--Hung-Jen Wang <[email protected]> wrote:
[...]
> Simply put, my problem is this: A (empty or non-empty) varlist needs to be
> specified by an -option-, and so the -option- should be able to
> accommodate both empty and non-empty varlist (for Stata 6).
Hung-Jen met the problem when he defined the syntax of a program as:
program define myexpl
version 6.0
syntax ...., ..... [myopt(varlist) .... ]
Stata 6 does not allow an empty varlist, so when the user tries
. myexpl, myopt()
Stata will complain that "myopt() invalid" because the option -myopt()- does
not contain a valid varlist.
To work around the problem, Hung-Jen may try -myopt(passthru)-. See the
following example:
program define myexpl
version 6
syntax [, myopt(passthru)]
if "`myopt'" != "" {
di "myopt is: `myopt'"
tokenize `myopt', parse("()")
local myopt `3'
summ `myopt'
}
else {
di "myopt() is empty"
}
end
-passthru- tells Stata to pass anything specified in the option into the
program, including nothing. Then the empty varlist will be allowed
. myexpl, myopt()
myopt() is empty
Here the local macro `myopt' is empty when we specify nothing in the
corresponding option. However, when -myopt()- is not empty, the local macro
`myopt' will contain the full name of the option, i.e,. "myopt(varlist)". We
then need to use -tokenize- to extract the real variable list out from this
string. For example,
. myexpl, myopt(price mpg)
myopt is: myopt(price mpg) <----- note what `myopt' contains
Variable | Obs Mean Std. Dev. Min Max
---------+-----------------------------------------------------
price | 74 6165.257 2949.496 3291 15906
mpg | 74 21.2973 5.785503 12 41
In fact, specifying a empty varlist within -myopt()- is equivalent to not
specifying this option at all:
. myexpl
myopt() is empty
Weihua Guan <[email protected]>
Stata Corp.
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