.-
help for ^expr^ and ^exprcmd^                      (STB-39: ip19)
.-

Expansion of expressions in commands
------------------------------------

    ^expr^ [, ^nod^etail]: cmd expr-list [^if^ exp] [^in^ range] [weight]
             [^using^ filename] [^,^ options]
    
    ^exprcmd^ [, ^nod^etail]


Description
-----------

^expr^ is a prefix command that permits a list of expressions expr-list instead 
of a list of variables in the command ^cmd^. Expressions should be seperated by 
white space; thus, expressions may not contain embedded spaces. ^expr^ only 
supports "regular" Stata commands (see below).

^exprcmd^ switches on expression expansion for all subsequent commands. In the 
command prompt, the string ^(expr)^ is displayed. To suppress expression 
expansion in a command, prefix it with a period (^.^). To switch expression 
expansion, issue the period ^.^ as a single command.


Options
-------

^detail^ specifies that the expressions and the expanded command are displayed. 


Technical remarks
-----------------

The "regular" syntax of Stata commands is

    cmd varlist [if expr] [in range] [weight], options 

Due to limitations of the Stata parser, the command string may not contain 
double quotes.

^expr^ parses the expr-list on spaces, it then verifies whether a token can 
be interpreted as a varlist (using ^unabbrev^), and otherwise tries to 
interpret the token as an expression. Thus, ^x1-x2^ is interpreted as the 
list of variables between ^x1^ and ^x2^, not as the arithmetic difference 
between the variables ^x1^ and ^x2^. To fool ^expr^ in adopting the second
interpretation, use ^x1-1*x2^ or ^x1+(-x2)^.
    Note that this ambiguity in Stata syntax would become even larger with 
my "enhanced" varlist (see ^unabrev2^) where ^x1*x2^ would be interpreted 
as the list of variables with ^names^ that begin with ^x1^ and end with ^x2^, 
and not as the product of ^x1^ and ^x2^. 

^expr^ generates variables _Expr<n>, n=0,1,..., for non-atomic expressions.  
These variables are properly labeled using the defining expressions and are 
kept in the data so that, e.g., post-estimation commands such as ^predict^, 
^test^, ^lfit^ etc should work properly.  After estimation commands, you
may want to use ^diest^ for more understandable output. All variables named 
_Expr<n> are silently dropped by ^expr^.


Examples
--------

    . ^expr: summ x1 x1^^2 x1^^3+sin(x2/x3)^ 
    . ^expr: summ x1^                                 (^expr^ has no effect here)
    . ^expr: reg y x1 ln(abs(x1)), nocons^
    
    . ^exprcmd^                                  (switch on expression expansion)
    (expr) . ^summ x1 x1^^2 x1^^3+sin(x2/x3)^ 
    (expr) . ^reg y x1 ln(abs(x1)), nocons^
    . ^.^                                              (end expression expansion)


Author
------

     Jeroen Weesie
     Utrecht University
     Netherlands 
     email: weesie@@weesie.fsw.ruu.nl


Also See    
--------

    STB: STB-39 ip19
 Manual: [U] 20 Functions and expressions
On-line: help for @exp@, @diest@