Solved

Puzzle in sort

Posted on 2000-04-29
3
226 Views
Last Modified: 2010-03-05
Hi,

I am not familiar with perl, and has came across this line for a sort

foreach $key (sort {$info{$b} <=> $info{$a}}keys(%info))
{

I was told that this is used to sort a hash list by using other fields that the key, how is it so?

Thanks in Advance.
0
Comment
Question by:sata
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
3 Comments
 
LVL 84

Expert Comment

by:ozo
ID: 2762184
perldoc -f sort
perldoc -q sort
0
 
LVL 3

Accepted Solution

by:
guadalupe earned 80 total points
ID: 2762350
OK the deal is this ... first $a and $b are identifiers to the keys supplied by the keys fuction applied to the hash %info at the right of the statement.  The <=> is a numerical comparative operator... that means that we understand the values of the hash to be numeric.  If we wanted to do alphabetic comparisons we would change the <=> for cmp    Then  we yor comment about  
"hash list by using other fields that the key"

is so because you will notice that the values to the left and right of the comparison operator are indeed not the keys but the values...  as mentioned above $a and $b identify the keys and as in any hash operation $hash{key} will be the value, so
$info{$b} and $info{$a} are indeed the values of the hash corresponding to the keys   $a   and $b ...

the real magic is in the $a, $b and sort these mantain two distinct keys and keep sorting untill the whole thing is sorted and there is just perl magic which I don't pretend to understand the internal workings of but hey thats what perl is for...if you wanted it another way you would be using C!  The final thing:  foreach $key....
All the rest before returns a sorted list of keys so you just take them one at a time and go!!!  The only other thing is that if I remember correctly
$info{$b} <=> $info{$a} will give lesser to greater  either way if it orders the wrong way from wat you wnt you can always chnge it to:

$info{$a} <=> $info{$b}  and the same if you were using cmp:

$info{$a} cmp $info{$b}...

Hope this has made things a bit clearer!
0
 
LVL 1

Author Comment

by:sata
ID: 2765368
Thanks for the answer!!
0

Featured Post

Enroll in May's Course of the Month

May’s Course of the Month is now available! Experts Exchange’s Premium Members and Team Accounts have access to a complimentary course each month as part of their membership—an extra way to increase training and boost professional development.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Many time we need to work with multiple files all together. If its windows system then we can use some GUI based editor to accomplish our task. But what if you are on putty or have only CLI(Command Line Interface) as an option to  edit your files. I…
A year or so back I was asked to have a play with MongoDB; within half an hour I had downloaded (http://www.mongodb.org/downloads),  installed and started the daemon, and had a console window open. After an hour or two of playing at the command …
Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions. Consider email address RFCs: Look at HTML5 form input element (with type=email) regex pattern: T…

738 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question