[U] User's Guide

148 [ U ] 13 Functions and expressionsYou could have saved yourself some typing by typing just. generate y = exp(rnormal())Functions accept expressions as arguments.All functions are defined over a specified domain and return values within a specified range.Whenever an argument is outside a function’s domain, the function will return a missing value orissue an error message, whichever is most appropriate. For example, if you supplied the log()function with an argument of zero, the log(0) would return a missing value because zero is outsidethe natural logarithm function’s domain. If you supplied the log() function with a string argument,Stata would issue a “type mismatch” error because log() is a numerical function and is undefinedfor strings. If you supply an argument that evaluates to a value that is outside the function’s range,the function will return a missing value. Whenever a function accepts a string as an argument, thestring must be enclosed in double quotes, unless you provide the name of a variable that has a stringstorage type.13.4 System variables ( variables)Expressions may also contain variables (pronounced “underscore variables”), which are built-insystem variables that are created and updated by Stata. They are called variables because their namesall begin with the underscore character, ‘ ’.Thevariables 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 errorsbelow for a complete description.cons is always equal to the number 1 when used directly and refers to the intercept term whenused indirectly, as in b[ cons].n contains the number of the current observation.N contains the total number of observations in the dataset or the number of observations in thecurrent by() group.rc contains the value of the return code from the most recent capture command.[eqno] se[varname] contains the value (to machine precision) of the standard error of the coefficienton 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 below for acomplete description.13.5 Accessing coefficients and standard errorsAfter fitting a model, you can access the coefficients and standard errors and use them in subsequentexpressions. Also see [R] predict (and [U] 20 Estimation and postestimation commands) for aneasier way to obtain predictions, residuals, and the like.