RRDGRAPH_RPN(1)                     rrdtool                    RRDGRAPH_RPN(1)



NAME
       rrdgraph_rpn - About RPN Math in rrdtool graph

SYNOPSIS
       RPN expression:=vname|operator|value[,RPN expression]

DESCRIPTION
       If you have ever used a traditional HP calculator you already know RPN.
       The idea behind RPN is that you have a stack and push your data onto
       this stack. Whenever you execute an operation, it takes as many ele-
       ments from the stack as needed. Pushing is done implicitly, so whenever
       you specify a number or a variable, it gets pushed onto the stack auto-
       matically.

       At the end of the calculation there should be one and only one value
       left on the stack.  This is the outcome of the function and this is
       what is put into the vname.  For CDEF instructions, the stack is pro-
       cessed for each data point on the graph. VDEF instructions work on an
       entire data set in one run. Note, that currently VDEF instructions only
       support a limited list of functions.

       Example: "VDEF:maximum=mydata,MAXIMUM"

       This will set variable "maximum" which you now can use in the rest of
       your RRD script.

       Example: "CDEF:mydatabits=mydata,8,*"

       This means:  push variable mydata, push the number 8, execute the oper-
       ator *. The operator needs two elements and uses those to return one
       value.  This value is then stored in mydatabits.  As you may have
       guessed, this instruction means nothing more than mydatabits = mydata *
       8.  The real power of RPN lies in the fact that it is always clear in
       which order to process the input.  For expressions like "a = b + 3 * 5"
       you need to multiply 3 with 5 first before you add b to get a. However,
       with parentheses you could change this order: "a = (b + 3) * 5". In
       RPN, you would do "a = b, 3, +, 5, *" without the need for parentheses.

OPERATORS
       Boolean operators
           LT, LE, GT, GE, EQ, NE

           Pop two elements from the stack, compare them for the selected con-
           dition and return 1 for true or 0 for false. Comparing an unknown
           or an infinite value will always result in 0 (false).

           UN, ISINF

           Pop one element from the stack, compare this to unknown respec-
           tively to positive or negative infinity. Returns 1 for true or 0
           for false.

           IF

           Pops three elements from the stack.  If the element popped last is
           0 (false), the value popped first is pushed back onto the stack,
           otherwise the value popped second is pushed back. This does,
           indeed, mean that any value other than 0 is considered to be true.

           Example: "A,B,C,IF" should be read as "if (A) then (B) else (C)"



       Comparing values
           MIN, MAX

           Pops two elements from the stack and returns the smaller or larger,
           respectively.  Note that infinite is larger than anything else.  If
           one of the input numbers is unknown then the result of the opera-
           tion will be unknown too.

           LIMIT

           Pops two elements from the stack and uses them to define a range.
           Then it pops another element and if it falls inside the range, it
           is pushed back. If not, an unknown is pushed.

           The range defined includes the two boundaries (so: a number equal
           to one of the boundaries will be pushed back). If any of the three
           numbers involved is either unknown or infinite this function will
           always return an unknown

           Example: "CDEF:a=alpha,0,100,LIMIT" will return unknown if alpha is
           lower than 0 or if it is higher than 100.



       Arithmetics
           +, -, *, /, %

           Add, subtract, multiply, divide, modulo

           SIN, COS, LOG, EXP, SQRT

           Sine and cosine (input in radians), log and exp (natural loga-
           rithm), square root.

           ATAN

           Arctangent (output in radians).

           ATAN2

           Arctangent of y,x components (output in radians).  This pops one
           element from the stack, the x (cosine) component, and then a sec-
           ond, which is the y (sine) component.  It then pushes the arctan-
           gent of their ratio, resolving the ambiguity between quadrants.

           Example: "CDEF:angle=Y,X,ATAN2,RAD2DEG" will convert "X,Y" compo-
           nents into an angle in degrees.

           FLOOR, CEIL

           Round down or up to the nearest integer.

           DEG2RAD, RAD2DEG

           Convert angle in degrees to radians, or radians to degrees.

       Set Operations
           SORT, REV

           Pop one element from the stack.  This is the count of items to be
           sorted (or reversed).  The top count of the remaining elements are
           then sorted (or reversed) in place on the stack.

           Example: "CDEF:x=v1,v2,v3,v4,v5,v6,6,SORT,POP,5,REV,POP,+,+,+,4,/"
           will compute the average of the values v1 to v6 after removing the
           smallest and largest.

           AVG

           Pop one element (count) from the stack. Now pop count elements and
           build the average, ignoring all UNKNOWN values in the process.

           Example: "CDEF:x=a,b,c,d,4,AVG"

           TREND

           Create a "sliding window" average of another data series.

