RPNTUTORIAL(1) rrdtool RPNTUTORIAL(1)
NAME
rpntutorial  Reading RRDtool RPN Expressions by Steve Rader
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, 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 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 expres
sion 2,1,GT evaluates to 1.
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.
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 expres
sion "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 bizarre to me. But when
I read "if 1 then 10 else 100" its 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 Condition then Y else Z". So its important
to be able to read IF operators along with the LT, LE, GT, GE and EQ
operators.
Some Examples
While compound expressions can look overly complex, they can be consid
ered 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 its 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 and is replaced by 5. This results in the
stack "1,5,+". Finally, "1,5,+" is evaluated resulting in the answer
6. For convenience, its 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
Lets use that notation to conveniently solve some complex RPN expres
sions 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.
Lets read a complex RPN expression that also has the traditional mul
tiplication 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 lets 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 ( lets call this 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 with its verbose
description againg andvoila!you have a easily readable description
of the expression:
if input > 10 then 10 else input
Finally, lets 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
where A is "input * 8"
2) A,7000,GT,7000,input,8,*,IF eval is A,7000,GT result is B
where B is "if ((input * 8) > 7000) then true"
3) B,7000,input,8,*,IF eval is input,8,* result is C
where C is "input * 8"
4) B,7000,C,IF
At last we have a readable decoding of the complex RPN expression with
a variable:
if ((input * 8) > 7000) then 7000 else (input * 8)
Exercises
Exercise 1:
Compute "3,2,*,1,+ and "3,2,1,+,*" by hand. Rewrite them in tradi
tional 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 traditional notation to show these expressions are not the same.
Write an expression thats 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. Parentheses 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 was desirable for the RRDtool developers to implement
RPN notation instead of traditional mathematical notation.
Answer 4:
The algorithm that implements traditional mathematical
notation is more complex then algorithm used for RPN.
So implementing RPN allowed Tobias Oetiker to write less
code! (The code is also less complex and therefore less
likely to have bugs.)
AUTHOR
Steve Rader
1.2.15 20060714 RPNTUTORIAL(1)
