<> capture graph describe Graph tempname gdate local `gdate' = "`r(command_date)' `r(command_time)'" <> The general scheme for writing commands will be something like this (see `help language`, this is a pared down version): ```` [prefix:] command [varlist] [if] [, options] ```` For example, try: ```{stata, eval=FALSE} help summarize ``` and you will see ```` summarize [varlist] [if] [in] [weight] [, options] ```` The name of the command is *summarize*. It can be abbreviated as short as *su* (although *sum* is more common in practice). This is indicated by the underlining. This command takes ("accepts") a *variable list*. The square brackets indicate that the variable list is actually optional for this command, as is everything besides the command name, *summarize*. Having loaded a data set, we can try out this command ``` <> summarize <> ``` <> capture graph describe Graph local checkdate = "`r(command_date)' `r(command_time)'" <> <> <> <> <> local `gdate' = "`r(command_date)' `r(command_time)'" <> ## *Varlist* Next we add a variable list. There are a number of ways we can write lists of variables in Stata. (See `help varlist`.) - write out the variable name(s) - use wildcards - use a first-to-last (sequential) list - the keyword `_all` ``` <> summarize price mpg // spelled-out list summarize mpg - trunk // first-to-last list summarize *n // wildcard list summarize _all // or use " * " <> ``` <> capture graph describe Graph local checkdate = "`r(command_date)' `r(command_time)'" <> <> <> <> <> local `gdate' = "`r(command_date)' `r(command_time)'" <> Some languages have indexed lists, like *myvar1-myvar4*, meaning the four variables myvar1, myvar2, myvar3, and myvar4, regardless of where they are located in the data set. Not Stata. In Stata this would be a first-to-last (positional) list. It might be the list you want, or it might not - it depends how the data set was constructed. ### Exercise How else could you specify the list with myvar1, myvar2, myvar3, and myvar4? (State any assumptions you make.) ## *If* An `if` condition allows your command to act on a *subset* of the observations in your data set. (See `help if`.) The syntax for most commands can be ```` command if expression ```` where *expression* is something that is true or false for each observation in the data set. For example, to see the average price of a foreign car in this data set we might try ``` <> summarize price if foreign==1 <> ``` <> capture graph describe Graph local checkdate = "`r(command_date)' `r(command_time)'" <> <> <> <> <> local `gdate' = "`r(command_date)' `r(command_time)'" <>