From: ale@...
Date: 2001-09-01T18:37:16+09:00
Subject: [ruby-talk:20661] Re: replacing items in arrays


Hello Tobias,

On Sat, 1 Sep 2001, Tobias Reif wrote:

> 1.
> why doesn't the second block work?
> 
>      line[line_index] = '<td>' + line[line_index] + '</td>'

vs.

>      item = line[line_index]
>      item = '<td>' + item + '</td>'

Well, the reason is the same as with:

 ruby -e'a=[1,2,3]; b = a[1]; b = 4; p a; a[1] = 4; p a;'
 [1, 2, 3]
 [1, 4, 3]

The objects for the above are ('->' is a notation for a reference):

a  -> [-> 1,
       -> 2,
       -> 3
      ]

Statement b = a[1];

b  ->     2    (b starts to refer the Fixnum object with content 2
                and the object is exactly the same as the one
                a[1] is referring to)

statement b = 4;

b  ->     4

And when you print array a, it's still an array with element 0 referring
to an Fixnum object of 1, element 1 referring to a Fixnum object of 2, and
likewise for element 2.

Statement a[1] = 4 alters the array to be

a  -> [-> 1,
       -> 4,
       -> 3
      ]

The first block version first generates a value by
  '<td>' + a[2] + '</td>'
and then says a[2] = new_value, where as the latter block version never
alters a[2], thus the table remains the same.

Beware there's always danger when you use exclamation-methods, 

  a=["a", "b", "c"];
  b = a[1]
  b.gsub(/b/, "bb")
  p a
  
  b = a[1]
  b.gsub!(/b/, "bb")
  p a

=>

  ["a", "b", "c"]
  ["a", "bb", "c"]


and one has to be careful even with non-exclamated ones if they clearly
alter the state of an object:

  a = ["a", "b", "c"]
  b = a[1]
  b += "b"
  p a

  b = a[1]
  b.concat "b"
  p a

> 2.
> wouldn't it be nice to be able to just say:
> 
> Array#replace_each:
> -----
> an_array = ['one', 'two']
> 
> an_array.replace_each do |item|
>   'number ' + item
> end
> -----
> -> ['number one', 'number two']
> 
> Or did I overlook an equivalent?

  a = ["one", "two"]
  a.collect! { |e| "number " + e }
  p a

=>

  ["number one", "number two"]


     - Aleksi