Re: st: Error with -round()-

[Nick Cox <njcoxstata@gmail.com>]

There is no bug here, merely a surprise if you don't understand what
is going on. In fact, this is probably the most reported bug that is
just a surprise at first sight in the history of Stata. I'd estimate
reports on Statalist that are of the order of one hundred over several
years.

. local value = round(.8125,.01)

. di "`value'"
.8100000000000001

. di `value'
.81

In the first example, you are seeing the decimal approximation to a
binary approximation to .81. A little thought shows that .81, although
a simple decimal, doesn't have an exact binary equivalent, but Stata
does the best it can.

When you insist on -display- as a string Stata is as literal as it knows.

In the second example, -display-'s role is to display nicely and the
default numeric format is sufficient to round off the minute detail.

The positive answer, although it may seem paradoxical at first sight,
is that in Stata rounding of fractions is a string manipulation, not a
numeric manipulation.  (Rounding to integers is safe either way, as
integers within Stata's limits clearly can be held exactly as binaries
and as decimals.)

local value : di %3.2f .8125
di "`value'"

di "`: di %3.2f .8125"

are ways to ensure the correct result.

For much, much more -search precision- and home in on William Gould's
blog posts, the manual entries and three Stata Journal papers on this
subject.

Nick
njcoxstata@gmail.com


On 9 March 2014 02:46, Matt Vivier <mvivier@remedypartners.com> wrote:
> Hello all,
>
> I'm using:
> Stata/IC 12.1 for Windows (64-bit x86-64)
> Revision 06 Feb 2012
>
> I'm very hesitant to suggest that there is a bug in Stata itself, but
> I am getting an unexpected result when using -round()- on some
> numbers, and have been unable to find an explanation for it.
>
> I'm pulling information out of my dataset, and writing it to a table
> in an HTML file, and wanted to round the values down to something of
> reasonable length. The vast majority of the time, -round()- functions
> as expected, but it occasionally seems to misfire.
>
> This can be replicated with:
> local value = round(.8125,.01)
> di "`value'"
>
> However, if you type
> di `value'
>
> You see the expected result.
> Unfortunately, (I believe) I have to enclose it in quotes to write it
> to the HTML file as such:
> file write html `"<TD>`value'</TD>"'
>
> My questions are:
> Can anyone replicate this issue?
> If so, any thoughts on what causes it? It seems to be caught up in
> treating `value' like a string due to the quotes.
> Any thoughts on a workaround?
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