Description Paul Rogers 2008-04-04 01:16:22 EDT

Created attachment 94810 [details]
Product Sales by Country.rptdesign

Open the attached report (or build your own.) Look at the Change column for a country, say Australia. This field has a value computed using the

Finance.percent( )

function provided by BIRT. The result is a percent. That is 100 percent means 1 (number). Finance.percent( 2, 1 ) returns 50. (Note the strange reversal of numerator and denominator: one thinks of division as num/denom, but the percent function uses percent( denom, num ). Hard to remember, but it is too late to change.)

Now, look at the style for this item. It uses BIRT's percent formatting.

Preview the report. The field should show the percentage change as a percent. But, it shows 100 times the change. The reason appears to be that the percentage formatting feature assumes percents are represented as ratios: .5 for 50% and so on.

The result is that one can choose to use one or the other feature, but not both together. This gets tricky because:

* One cannot specify a custom format so I can do my own percentage. (See Bug 225692.)

* I used percent because I need a "safe divide", one that handles a zero in the denominator. I assumed that Finance.percent handled this case.

The workaround is to use percent formatting (since I can't easily do custom formatting without writing messy code), and to do a roll-my-own safe divide:

if ( denom == 0 ) then 0;
else num / denom;

A suggestion for resolution is to add a new method:

BirtComp.safeDivide( num, denom [, if0 ] )

Example:

BirtComp.safeDivide( 10 /* change */, 0 /* base */, 100 );

Here, I want to treat an increase from zero as a 100% increase. (Not mathematically correct, but a useful approximation for business people.)

BirtComp.safeDivide( a, b, null );

Here I want a null (undefined) value if b is 0.