5 I<E<lt>RPN expressionE<gt>> :=
6 I<E<lt>vnameE<gt>>|I<E<lt>operatorE<gt>>|I<E<lt>valueE<gt>>
7 [ , I<E<lt>RPN expressionE<gt>>]
11 If you have ever used a traditional HP calculator you already know
12 B<RPN>. The idea behind B<RPN> is that you have a stack and push
13 your data onto this stack. Whenever you execute an operation, it
14 takes as many elements from the stack as needed. Pushing is done
15 implicit so whenever you specify a number or a variable, it gets
18 At the end of the calculation there should be one and exactly one
19 value left on the stack. This is the outcome of the function and
20 this is what is put into the I<vname>. For B<CDEF> instructions,
21 the stack is processed for each data point on the graph. B<VDEF>
22 instructions work on an entire data set in one run.
24 Example: C<CDEF:mydatabits=mydata,8,*>
26 This means: push variable I<mydata>, push the number 8, execute
27 the operator I<+>. The operator needs two elements and uses those
28 to return one value. This value is then stored in I<mydatabits>.
29 As you may have guessed, this instruction means nothing more than
30 I<mydatabits = mydata * 8>. The real power of B<RPN> lies in the
31 fact that it is always clear in which order to process the input.
32 For expressions like C<a = b + 3 * 5> you need to multiply 3 with
33 5 first before you add I<b> to get I<a>. However, with parentheses
34 you could change this order: C<a = (b + 3) * 5>. In B<RPN>, you
35 would do C<a = b, 3, +, 5, *> and need no parentheses.
41 =item Boolean operators
43 B<LT, LE, GT, GE, EQ, NE>
45 Pop two elements from the stack, compare them for the selected condition
46 and return 1 for true or 0 for false. Comparing an I<unknown> or an
47 I<infinite> value will always result in 0 (false).
51 Pop one element from the stack, compare this to I<unknown> respectively
52 to I<positive or negative infinity>. Returns 1 for true or 0 for false.
56 Pops three elements from the stack. If the last element is 0 (false),
57 the first value is pushed back onto the stack, otherwise the second
58 popped value is pushed back. This does, indeed, mean that any value
59 other than 0 is considered true.
60 I<Note: Should this change? It should IMHO as all the other functions
61 would return unknown if A,B or C were unknown>
63 Example: C<A,B,C,IF> should be read as C<if (A) then (B) else (C)>
67 =item Comparing values
71 Pops two elements from the stack and returns the lesser or larger.
72 The two numbers shouldn't be I<infinite> or I<unknown>, if they are
73 that value is pushed back onto the stack as the result.
77 Pops two elements from the stack and uses them to define a range.
78 Then it pops another element and if it falls inside the range, it
79 is pushed back. If not, an I<unknown> is pushed.
81 The range defined includes the two boundaries (so: a number equal
82 to one of the boundaries will be pushed back). If any of the three
83 numbers involved is either I<unknown> or I<infinite> this function
84 will always return an I<unknown>
86 Example: C<CDEF:a=alpha,0,100,LIMIT> will return I<unknown> if
87 alpha is lower than 0 or if it is higher than 100.
95 Add, subtract, multiply, divide, modulo
99 Sine, cosine (input in radians), log, exp (natural logarithm)
103 Round down,up to the nearest integer
111 Pushes an unknown value on the stack
115 Pushes a positive or negative infinite value on the stack. When
116 such a value is graphed, it appears at the top or bottom of the
117 graph, no matter what the actual value on the y-axis is.
121 Pushes an I<unknown> value if this is the first value of a data
122 set or otherwise the result of this B<CDEF> at the previous time
123 step. This allows you to do calculations across the data. This
124 function cannot be used in B<VDEF> instructions.
130 Time inside RRDtool is measured in seconds since the epoch. This
131 epoch is defined to be S<C<Thu Jan 1 00:00:00 UTC 1970>>.
139 Pushes the current time on the stack.
145 Pushes the time the currently processed value was taken onto the stack.
151 Takes the time as defined by B<TIME>, applies the time zone offset
152 valid at that time including daylight saving time if your OS supports
153 it, and pushes the result on the stack. There is an elaborate example
154 in the examples section on how to use this.
158 For B<VDEF> operations, B<TIME> and B<LTIME> have a different meaning
159 I<not yet implemented>. As the B<VDEF> statement does not work per
160 value but rather on a complete time series, there is no such thing as
161 the currently processed value. However, if you have used an operator
162 that returned a time component and would like to have this available
163 in the value component in stead (so you can use it as a number), you
164 can use B<TIME> or B<LTIME> for that.
168 =item Processing the stack directly
172 Duplicate the top element, remove the top element, exchange the two
177 =item Selecting characteristics
179 These operators work only on B<VDEF> statements.
180 I<We can make most of them work at DEF and CDEF statements. If we do
181 so, we have a moving (not rolling!) average, max,min etcetera>
187 =item MAXIMUM, MINIMUM, AVERAGE
189 Return the corresponding value, MAXIMUM and MINIMUM also return
190 the first occurance of that value in the time component.
192 Example: C<VDEF:avg=mydata,AVERAGE>
198 Return the last,first value including its time. The time for
199 FIRST is actually the start of the corresponding interval, where
200 LASTs time component returns the end of the corresponding interval.
202 Example: C<VDEF:first=mydata,FIRST>
208 Returns the rate from each defined timeslot multiplied with the
209 step size. This can for instance return total bytes transfered
210 when you have logged bytes per second. The time component returns
211 the amount of seconds
213 Example: C<VDEF:total=mydata,TOTAL>
219 Should follow a B<DEF> or B<CDEF> I<vname>. This I<vname> is popped,
220 another number is popped which is a certain percentage (0..100). The
221 data set is then sorted and the value returned is chosen such that
222 I<percentage> percent of the values is lower or equal than the result.
223 I<Unknown> values are considered lower than any finite number for this
224 purpose so if this operator returns an I<unknown> you have quite a lot
225 of them in your data. B<Inf>inite numbers are lesser, or more, than the
226 finite numbers and are always more than the I<Unknown> numbers.
228 Example: C<VDEF:perc95=mydata,95,PERCENT>