Welcome to Linux Knowledge Base and Tutorial
"The place where you learn linux"
Linux Magazine - Missing Anything?

 Create an AccountHome | Submit News | Your Account  

Tutorial Menu
Linux Tutorial Home
Table of Contents

· Introduction to Operating Systems
· Linux Basics
· Working with the System
· Shells and Utilities
· Editing Files
· Basic Administration
· The Operating System
· The X Windowing System
· The Computer Itself
· Networking
· System Monitoring
· Solving Problems
· Security
· Installing and Upgrading
· Linux and Windows

Glossary
MoreInfo
Man Pages
Linux Topics
Test Your Knowledge

Site Menu
Site Map
FAQ
Copyright Info
Terms of Use
Privacy Info
Disclaimer
WorkBoard
Thanks
Donations
Advertising
Masthead / Impressum
Your Account

Communication
Feedback
Forums
Private Messages
Surveys

Features
HOWTOs
News Archive
Submit News
Topics
User Articles
Web Links

Google
Google


The Web
linux-tutorial.info

Who's Online
There are currently, 186 guest(s) and 0 member(s) that are online.

You are an Anonymous user. You can register for free by clicking here

  

rpntutorial




DESCRIPTION

       This tutorial should help you get to grips with rrdtool
       RPN expressions as seen in CDEF arguments of rrdtool
       graph.


Reading Comparison Operators

       The LT, LE, GT, GE and EQ RPN logic operators are not as
       tricky as they appear.  These operators act on the two
       values on the stack preceding them (to the left).  Read
       these two values on the stack from left to right inserting
       the operator in the middle.  If the resulting statement is
       true, the replace the three values from the stack with
       "1".  If the statement if false, replace the three values
       with "0".

       For example think about "2,1,GT".  This RPN expression
       could be read as "is two greater than one?"  The answer to
       that question is "true".  So the three values should be
       replaced with "1".  Thus the RPN expression 2,1,GT evalu­
       ates to 1.

       Now also consider "2,1,LE".  This RPN expression could be
       read as "is two less than or equal to one?".   The natural
       response is "no" and thus the RPN expression 2,1,LE evalu­
       ates to 0.


Reading the IF Operator

       The IF RPN logic operator can be straightforward also.
       The key to reading IF operators is to understand that the
       condition part of the traditional "if X than Y else Z"
       notation has *already* been evaluated.  So the IF operator
       acts on only one value on the stack: the third value to
       the left of the IF value.  The second value to the left of
       the IF corresponds to the true ("Y") branch.  And the
       first value to the left of the IF corresponds to the false
       ("Z") branch.  Read the RPN expression "X,Y,Z,IF" from
       left to right like so: "if X then Y else Z".

       For example, consider "1,10,100,IF".  It looks bizzare to
       me.  But when I read "if 1 then 10 else 100" it's crystal
       clear: 1 is true so the answer is 10.  Note that only zero
       is false; all other values are true.  "2,20,200,IF" ("if 2
       then 20 else 200") evaluates to 20.  And "0,1,2,IF" ("if 0
       then 1 else 2) evaluates to 2.

       Notice that none of the above examples really simulate the
       whole "if X then Y else Z" statement.  This is because
       computer programmers read this statement as "if Some Con­
       dition then Y else Z".  So it's important to be able to
       read IF operators along with the LT, LE, GT, GE and EQ
       operators.
       sake, it's useful to write this set of operations as:

        1) 1,2,3,+,+    eval is 2,3,+ = 5    result is 1,5,+
        2) 1,5,+        eval is 1,5,+ = 6    result is 6
        3) 6

       Let's use that notation to conviently solve some complex
       RPN expressions with multiple logic operators:

        1) 20,10,GT,10,20,IF  eval is 20,10,GT = 1     result is 1,10,20,IF

       read the eval as pop "20 is greater than 10" so push 1

        2) 1,10,20,IF         eval is 1,10,20,IF = 10  result is 10

       read pop "if 1 then 10 else 20" so push 10.  Only 10 is
       left so 10 is the answer.

