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add 0,7000 to fix #246
[rrdtool.git]
/
doc
/
rpntutorial.pod
diff --git
a/doc/rpntutorial.pod
b/doc/rpntutorial.pod
index
246eb81
..
916f044
100644
(file)
--- a/
doc/rpntutorial.pod
+++ b/
doc/rpntutorial.pod
@@
-1,13
+1,11
@@
=head1 NAME
=head1 NAME
-rpntutorial - Reading RRDTtool RPN Expressions by Steve Rader
-
-=for html <div align="right"><a href="rpntutorial.pdf">PDF</a> version.</div>
+rpntutorial - Reading RRDtool RPN Expressions by Steve Rader
=head1 DESCRIPTION
=head1 DESCRIPTION
-This tutorial should help you get to grips with
rrd
tool RPN expressions
-as seen in CDEF arguments of
rrd
tool graph.
+This tutorial should help you get to grips with
RRD
tool RPN expressions
+as seen in CDEF arguments of
RRD
tool graph.
=head1 Reading Comparison Operators
=head1 Reading Comparison Operators
@@
-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
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, the
n
replace the three values from the
stack with "1". If the statement if false, replace the three values
with "0".
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.
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"
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
=head1 Reading the IF Operator
@@
-40,7
+38,7
@@
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".
("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 biz
za
re to me.
+For example, consider "1,10,100,IF". It looks biz
ar
re 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.
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.
@@
-57,28
+55,28
@@
GT, GE and EQ operators.
While compound expressions can look overly complex, they can be
considered elegantly simple. To quickly comprehend RPN expressions,
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
,
-
w
hen 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
.
+
W
hen 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")
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
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,+
2) 1,5,+ eval is 1,5,+ = 6 result is 6
3) 6
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
+Let's use that notation to conv
en
iently 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
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
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
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
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":
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"
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
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 a
nd--voila!--you have a
easily readable description
+read "if A then 10 else input". Now replace A
with
it's verbose
+description a
gain and--voila!--you have an
easily readable description
of the expression:
if input > 10 then 10 else input
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
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
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.
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,*
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 tradition
al
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:
uses the LE and DIV operators.
Answer 2:
@@
-175,7
+173,7
@@
Answer 3:
Traditional mathematic expressions are evaluated by
doing multiplication and division first, then addition and
Traditional mathematic expressions are evaluated by
doing multiplication and division first, then addition and
- subtraction. P
erentenc
es are used to force the evaluation of
+ subtraction. P
arenthes
es 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.
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.
@@
-183,7
+181,7
@@
Answer 3:
Exercise 4:
Exercise 4:
-Explain why it
i
s desirable for the RRDtool developers to implement
+Explain why it
wa
s desirable for the RRDtool developers to implement
RPN notation instead of traditional mathematical notation.
Answer 4:
RPN notation instead of traditional mathematical notation.
Answer 4:
@@
-197,4
+195,4
@@
Answer 4:
=head1 AUTHOR
=head1 AUTHOR
-
steve r
ader E<lt>rader@wiscnet.netE<gt>
+
Steve R
ader E<lt>rader@wiscnet.netE<gt>