2004 is a leap year. This raises a small Stata problem,
how to determine if a year is a leap year using Stata
syntax.
The criteria for a year to be a leap year are
YES if 1. year divisible by 4
(but NOT if 2. year divisible by 100
(but YES if 3. year divisible by 400))
and NO otherwise.
If a leap year is a first-order correction,
then the third criterion is an example of a third-order
correction. Scientists and engineers often use such
ideas, but they seem in shorter supply in the everyday
world. (I owe this remark to some Stata friend, but
I have forgotten who it was. If you recognise your
remark, remind me privately.)
Naturally everybody knows criterion 1. but criteria
2. and 3. are occasionally forgotten. For example,
Excel has 1900 as a leap year: allegedly this
was to provide bug-for-bug compatibility with
Lotus 1-2-3. This is documented openly by Microsoft.
In Stata: suppose -year- is a variable. An indicator
1 for leap year and 0 otherwise is then
given by
(mod(year,4) == 0 & mod(year,100) != 0) | mod(year,400) == 0
as -mod(,)- provides the remainder left over from division.
The nested -cond()- solution, to please David Kantor, is
cond(mod(year,400) == 0, 1,
cond(mod(year,100) == 0, 0,
cond(mod(year,4) == 0, 1,
0)))
-- the layout of which may be important to you, if not
to Stata.
To make sure you have balanced parentheses, exploit the
pertinent function in a decent text editor: in the
Stata do-file editor it's "Balance", Ctrl-B; in Vim
it is the % key; etc.
Given daily dates, a feature of a leap year is clearly
that there is a February 29, so
mdy(2,29,2004) < .
is true. More explicitly: -mdy(2,29,?)- is missing
if ? is not a leap year, but non-missing otherwise;
in the latter case, it is less than missing. You
could also do that as
!mi(mdy(2,29,2004))
Similarly, there are 366 days in a leap year, so
doy(mdy(12,31,2004)) == 366
is true. We could vectorise both of those calculations
to say
mdy(2,29,year) < .
or
doy(mdy(12,31,year)) == 366
Any other amusing or neat solutions? Especially neater
ones?
Nick
[email protected]
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