       Let's read a complex RPN expression that also has the tra­
       ditional multiplication operator:

        1) 128,8,*,7000,GT,7000,128,8,*,IF  eval 128,8,*       result is 1024
        2) 1024,7000,GT,7000,128,8,*,IF     eval 1024,7000,GT  result is 0
        3) 0,128,8,*,IF                     eval 128,8,*       result is 1024
        4) 0,7000,1024,IF                                      result is 1024

       Now let's go back to the first example of multiple logic
       operators but replace the value 20 with the variable
       "input":

        1) input,10,GT,10,input,IF  eval is input,10,GT  result is A

       Read eval as "if input > 10 then true" and replace
       "input,10,GT" with "A:

        2) A,10,input,IF            eval is A,10,input,IF

       read "if A then 10 else input".  Now replace A it's ver­
       bose description and--voila!--you have a easily readable
       description of the expression:

        if input > 10 then 10 else input

       Lastly, let's to back the first most complex example and
       replace the value 128 with "input":

        1) input,8,*,7000,GT,7000,input,8,*,IF  eval input,8,*     result is A

       where A is "input * 8"

        2) A,7000,GT,7000,input,8,*,IF          eval is A,7000,GT  result is B

       Exercise 1:

       Compute "3,2,*,1,+ and "3,2,1,+,*" by hand.  Rewrite them
       in traditional notation.  Explain why they have different
       answers.

       Answer 1:

           3*2+1 = 7 and 3*(2+1) = 9.  These expressions have
           different answers because the altering of the plus and
           times operators alter the order of their evaluation.

       Exercise 2:

       One may be tempted to shorten the expression

        input,8,*,56000,GT,56000,input,*,8,IF

       by removing the redundant use of "input,8,*" like so:

        input,56000,GT,56000,input,IF,8,*

       Use tradition notation to show these expressions are not
       the same.  Write an expression that's equivalent to the
       first expression but uses the LE and DIV operators.

       Answer 2:

           if (input <= 56000/8 ) { input*8 } else { 56000 }
           input,56000,8,DIV,LT,input,8,*,56000,IF

       Exercise 3:

       Briefly explain why traditional mathematic notation
       requires the use of parentheses.  Explain why RPN notation
       does not require the use of parentheses.

       Answer 3:

           Traditional mathematic expressions are evaluated by
           doing multiplication and division first, then addition and
           subtraction.  Perentences are used to force the evaluation of
           addition before multiplication (etc).  RPN does not require
           parentheses because the ordering of objects on the stack
           can force the evaluation of addition before multiplication.

       Exercise 4:

       Explain why it is desirable for the RRDtool developers to
       implement RPN notation instead of traditional mathematical
       notation.

  
Show your Support for the Linux Tutorial

Purchase one of the products from our new online shop. For each product you purchase, the Linux Tutorial gets a portion of the proceeds to help keep us going.


Login
Nickname

Password

Security Code
Security Code
Type Security Code


Don't have an account yet? You can create one. As a registered user you have some advantages like theme manager, comments configuration and post comments with your name.

Help if you can!


Amazon Wish List

Did You Know?
The Linux Tutorial can use your help.


Friends



Tell a Friend About Us

Bookmark and Share



Web site powered by PHP-Nuke

Is this information useful? At the very least you can help by spreading the word to your favorite newsgroups, mailing lists and forums.
All logos and trademarks in this site are property of their respective owner. The comments are property of their posters. Articles are the property of their respective owners. Unless otherwise stated in the body of the article, article content (C) 1994-2013 by James Mohr. All rights reserved. The stylized page/paper, as well as the terms "The Linux Tutorial", "The Linux Server Tutorial", "The Linux Knowledge Base and Tutorial" and "The place where you learn Linux" are service marks of James Mohr. All rights reserved.
The Linux Knowledge Base and Tutorial may contain links to sites on the Internet, which are owned and operated by third parties. The Linux Tutorial is not responsible for the content of any such third-party site. By viewing/utilizing this web site, you have agreed to our disclaimer, terms of use and privacy policy. Use of automated download software ("harvesters") such as wget, httrack, etc. causes the site to quickly exceed its bandwidth limitation and are therefore expressly prohibited. For more details on this, take a look here

PHP-Nuke Copyright © 2004 by Francisco Burzi. This is free software, and you may redistribute it under the GPL. PHP-Nuke comes with absolutely no warranty, for details, see the license.
Page Generation: 0.09 Seconds