a sorted sequence of the items in coll, where the sort

  order is determined by comparing (keyfn item).  If no comparator is

  supplied, uses compare.  comparator must implement

  java.util.Comparator.  Guaranteed to be stable: equal elements will

  not be reordered.  If coll is a Java array, it will be modified.  To

  avoid this, sort a copy of the array.

nil

<sheng.p...@gmail.com> 于2019年5月6日周一 上午10:48写道：

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[se...@corfield.org]

(sort-by #{:age} …) will use the set #{:age} as the keyfn, and both (#{:age} {:age 3, :name “luo”}) and (#{:age} {:age 1, :name “sheng”}) both return :age – because both maps contain the key in the set. As far as sort-by is concerned, both hash maps compare equal.

 

What you want is (sort-by :age …) so you the keyfn pulls the value corresponding to :age out of the hash maps. That will produce 3 from {:age 3, :name “luo”} and 1 from {:age 1, :name “sheng”} so they will sort appropriately.

 

Sean Corfield -- (970) FOR-SEAN -- (904) 302-SEAN
An Architect's View -- http://corfield.org/

"If you're not annoying somebody, you're not really alive."
-- Margaret Atwood

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[sheng.p...@gmail.com]

Thanks. What a newbie question.

在 2019年5月6日星期一 UTC+8上午11:34:36，se...@corfield.org写道：

(sort-by #{:age} …) will use the set #{:age} as the keyfn, and both (#{:age} {:age 3, :name “luo”}) and (#{:age} {:age 1, :name “sheng”}) both return :age – because both maps contain the key in the set. As far as sort-by is concerned, both hash maps compare equal.

 

What you want is (sort-by :age …) so you the keyfn pulls the value corresponding to :age out of the hash maps. That will produce 3 from {:age 3, :name “luo”} and 1 from {:age 1, :name “sheng”} so they will sort appropriately.

 

Sean Corfield -- (970) FOR-SEAN -- (904) 302-SEAN
An Architect's View -- http://corfield.org/

"If you're not annoying somebody, you're not really alive."
-- Margaret Atwood

 

From: sheng....@gmail.com
Sent: Sunday, May 5, 2019 7:48 PM
To: Clojure
Subject: results from sort-by are not sorted

 

Hey there, 

 

in my lein repl, 

 

(sort-by #{:age} [{:age 3,:name "luo"},{:age 1,:name "sheng"}]) 

 

returns 

 

({:age 3, :name "luo"} {:age 1, :name "sheng"}) 

 

rather than 

 

({:age 1, :name "sheng"}, {:age 3, :name "luo"}). 

 

Why?

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[Justin Smith]

minor nitpick to the answer Sean provided: #{:age} as a function returns :age for an argument equal to :age and nil for all other inputs, including a hash map containing that key.

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[Sean Corfield]

Good catch, thank you! And that was my _second_ edit (my first draft was also wrong in a different way). Lesson: just try this stuff in the REPL to see what _really_ happens! 😊

 

Sean Corfield -- (904) 302-SEAN

An Architect's View -- http://corfield.org/

"If you're not annoying somebody, you're not really alive."
-- Margaret Atwood

 

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From: clo...@googlegroups.com <clo...@googlegroups.com> on behalf of Justin Smith <noise...@gmail.com>
Sent: Monday, May 6, 2019 1:53:48 PM
To: Clojure
Subject: Re: results from sort-by are not sorted