__Title__

**[U] 13.4 System variables (_variables)**

__Description__

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,
and multinomial logit). See **[U] 13.5 Accessing coefficients and**
**standard errors** for a 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 **[P] capture**.

**[***eqno***]_se[***varname***]** contains the value (to machine precision) of the
standard error of the coefficient on *varname* from the most recently
fit model (such as ANOVA, regression, Cox, logit, probit, and
multinomial logit). See **[U] 13.5 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__

**. display _pi**
**. display cos(_pi)**
**. range x 0 4*_pi 100**

**. sysuse auto, clear**
**. generate price2 = price[_n-1]**
**. regress price mpg foreign rep78**
**. display _b[mpg]**

**. mlogit rep78 gear_ratio displacement foreign**
**. display [5]_b[foreign]**

**. webuse dollhill2, clear**
**. by age (smokes), sort: generate wgt=pyears[_N]**