Feature #8887
Updated by nobu (Nobuyoshi Nakada) about 10 years ago
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
*
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*
```
Any comments?