st: generate variable versus define scalar, with conditional statement

[annoporci <annoporci@gmail.com>]

I learned something today I'd like to share with Statalist.

The way to conditionally define a variable and a scalar are different. An
example is given below.


clear all
sysuse sp500

/// generate a new variable with missing entries except at the given date
mdy(1,2,2001):
gen newvariable = open if date == mdy(1,2,2001)

/// define a new scalar by the value of variable open at the given date
mdy(1,2,2001):
if date == mdy(1,2,2001) scalar newscalar = open
scalar list

/// THIS IS NOT THE WAY:
scalar newscalar = open if date == mdy(1,2,2001)

The above, incorrect way, is intuitive for a beginner (like myself), it is
similar to the syntax to generate new variables, but it won't work.

This is trivial for 99% of Statalist users, but may be a useful tip for
the remaining 1%. And if not, almost surely for a future incarnation of
myself scanning through Statalist for hints when the following error
message displays:

      if not allowed

When I submitted *stata scalar "if not allowed"* Google returned a  
solution, in a more complicated setting:

http://statalist.1588530.n2.nabble.com/scalar-do-not-allow-if-td5748129.html

--
Patrick Toche.
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