Calculation of Previous field - crystal-reports

Question

New to CR and use CR v10 and SQL Server 2000.
For the first record i.e Beginning Balance , the calculation is sum(field) from the input date, which I have calculated in SP as BegDateSum
But for the rest of the records under a group, the calculation should be previous(balance)+IN+OUT
Sample has been given:
Date Doc Descrip IN OUT Balance
Group Header-------- Beginning Balance-------------- 50 <---- sum(field) from my inputdate
3/2/2012 A -1 0 49 <-- (50+(-1)+0)
4/2/2012 B -2 0 47 <-- (49+(-2)+0)
5/2/2012 C 0 3 50
6/2/2012 D -2 3 51
How do I achieve this?
I am not sure whether to use running total, in case I have to how to do it.

Answer 1

A running total field won't work in this case, they are designed to add up (or count, or average, etc) one field and give you the sub-totals automatically. But, we can do some custom functions that will give the results you need. Assuming that your initial 50 is a static value, you would set a variable to that amount, and then add the IN and OUT values as you go along (printing that result of that).
First, initialize the value in the report header with a formula like:
WhilePrintingRecords;
Global NumberVar Balance;
Balance := 50;
""; //print nothing on the screen
Then, the formula to calculate and show the new balance, in the bar where the data is:
WhilePrintingRecords;
Global NumberVar Balance;
Balance := Balance + {tableName.IN} + {tableName.OUT};
The last line both calculates the new value, and tells what the result of the formula should be.
If the "50" is calculated somehow, then that will have to be done before the formula that calculates the new balance. If it is based off of the first record read in, you'll want to use a formula that includes If PreviousIsNull({tableName.Balance}) Then ..., that is usually a good indicator of the first record in the data set (unless that field can be null).