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Re: assignment inside boolean expression [message #20612 is a reply to message #20609]

Mon, 10 July 2000 00:00

Liam E. Gumley
Messages: 378
Registered: January 2000

Senior Member

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

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[Message index]

		Re: assignment inside boolean expression By: Jeffrey Jones on Tue, 11 July 2000 00:00
		Re: assignment inside boolean expression By: Ben Tupper on Tue, 11 July 2000 00:00
		Re: assignment inside boolean expression By: promashkin on Tue, 11 July 2000 00:00
		Re: assignment inside boolean expression By: Craig Markwardt on Tue, 11 July 2000 00:00
		Re: assignment inside boolean expression By: K. Bowman on Tue, 11 July 2000 00:00
		Re: assignment inside boolean expression By: Ben Tupper on Tue, 11 July 2000 00:00
		Re: assignment inside boolean expression By: promashkin on Mon, 10 July 2000 00:00
		Re: assignment inside boolean expression By: Liam E. Gumley on Mon, 10 July 2000 00:00
		Re: assignment inside boolean expression By: Liam E. Gumley on Mon, 10 July 2000 00:00

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