st: The accuracy of the float data type

[R Zhang <r05zhang@gmail.com>]

Dear Statalist,

my data structure is as follows

firmID    segmentID   sales year
1001       1               25.395     1990
1001       1                32.007     1991

............

a firm can operate in multiple segments as identified by  segmentID .
I wanted to identify the largest segment by sales,so I used

bysort firmID year : egen maxsale=max(sales)

then I did
gen PriSIC=0
replace PriSIC=1 if sales=maxsale

I got
firmID    segmentID   sales year                  maxsale    prisic
1001       1               25.395     1990            25.395         0
1001       1                32.007     1991            32.007       0

I could not figure out why prisic is 0, so I compute the diffderence
(sales-maxsale), it shows a very small negative number , and the data
dictionary shows sales format float %12.0g, and maxsale format float
%9.0g

what should I do to correct this?

thanks!!!

Rochelle
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