From: Robert Klemme <shortcutter@...>
Date: 2008-03-29T01:15:05+09:00
Subject: Re: numbers of a record in an array

On 28.03.2008 03:41, Charles Calvert wrote:
> On Tue, 25 Mar 2008 05:23:50 -0500, Robert Klemme wrote:
> 
>> 2008/3/25, Jesús Gabriel y Galán <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.

Probably just a spelling error: it should read "for each e in a ...". 
Your pseudo code has it actually correct.

>  I would pseudocode it as this:
> 
> cnt = Hash.new(0)
> foreach (e in a)
> {
> 	cnt[e] += 1
> }

Btw, it's not far from pseudo code to Ruby code:

irb(main):003:0> cnt = Hash.new 0
=> {}
irb(main):004:0> for e in a do
irb(main):005:1*   cnt[e] += 1
irb(main):006:1> end
=> ["a", "b", "a", "c", "b", "a", "c", "c"]
irb(main):007:0> cnt
=> {"a"=>3, "b"=>2, "c"=>3}
irb(main):008:0>

:-)

> 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.

As Todd said, the return value of the block is the next iteration's 
accumulator:

irb(main):009:0> (1..5).inject(0) {|*a| p a;a.first+10}
[0, 1]
[10, 2]
[20, 3]
[30, 4]
[40, 5]
=> 50

This allows elegant code, as in

irb(main):010:0> (1..5).inject(0){|s,x| s+x}
=> 15

which would not work if s was the same instance for every iteration.

Kind regards

	robert