fix calc of fahrenheit ...

git-svn-id: svn://svn.oetiker.ch/rrdtool/branches/1.2/program@842 a5681a0c-68f1-0310-ab6d-d61299d08faa

diff --git a/doc/cdeftutorial.pod b/doc/cdeftutorial.pod
index bfe94ad..fc24a34 100644 (file)
--- a/doc/cdeftutorial.pod
+++ b/doc/cdeftutorial.pod
@@ -721,9 +721,12 @@ If you do so, you won't be able to use these next GPRINTs:
 
 =head2 Degrees Celsius vs. Degrees Fahrenheit
 
+To convert Celsius into Fahrenheit use the formula
+F=9/5*C+32
+
    rrdtool graph demo.png --title="Demo Graph" \
       DEF:cel=demo.rrd:exhaust:AVERAGE \
-      CDEF:far=cel,32,-,0.55555,* \
+      CDEF:far=9,5,/,cel,*,32,+ \
       LINE2:cel#00a000:"D. Celsius" \
       LINE2:far#ff0000:"D. Fahrenheit\c"
 
@@ -731,19 +734,17 @@ This example gets the DS called "exhaust" from database "demo.rrd"
 and puts the values in variable "cel". The CDEF used is evaluated
 as follows:
 
-   CDEF:far=cel,32,-,0.5555,*
-   1. push variable "cel"
-   2. push 32
-   3. push function "minus" and process it
-      The stack now contains values that are 32 less than "cel"
-   4. push 0.5555
-   5. push function "multiply" and process it
-   6. the resulting value is now "(cel-32)*0.55555"
-
-Note that if you take the Celsius to Fahrenheit function you should
-be doing "5/9*(cel-32)" so 0.55555 is not exactly correct. It is close
-enough for this purpose and it saves a calculation.
-
+   CDEF:far=9,5,/,cel,*,32,+
+   1. push 9, push 5
+   2. push function "divide" and process it
+      the stack now contains 9/5
+   3. push variable "cel"
+   4. push function "multiply" and process it
+      the stack now contains 9/5*cel
+   5. push 32
+   6. push function "plus" and process it
+      the stack contains now the temperature in Fahrenheit
+   
 =head2 Changing unknown into zero
 
    rrdtool graph demo.png --title="Demo Graph" \