From: sempsteen <sempsteen@...>
Date: 2006-11-25T22:37:55+09:00
Subject: Re: coding practise

correction:
this also does two method calls
puts hsh['id'] if hsh['id'] != nil

this is the one which i wanted to write, only one method call:
result = hsh['id']
puts result if result != nil

On 11/25/06, sempsteen <sempsteen@gmail.com> wrote:
> Thanks for your replies Gareth and Paul.
> I've improved return statement as you suggested Paul.
> this didn't worked:
> return self == t2 and self != t1
> but this:
> return (self == t2 and self != t1)
> and this:
> self == t2 and self != t1
> worked.
>
> I've also set the sums' initial values to 1 and start the loops from 2
> which is more reasonable.
> Paul, i understood your code.
> Amicable numbers was just an example. I actually want to know where to
> use the method pairs like:
> has_key? [],
> has_friend?, friend
> has_sth?, give_that
> ...
>
> When we call "has_friend?" method it does the same job as "friend"
> method except that it returns true or false not the amicable number.
> When we call "Hash#has_key?" method it does the same job as "Hash#[]"
> method except that it returns true or false, not the value of the
> given Hash index.
>
> So if they both do the same job why do i need to search for an
> existing value and then call another method that gives the value like:
> hsh = {'id' => 10, 'lang' => 'Ruby'}
> puts hsh['id'] if hsh.has_key?('id')
>
> I can write it like this:
> puts hsh['id'] if hsh['id'] != nil
>
> I hope i could explain it.
>
> On 11/25/06, Paul Lutus <nospam@nosite.zzz> wrote:
> > sempsteen wrote:
> >
> > > Hi all,
> > > First of all sorry for my english.
> > > I'm a Ruby newbie, trying to learn the language from the book, "The
> > > Pragmatic Programmer's Guide". I loved the language very much. Now i
> > > have some question marks about some issues.
> > > If you help me understand this concept i'll be very happy.
> > >
> > > 1-) What does "Hash#has_key?" actually do?
> >
> > It returns true if provided with a key that is present in the hash.
> >
> > > Why do we need such a method in spite of using the result of "Hash#[]"
> > > method which will return nil for a non-present key.
> >
> > The method has_key? is faster than using the key to find and return a value,
> > which is what Hash#[] must do.
> >
> > > 2-) If we go ahead by the same manner is this a correct way of writing
> > > a program that finds amicable numbers:
> > >
> > > class Fixnum
> > >    def has_friend?
> > >       t1, t2 = 0, 0
> > >       1.upto(self / 2) {|i| t1 += i if self % i == 0}
> > >       1.upto(t1 / 2) {|i| t2 += i if t1 % i == 0}
> >
> > For this section:
> >
> > >       if self == t2 and self != t1
> > >          return true
> > >       else
> > >          return false
> >
> > Use this:
> >
> > return self == t2 and self != t1
> >
> > This produces the same result.
> >
> > Also, in each of your loops you are testing whether a particular number can
> > be divided by one with no remainder. The answer is always yes, so for each
> > calculated value skip this test (start with 2 not 1) and set the initial
> > value equal to 1.
> >
> > An article about amicable numbers, with some facts that may improve your
> > method of calculating them:
> >
> > http://en.wikipedia.org/wiki/Amicable_number
> >
> > About the general topic, it is more efficient to compile an array of divisor
> > sums and compare in that fashion than to test each number separately as you
> > are doing. Like this:
> >
> > -----------------------------------------------
> >
> > #!/usr/bin/ruby -w
> >
> > hash = {}
> >
> > max = 10000
> >
> > 2.upto(max) do |i|
> >    sum = 1
> >    2.upto(i/2) do |j|
> >       sum += j if (i % j) == 0
> >    end
> >    hash[i] = sum
> > end
> >
> > hash.keys.sort.each do |i|
> >    a = hash[i]
> >    b = hash[a]
> >    puts "#{a} <-> #{b}" if a != b && b == i
> > end
> >
> > -----------------------------------------------
> >
> > Output:
> >
> > 284 <-> 220
> > 220 <-> 284
> > 1210 <-> 1184
> > 1184 <-> 1210
> > 2924 <-> 2620
> > 2620 <-> 2924
> > 5564 <-> 5020
> > 5020 <-> 5564
> > 6368 <-> 6232
> > 6232 <-> 6368
> >
> > --
> > Paul Lutus
> > http://www.arachnoid.com
> >
> >
>
>