           Usage: CDEF:smoothed=x,1800,TREND

           This will create a half-hour (1800 second) sliding window average
           of x.  The average is essentially computed as shown here:

                            +---!---!---!---!---!---!---!---!--->
                                                                now
                                  delay     t0
                            <--------------->
                                    delay       t1
                                <--------------->
                                         delay      t2
                                    <--------------->

                Value at sample (t0) will be the average between (t0-delay) and (t0)
                Value at sample (t1) will be the average between (t1-delay) and (t1)
                Value at sample (t2) will be the average between (t2-delay) and (t2)

       Special values
           UNKN

           Pushes an unknown value on the stack

           INF, NEGINF

           Pushes a positive or negative infinite value on the stack. When
           such a value is graphed, it appears at the top or bottom of the
           graph, no matter what the actual value on the y-axis is.

           PREV

           Pushes an unknown value if this is the first value of a data set or
           otherwise the result of this CDEF at the previous time step. This
           allows you to do calculations across the data.  This function can-
           not be used in VDEF instructions.

           PREV(vname)

           Pushes an unknown value if this is the first value of a data set or
           otherwise the result of the vname variable at the previous time
           step. This allows you to do calculations across the data. This
           function cannot be used in VDEF instructions.

           COUNT

           Pushes the number 1 if this is the first value of the data set, the
           number 2 if it is the second, and so on. This special value allows
           you to make calculations based on the position of the value within
           the data set. This function cannot be used in VDEF instructions.

       Time
           Time inside RRDtool is measured in seconds since the epoch. The
           epoch is defined to be "Thu Jan 1 00:00:00 UTC 1970".

           NOW

           Pushes the current time on the stack.

           TIME

           Pushes the time the currently processed value was taken at onto the
           stack.

           LTIME

           Takes the time as defined by TIME, applies the time zone offset
           valid at that time including daylight saving time if your OS sup-
           ports it, and pushes the result on the stack.  There is an elabo-
           rate example in the examples section below on how to use this.

       Processing the stack directly
           DUP, POP, EXC

           Duplicate the top element, remove the top element, exchange the two
           top elements.



VARIABLES
       These operators work only on VDEF statements. Note that currently ONLY
       these work for VDEF.

       MAXIMUM, MINIMUM, AVERAGE
           Return the corresponding value, MAXIMUM and MINIMUM also return the
           first occurrence of that value in the time component.

           Example: "VDEF:avg=mydata,AVERAGE"

       LAST, FIRST
           Return the last/first value including its time.  The time for FIRST
           is actually the start of the corresponding interval, whereas LAST
           returns the end of the corresponding interval.

           Example: "VDEF:first=mydata,FIRST"

       TOTAL
           Returns the rate from each defined time slot multiplied with the
           step size.  This can, for instance, return total bytes transfered
           when you have logged bytes per second. The time component returns
           the number of seconds.

           Example: "VDEF:total=mydata,TOTAL"

       PERCENT
           This should follow a DEF or CDEF vname. The vname is popped,
           another number is popped which is a certain percentage (0..100).
           The data set is then sorted and the value returned is chosen such
           that percentage percent of the values is lower or equal than the
           result.  Unknown values are considered lower than any finite number
           for this purpose so if this operator returns an unknown you have
           quite a lot of them in your data.  Infinite numbers are lesser, or
           more, than the finite numbers and are always more than the Unknown
           numbers.  (NaN < -INF < finite values < INF)

           Example: "VDEF:perc95=mydata,95,PERCENT"

       LSLSLOPE, LSLINT, LSLCORREL
           Return the parameters for a Least Squares Line (y = mx +b) which
           approximate the provided dataset.  LSLSLOPE is the slope (m) of the
           line related to the COUNT position of the data.  LSLINT is the
           y-intercept (b), which happens also to be the first data point on
           the graph. LSLCORREL is the Correlation Coefficient (also know as
           Pearson's Product Moment Correlation Coefficient).  It will range
           from 0 to +/-1 and represents the quality of fit for the approxima-
           tion.

           Example: "VDEF:slope=mydata,LSLSLOPE"

SEE ALSO
       rrdgraph gives an overview of how rrdtool graph works.  rrdgraph_data
       describes DEF,CDEF and VDEF in detail.  rrdgraph_rpn describes the RPN
       language used in the ?DEF statements.  rrdgraph_graph page describes
       all of the graph and print functions.

       Make sure to read rrdgraph_examples for tips&tricks.

AUTHOR
       Program by Tobias Oetiker <tobi@oetiker.ch>

       This manual page by Alex van den Bogaerdt <alex@ergens.op.het.net>



1.2.19                            2007-02-01                   RRDGRAPH_RPN(1)
