## Counting Frequency of Values in an Array (And Sorting by Frequency?)

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

Is there no method for an array that will tell me the # of occurrences
for an item?

IE: ["a", "a", "a", "b", "c", "c"].count("a") #producing 3 ?

I almost thought that rindex would do the trick when looking at the
class docs but.. the example was just engineered to trick me :-(

I realize I could pass these to a block and count but.. wanted to make
sure it didn't exist. If not, why? Thank you.. ( I did search btw.. no
avail )

Also, what's the best way of printing out each unique item and the
number of times it occurs, sorted by numerically by the number of
times it occurs?

IE: in my example above, i'd like to see (sorted by occurrence
greatest to least)
#desired output:
a: 3
c: 2
b: 1

Or sorted from least to greatest:
#desired output:
b: 1
c: 2
a: 3

I was able to hack it by using a hash doing various things to it.. but
it didn't seem "rubyish".

Thank you for any input.

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

What could be more ruby-ish than monkeypatching a built-in class and
using inject in the process?

class Array
def counts
inject( Hash.new(0) ){ |hash,element|
hash[ element ] +=1
hash
}
end
def counts_up
counts.sort_by{ |k,v| v }
end
def counts_down
counts.sort_by{ |k,v| -v }
end
end
a = ["a", "a", "a", "b", "c", "c"]
p a.counts, a.counts_up, a.counts_down
#=> {"a"=>3, "b"=>1, "c"=>2}
#=> [["b", 1], ["c", 2], ["a", 3]]
#=> [["a", 3], ["c", 2], ["b", 1]]

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

array.select { |i| i == 'a' }.length
or array.inject(0) { |count, item| count += 1 if item == 'a'; count }

As you say:
puts array.inject(Hash.new(0)) { |hash, item| hash[item] += 1
hash }.sort_by { |k, v| v }.map { |k, v| "#{k}:#{v}" }

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

Hi,

At Thu, 12 Oct 2006 11:52:19 +0900,
x1 wrote in [ruby-talk:219218]:

FYI, Enumerable in 1.9 has that method.

--

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

Wow Capaldo, that worked! I fear for the next java programmer who has
to make sense of my code when I leave ;-)

To reverse sort, I added .reverse.. Here's the final product:
puts ["a", "a", "a", "b", "c", "c"].inject(Hash.new(0)) { |hash, item|
hash[item] += 1
hash }.sort_by { |k, v| v }.reverse.map { |k, v| "#{k}:#{v}" }

Thanks so much.

Nakada, I look forward to using it in 1.9 :-)

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

and.. so you're aware.. my hacky code was something like this:

items = {}
["a", "a", "a", "b", "c", "c"].each do |i|
if items.include? i
items[i] += 1
else
items[i] = 1
end
end

items.sort {|a,b| a<=>b}.reverse.each do |a, b|
puts a + ":" + b.to_s
end

horrific eh?

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

items = Hash.new(0)
["a","a","a","b","c","c"].each do |i|
items[i] += 1
end
items.sort_by {|key,value| -value}.each do |key, value|
puts "#{a}:#{b}"
end

Slightly less horrific?

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

Hi,
You can use ["a", "a", "a", "b", "c", "c"].grep("a").size

array.uniq.sort_by{|x|array.grep(x).size}.reverse.each{|x|puts "#{x}:
#{array.grep(x).size}"}

array.uniq.sort_by{|x|array.grep(x).size}.each{|x|puts "#{x}:
#{array.grep(x).size}"}

Regards,
Park Heesob

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

Ah ok.. I'm with ya. Meet "sort_by", my new friend. Thanks again :-D

### Counting Frequency of Values in an Array (And Sorting by Frequency?)

Smalltalk has a collection class called Bag, which is an unordered
collection of objects which keeps track of the number of occurences of
each equal element in the collection, so you can do something like:

bag <- Bag.new
bag.occurencesOf: 1 => 2
bag.occurencesOf: 1 => 1
bag.remove: 1
bag.occurrencesOf: 1 => 1

Now Bag is kind of the black sheep of the Smalltalk collection
classes. Most Smalltalkers would either never use it or overuse it.
The only use I could think of was to implement a histogram.

--
Rick DeNatale

My blog on Ruby
http://www.yqcomputer.com/