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Re: st: xline

From   n j cox <>
Subject   Re: st: xline
Date   Wed, 31 Jan 2007 16:17:39 +0000

Good answers have already been posted, but the question
remains of why you thought that the -xline()- option
might perform in the way you wanted, so that misunderstanding
needs elucidation.

There are two issues here.

The first is that the on-line _help_, here for -axis label options-, specifies what kind of input is acceptable. The trick that Michael Blasnik specified


is not explicit in that help, nor should it have been -- unless it is considered acceptable to spell out almost everywhere
in the help that you can use that trick to specify input. (It could in
principle be relevant to _every_ command.) By the way, I say "trick" but that is just light-hearted. There is no dodge or work-around: it is
a perfectly straightforward device in Stata.

Understanding why Michael's trick works is perhaps helped by thinking that there are two steps to take even when the input is something very simple such as

scatter y t, xline(`=d(5jan2007)')

* One engine, which you might think of as "Stata" or "the interpreter",
looks at what you typed and does various checks and preliminary processing. These include checking that the command exists, that the
syntax is correct and (pertinently here) carrying out any evaluation of global or local macros or equivalent expressions. So Stata is instructed to evaluate d(5jan2007) before passing the command on for execution. The answer is 17171.

* Another engine, which in this case is -scatter-, sees not what you
typed but

scatter y t, xline(17171)

which is in this case legal as far as it is concerned (and matches one of the possibilities specified in the help). -scatter- in turn calls
other programs and so on and so forth but that is beyond the story here.

The second issue is precisely when will Stata _evaluate_ what you give it? The main answers that spring to mind are

* When you refer to local or global macros. You type the name; Stata inserts the contents in place of the name.

* When you specify evaluation by inserting an expression after an equals sign (using material allowed in that context). `= <stuff>' you can type anywhere you like in commands (whether the results will be what you want is a different matter).

* When it is the command's job to evaluate. -display- is one conspicuous

That's unlikely to be a complete list, but it may help understanding.


Eric G. Wruck

I am working on a graph & want to insert vertical lines for various
dates, time being the x-axis. It seems xline will not accept
expressions or variable names, as I have tried, for example:




where scan1 was previously defined in the .do file with

gen scan1 = d(5jan2007)

The only way I get the thing to work is to have xline(17171), but I
don't want to have to look up all the dates by hand. Does someone
know an easier way to handle this?
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