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Re: st: problem with blabla if baz == float(#)


From   Joseph Coveney <jcoveney@bigplanet.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: problem with blabla if baz == float(#)
Date   Tue, 19 Nov 2002 09:55:05 +0900

Guillaume Frechette had posted on a problem related to numerical precision in 
expressions with arguments of certain noninteger numbers.  I posted an admittedly 
awkward approach using tolerances.  Nick Cox posted an alternative, exploiting Stata's 
consistency in internally representing precision to define categories using -egen, 
group()- that contain like noninteger numbers.  Guillaume followed up with what seems 
like a more elegant solution:

----------------------------begin excerpted posting-----------------------------------------------

Nick proposed one solution, here's another one which I just found:

. tab baz if baz==1-float(1/3)-float(1/3)

        baz |      Freq.     Percent        Cum.
------------+----------------------------------
   .3333333 |       1257      100.00      100.00
------------+----------------------------------
      Total |       1257      100.00

---------------------------end excerpted posting--------------------------------------------------

Guillaume's solution works for single-precision (see 
www.stata.com/support/faqs/data/float.html), but an attempt to use it with double-
precision variables doesn't work.  Omitting the -float()- function, however, does seems 
to work with double-precision variables, in a manner exactly the opposite of that with 
single-precsion variables (see log file below).  Stata's representation of precision 
doesn't seem consistent here for some reason.

Joseph Coveney

--------------------------------------begin log--------------------------------------------------------

. set obs 850
obs was 0, now 850

. generate double foo=1.0/3.0

. generate double bar=1.0/3.0

. generate double baz=1.0-foo-bar

. * next line from Guillaume Frechette's posting
. tab baz if baz==1-float(1/3)-float(1/3)
no observations

. tabulate baz if baz==1-1/3-1/3

        baz |      Freq.     Percent        Cum.
------------+-----------------------------------
   .3333333 |        850      100.00      100.00
------------+-----------------------------------
      Total |        850      100.00

. clear

. set obs 850
obs was 0, now 850

. generate float foo=1.0/3.0

. generate float bar=1.0/3.0

. generate float baz=1.0-foo-bar

. tab baz if baz==1-float(1/3)-float(1/3)

        baz |      Freq.     Percent        Cum.
------------+-----------------------------------
   .3333333 |        850      100.00      100.00
------------+-----------------------------------
      Total |        850      100.00

. tabulate baz if baz==1-1/3-1/3
no observations

--------------------------------------begin log--------------------------------------------------------
 
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