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Re: st: gsort issue
I'm with Fred & Nick. After using Stata exclusively for
over a decade, I still find that my intuition about how
Stata will treat a missing value to come short suprisingly
often, and this is another example of where it doesn't do
what I would think.
I've often wished for a "negative" missing (a la -99)
value, so I could simply:
replace x = -abs(x) if missing(x)
so that in contexts where I would rather treat missing
as the smallest rather than largest value, I could do so
in one stroke instead of trying to trick, eg, -bys y (x)-
into doing what I want.
j
Nick Cox wrote:
I don't use -gsort- much, as I usually prefer to work out
my own -sort- order without wanting to re-discover
the precise idiosyncratic syntax of -gsort-. (I've
got a blind spot on -recode- for the same kind of reason.)
(That's not on a par with B*ll G???d, who
can write the equivalent of an -egen- function
several times faster than it takes to find out
whether that function already exists.)
But -- to the point -- while what Brian says is a fair
answer it seems to me to point to a missing option on -gsort-.
-reallydowantmissingfirst- would not be very Stataish
as a name, but Fred Wolfe's want and need seemed very reasonable
to me.
Nick
[email protected]
Brian P. Poi
On Thu Jul 5 06:58:30 2007, Fred Wolfe wrote:
Is there a problem with gsort (Stata 10 and below) or am I
misunderstanding something?
I have a variable called -phdif-. I want the greatest value of that
variable to appear in the last observation. There are
missing values,
so I use -gsort- with the -mfirst- option.
...
. gsort phdif
. l phdif in 1,clean
phdif
1. 1
. l phdif in l,clean
phdif
169914. .
The problem appears to be that missings are still last even
though I
used the -mfirst- option.
Any suggestions? Is this a problem or am I thinking about this
incorrectly?
The "mfirst" option of -gsort- applies only to variables sorted in
descending order.
Stata stores missing values as extremely large numbers, so if
a variable
is sorted in descending order, missing values should appear
first in the
list since they are greater than all non-missing values.
-gsort-, however, tries to be helpful when sorting in
descending order by
putting the missing values at the end of the list, assuming
that the user
really cares about the large real values of the variable, not
the missing
values.
The "mfirst" option tells -gsort- to put the missing values
first in the
list instead of trying to be helpful by putting them at the
end of the
list.
If you want to get the missing values to appear first when doing an
ascending sort, one way to proceed is to create a 0/1
variable equal to 0
if the variable of interest contains missing and 1 otherwise
and then sort
by the indicator variable and the variable of interest:
. sysuse auto
. generate missrep78 = cond(missing(rep78), 0, 1)
. gsort missrep78 rep78
. list rep78 in 1/7, sep(0)
+-------+
| rep78 |
|-------|
1. | . |
2. | . |
3. | . |
4. | . |
5. | . |
6. | 1 |
7. | 1 |
+-------+
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