# [rrd-users] Re: Math functions in RRD-graph

Alex van den Bogaerdt alex at slot.hollandcasino.nl
Tue Feb 5 00:29:53 MET 2002

```Edwin Nadorp wrote:

> I calculate the diverence by a - b = c
> But when b is greater then a the result is an negative number. I dont
> like that.
>
> So my question:
>
> How can i calculate the line by the folowing function : abs(a - b) =
> c ??

You want to have
c == a - b   for a>b
and  c == b - a   in other cases

CDEF:c=a,b,GT,a,b,-,b,a,-,IF

Alternatively, you can do:
c == a - b; if (c<0) c=c*-1 else c=c*1;
CDEF:c=a,b,-,DUP,0,LT,-1,1,IF,*

this does: calculate a-b; duplicate; use the duplicate to check
against being less than zero; multiply with -1 if so, with 1 if not.

These examples do not check for being the INF, -INF or UNKN value.

cheers,
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