Re: assignment inside boolean expression

["Liam E.Gumley" <Liam.Gumley(at)ssec.wisc.edu>]

Patrick Broos wrote:
> I was wondering if it's common knowlege that one can put an IDL
> assignment inside
> a boolean expression (like in the C language).  For example
> 
> if (v = 0) then ...    assigns v and does not execute the "then"
> statement, while
> if (v = 1) then ...    assigns v and does execute the then.
> 
> Just as in C I find this leads to really nasty bugs.

Curious: I've never even considered using this syntax.

Enclosing a statement inside parentheses turns it into an expression,
which has a type and a value, e.g.

IDL> help, (v = 100)
<Expression>    INT       =      100

The variables in the right hand side of the statement must necessarily
be defined:
IDL> help, (zv = tv + vt)
% Variable is undefined: VT.
% Execution halted at:  $MAIN$                 

If you take the following statements:

IDL> if (v = 0) then print, 'True'
IDL> help, v
V               INT       =        0

IDL> if (v = 1) then print, 'True'
True
IDL> help, v
V               INT       =        1

and remove the parentheses, the equivalent code is

IDL> v = 0
IDL> if (v) then print, 'True'     
IDL> help, v
V               INT       =        0

IDL> v = 1
IDL> if (v) then print, 'True'
True
IDL> help, v
V               INT       =        1

Recall that in IDL, integers with odd non-zero values are Boolean
'True'. Beware of floats and doubles though, where any non-zero value is
Boolean 'True'.

Cheers,
Liam.
http://cimss.ssec.wisc.edu/~gumley