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**PROGRAM:**

**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 expression 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 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 bizarre 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.

**Some** **Examples**

While compound expressions can look overly complex, they can be considered elegantly

simple. To quickly comprehend 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.

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, 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 conveniently 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, 7000,128,8,*,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 ( 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 again

and--voila!--you have an easily readable description of the expression:

if input > 10 then 10 else input

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

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 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 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:

if (input <= 56000/8 ) { input*8 } else { 56000 }

input,56000,8,DIV,LE,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.)

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