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help for System Variables                                   manual:  [U] 16.4 System variables (_variables)
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System Variables

    Expressions may also contain _variables (pronounced "underscore variables").  These are built in
    system variables that are created and updated by Stata.  They are called _variables because their
    names all begin with the underscore '_' character.

    The _variables are

    [eqno]_b[varname] (synonym:  [eqno]_coef[varname]) contains the value (to machine precision) of the
        coefficient on varname from the most recently fitted model (such as ANOVA, regression, Cox,
        logit, probit, multinomial logit, and the like).  See [U] 16.5 Accessing coefficients and
        standard errors for a complete description.

    _cons is always equal to the number 1 when used directly and refers to the intercept term when used
        indirectly, as in _b[_cons].

    _n contains the number of the current observation.

    _N contains the total number of observations in the dataset.

    _pi contains the value of pi to machine precision.

    _rc contains the value of the return code from the most recent capture command; see help capture.

    [eqno]_se[varname] contains the value (to machine precision) of the standard error of the coefficient
        on varname from the most recently fitted model (such as ANOVA, regression, Cox, logit, probit,
        multinomial logit, and the like).  See [U] 16.5 Accessing coefficients and standard errors for a
        complete description.

    The _n and _N variables are useful for indexing observations or generating sequences of numbers.  _n
    can act as a running counter within a by group and _N acts as the total number within each by group.

    _rc is useful for programmers, particularly if you want to test a command internal to a program   without having the program terminate.

Examples

    . by famid: gen sibs = _N
    . by famid: gen birtho = _N - _n + 1
    . display _pi
    . display cos(_pi)
    . gen x = cos(_pi/4*_n)
    . gen price2 = price[_n-1]

    . mvreg len disp gear_ratio = turn tru mpg wei
    . display [displ]_b[mpg]

    . mlogit rep gear_ratio turn tru
    . display [5]_b[turn]

Also see

    Manual:  [U] 16.4 System variables (_variables)

    Online:  help for by, generate, replace, subscripting

_rc 是一个暂元，记录了错误信息的代码，主要在编程中使用。举一个简单的例子：

. sysuse auto,clear
(1978 Automobile Data)

. drop price2   /*由于price2 变量不存在，所以返回错误代码 r(111),stata停止工作*/
variable price2 not found
r(111);

. capture drop price2 /*使用capture命令可以保证无论命令正确与否，stata都可以继续执行后续命令*/

. dis _rc  /*因为上面那个命令有问题，所以capture在其暂元_rc中记录了错误代码*/
111

. capture drop price  /*这条命令可以正确执行，所以返回的错误代码是 0 */

. dis _rc
0

. cap verify price<0  /*不同的情况下，返回的错误大麦是不同的*/

. dis _rc
199

* 实际编程过程中，我们这样使用 _rc 命令：

local var price2
capture drop `var'
if _rc != 0{
  dis "`var' not found!"
}