X-Git-Url: https://git.octo.it/?p=rrdtool.git;a=blobdiff_plain;f=doc%2Frpntutorial.pod;h=916f0448b5f18c71537bb90f821840c1ea101b73;hp=13ee0ecd04977d82cc1e60e7043f329ace689b56;hb=739bd5fa6279fbabcf081b749810ef36474bfb49;hpb=d828f3eccac8dbad7bfc14812e406377669baaa4 diff --git a/doc/rpntutorial.pod b/doc/rpntutorial.pod index 13ee0ec..916f044 100644 --- a/doc/rpntutorial.pod +++ b/doc/rpntutorial.pod @@ -2,8 +2,6 @@ rpntutorial - Reading RRDtool RPN Expressions by Steve Rader -=for html
PDF version.
- =head1 DESCRIPTION This tutorial should help you get to grips with RRDtool RPN expressions @@ -15,18 +13,18 @@ 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 +resulting statement is true, then 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 +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 evaluates to 1. -Now also consider "2,1,LE". This RPN expression could be read as "is +Now 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 evaluates to 0. +and thus the RPN expression 2,1,LE evaluates to 0. =head1 Reading the IF Operator @@ -57,15 +55,15 @@ GT, GE and EQ operators. While compound expressions can look overly complex, they can be considered elegantly simple. To quickly comprehend RPN expressions, -you must know the the algorithm for evaluating RPN expressions: -iterate searches from the left to the right looking for an operator, -when it's found, apply that operator by popping the operator and some +you must know the algorithm for evaluating RPN expressions: +iterate searches from the left to the right looking for an operator. +When it's found, apply that operator by popping the operator and some number of values (and by definition, not operators) off the stack. For example, the stack "1,2,3,+,+" gets "2,3,+" evaluated (as "2+3") -during the first iteration which is replaced by 5. This results in +during the first iteration and is replaced by 5. This results in the stack "1,5,+". Finally, "1,5,+" is evaluated resulting in the -answer 6. For convenience sake, it's useful to write this set of +answer 6. For convenience, it's useful to write this set of operations as: 1) 1,2,3,+,+ eval is 2,3,+ = 5 result is 1,5,+ @@ -78,7 +76,7 @@ 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 @@ -89,27 +87,27 @@ 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 + 3) 0,700,0,128,8,*,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 +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 + 1) input,10,GT,10,input,IF eval is input,10,GT ( lets call this A ) Read eval as "if input > 10 then true" and replace "input,10,GT" -with "A: - +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 verbose -description and--voila!--you have a easily readable description +read "if A then 10 else input". Now replace A with it's verbose +description again and--voila!--you have an 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 +Finally, let's go back to 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 @@ -141,7 +139,7 @@ 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 + different answers because the altering of the plus and times operators alter the order of their evaluation. @@ -155,8 +153,8 @@ 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 +Use traditional 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: @@ -183,7 +181,7 @@ Answer 3: Exercise 4: -Explain why it is desirable for the RRDtool developers to implement +Explain why it was desirable for the RRDtool developers to implement RPN notation instead of traditional mathematical notation. Answer 4: