From: Charles Calvert <cbciv@...>
Date: 2008-03-28T11:44:54+09:00
Subject: Re: numbers of a record in an array

On Tue, 25 Mar 2008 05:23:50 -0500, Robert Klemme wrote:

> 2008/3/25, Jes炭s Gabriel y Gal叩n <jgabrielygalan@gmail.com>:
>> On Tue, Mar 25, 2008 at 10:09 AM, Bu Mihai <mihai.bulhac@yahoo.com> wrote:
>>  > a have an array array=["a","b","c","a","b"]
>>  >  how can i find the numbers of "a", "b","c" items in the array?
>>
>>
>> Here's one way:
>>
>>  irb(main):006:0> a = %w{a b a c b a c c}
>>  => ["a", "b", "a", "c", "b", "a", "c", "c"]
>>  irb(main):007:0> h = Hash.new {|h,k| h[k] = 0}
>>  => {}
>>  irb(main):008:0> a.each {|x| h[x] += 1}
>>  => ["a", "b", "a", "c", "b", "a", "c", "c"]
>>  irb(main):009:0> h
>>  => {"a"=>3, "b"=>2, "c"=>3}
> 
> Here's another

Let's see if I understand this:

> irb(main):001:0> a = %w{a b a c b a c c}
> => ["a", "b", "a", "c", "b", "a", "c", "c"]

Creates an array of strings.

> irb(main):002:0> a.inject(Hash.new(0)){|cnt,e| cnt[e]+=1; cnt}
> => {"a"=>3, "b"=>2, "c"=>3}

Calls the method Enumerable::inject on the variable a, passing Hash.new(0)
as the initial value of memo, which is the accumulator.  For each e in cnt
(which is a reference to the hash returned by "Hash.new"), it increments
the count whose key is the value of e.  I would pseudocode it as this:

cnt = Hash.new(0)
foreach (e in a)
{
	cnt[e] += 1
}

What does the last "cnt" do, right before the closing brace, return a
reference to the hash? I tried removing that bit:

	a.inject(Hash.new(0)){|cnt,e| cnt[e]+=1}

and got the error:

	TypeError: can't convert String into Integer
        	from (irb):2:in `[]'
	        from (irb):2
        	from (irb):2:in `inject'
	        from (irb):2:in `each'
        	from (irb):2:in `inject'
	        from (irb):2
        	from :0

I'm guessing that the use of "cnt" is controlling the interpretation of
the type returned by cnt[e], but I don't understand how.

-- 
Charles Calvert             |  Software Design/Development
Celtic Wolf, Inc.           |  Project Management
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