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rpntutorial - Reading RRDTtool RPN Expressions by Steve Rader
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 two less than or equal to one?''. The natural response is ``no'' and thus the RPN expression 2,1,LE evaluates to 0.
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 Condition 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.
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 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 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 traditional 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 verbose 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
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)
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.
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.)
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