=include name =head1 SYNOPSIS IRPN expressionE> := IvnameE>|IoperatorE>|IvalueE> [ , IRPN expressionE>] =head1 DESCRIPTION If you have ever used a traditional HP calculator you already know B. The idea behind B is that you have a stack and push your data onto this stack. Whenever you execute an operation, it takes as many elements from the stack as needed. Pushing is done implicit so whenever you specify a number or a variable, it gets pushed automatically. At the end of the calculation there should be one and exactly one value left on the stack. This is the outcome of the function and this is what is put into the I. For B instructions, the stack is processed for each data point on the graph. B instructions work on an entire data set in one run. Example: C This will set variable "maximum" which you now can use in the rest of your RRD script. Example: C This means: push variable I, push the number 8, execute the operator I<+>. The operator needs two elements and uses those to return one value. This value is then stored in I. As you may have guessed, this instruction means nothing more than I. The real power of B lies in the fact that it is always clear in which order to process the input. For expressions like C you need to multiply 3 with 5 first before you add I to get I. However, with parentheses you could change this order: C. In B, you would do C and need no parentheses. =head1 OPERATORS =over 4 =item Boolean operators B Pop two elements from the stack, compare them for the selected condition and return 1 for true or 0 for false. Comparing an I or an I value will always result in 0 (false). B Pop one element from the stack, compare this to I respectively to I. Returns 1 for true or 0 for false. B Pops three elements from the stack. If the last element is 0 (false), the first value is pushed back onto the stack, otherwise the second popped value is pushed back. This does, indeed, mean that any value other than 0 is considered to be true. Example: C should be read as C Z<> =item Comparing values B Pops two elements from the stack and returns the lesser or larger. The two numbers shouldn't be I or I, if they are that value is pushed back onto the stack as the result. B 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 I 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 I or I this function will always return an I Example: C will return I if alpha is lower than 0 or if it is higher than 100. Z<> =item Arithmetics B<+, -, *, /, %> Add, subtract, multiply, divide, modulo B Sine, cosine (input in radians), log, exp (natural logarithm), square root B Arctangent. Output in radians. B Round down,up to the nearest integer Z<> =item Set Operations B Pop one element from the stack. This is the I of items to be sorted (or reversed). The top I of the remaining elements are then sorted (or reversed) in place on the stack. Example: C will compute the average of the values v1..v6 after removing the smallest and largest. B 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) =item Special values B Pushes an unknown value on the stack B 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. B Pushes an I value if this is the first value of a data set or otherwise the result of this B at the previous time step. This allows you to do calculations across the data. This function cannot be used in B instructions. B Pushes an I value if this is the first value of a data set or otherwise the result of vname variable at the previous time step. This allows you to do calculations across the data. This function cannot be used in B instructions. B 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 B instructions. Z<> =item Time Time inside RRDtool is measured in seconds since the epoch. This epoch is defined to be S>. B Pushes the current time on the stack. B