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rpntutorial - Reading RRDtool RPN Expressions by Steve Rader
-=for html <div align="right"><a href="rpntutorial.pdf">PDF</a> version.</div>
-
=head1 DESCRIPTION
This tutorial should help you get to grips with RRDtool RPN expressions
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
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:
+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.
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
Now let's go back to the first example of multiple logic operators,
but replace the value 20 with the variable "input":
-=for comment
-XXX wo kommt das A ploetzlich her? Hier braucht es einen Satz, dass A als
-XXX placeholder zum Lesbarmachen verwendet wird (shortcut).
-
- 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":
-
+
2) A,10,input,IF eval is A,10,input,IF
read "if A then 10 else input". Now replace A with it's verbose
-description againg and--voila!--you have a easily readable description
+description again and--voila!--you have a easily readable description
of the expression:
if input > 10 then 10 else input
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.