From: "Hanmac (Hans Mackowiak)" <hanmac@...>
Date: 2013-09-10T23:03:26+09:00
Subject: [ruby-core:57112] [ruby-trunk - Feature #8887] min(n), max(n),	min_by(n), max_by(n)


Issue #8887 has been updated by Hanmac (Hans Mackowiak).


hm i am curios in what order max(n) and max_by(n) should return the elements?

like [6, 0, 3, 3, 8, 3, 5, 0, 6].max(4):
should it return sort.last(n) #> [5, 6, 6, 8]
or is that better? [8, 6, 6, 5] because 8 is bigger than 6 ?

PS: min_by(n) and max_by(n) (and maybe sort_by(n)) should maybe implemented different without take, so that it max_by(n) picks only the first n biggest objects without sorting the entire test of the Enumerator?

also about your patch, e.min_by(n).size should respect also n and also enum_size, so it needs to return n when n < as size, and size when n > size (for size != nil)


----------------------------------------
Feature #8887: min(n), max(n), min_by(n), max_by(n)
https://bugs.ruby-lang.org/issues/8887#change-41723

Author: akr (Akira Tanaka)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


How about adding an optional argument, n, for Enumerable#{min,max,min_by,max_by} to
return minimum/maximum n elements as an array.

Example:
* [6, 0, 3, 3, 8, 3, 5, 0, 6].min(4) #=> [0, 0, 3, 3]
* [6, 0, 3, 3, 8, 3, 5, 0, 6].max(4) #=> [5, 6, 6, 8]
* [6, 0, 3, 3, 8, 3, 5, 0, 6].min_by(4) {|v| (v-5)**2 } #=> [5, 6, 6, 3]
* [6, 0, 3, 3, 8, 3, 5, 0, 6].max_by(4) {|v| (v-5)**2 } #=> [3, 8, 0, 0]

These methods are similar to sort follows first or last.

* e.min(n) is similar to e.sort.first(n)
* e.max(n) is similar to e.sort.last(n)
* e.min_by(n) {...} is similar to e.sort_by {...}.first(n)
* e.max_by(n) {...} is similar to e.sort_by {...}.last(n)

However e.min(n), e.max(n), e.min_by(n), e.max_by(n) are
less memory consuming and can be faster.
They use memory proportional to n, not e.
They doesn't sort whole e. 

I feel their use is not rare.
I found several use after searching.

[ruby-talk:123508], [ruby-list:40939], [ruby-talk:273980]
http://d.hatena.ne.jp/mjh/20101024/1287901875
http://stackoverflow.com/questions/11094874/get-top-n-elements-from-ruby-array-of-hash-values
http://www.math.kobe-u.ac.jp/~kodama/tips-ruby-sized_pqueue.html
https://bitbucket.org/sterlingcamden/topn

Also, e.max(n) can be used to implement weighted random sampling.

Pavlos S. Efraimidis, Paul G. Spirakis
Weighted random sampling with a reservoir
Information Processing Letters
Volume 97, Issue 5 (16 March 2006)

  % ./ruby -e '
  module Enumerable
    def wsample(n)
      self.max_by(n) {|v| rand ** (1.0/yield(v)) }
    end
  end
  e = (-20..20).to_a*10000
  a = e.wsample(20000) {|x|
    Math.exp(-(x/5.0)**2) # normal distribution
  }
  # a is 20000 samples from e.
  p a.length
  h = a.group_by {|x| x }
  -10.upto(10) {|x| puts "*" * (h[x].length/30.0).to_i if h[x] }
  '
  20000
  *
  ***
  ******
  ***********
  ******************
  *****************************
  *****************************************
  ****************************************************
  ***************************************************************
  ********************************************************************
  ***********************************************************************
  ***********************************************************************
  **************************************************************
  ****************************************************
  ***************************************
  ***************************
  ******************
  ***********
  *******
  ***
  *

Any comments?



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