Stata contains a trap for novice programmers and users of certain
commands such as nl. If a local or global macro contains a value that
is less than zero, care should be taken when using the exponentiation
operator.
Example:
. global A = -5
. display $A^2
-25
Woops. Apparently, exponentiation precedes negation. Since a macro
is basically just text, Stata takes 5, squares it, then applies the
negative sign. To do this properly, use parentheses:
. display ($A)^2
25
That's better. Or, consider using a scalar:
. scalar B = -4
. display B^2
16
For reference, here is Stata's order of evaluation for all operators,
from first to last:
! (or ~),^,- (negation),/,*,-(subtraction),+,!=(or ~=),>,<,<=.>=,==,& and |.
This trap (feature?) may very well be documented, though I couldn't
find it with Google. I may be missing something. I am running
version 8.2.
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/
