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Re: multi conditional for loops [message #20384 is a reply to message #20378]

Wed, 21 June 2000 00:00

Nigel Wade
Messages: 286
Registered: March 1998

Senior Member

K Mankoff wrote:
>
> Hi *,
> I am interested in coding with a c/c++ style multi-conditional for
> loop.
>
> c++ ex:
> for ( int i = 0; ( ( i < 10 ) && ( j <= 4 ) ); i++ )
> {
> /* do something */
> }
>
> Here is sample IDL code. It runs but produces very interesting
> results. (this is IDL5.3 on Linux RH6.2)
>
> PRO test
> j = 0
> FOR i = 0, ((j LE 4) AND 10) DO BEGIN
> j = j + 2
> PRINT, i, j
> ENDFOR
> END
>
> thanks for any info,
> ken.

The value of a conditional is either 0 or 1. So, the loop will execute
either 1 or 2 times depending on the value of the conditional.
(J < 4) => 1, 1 AND 10 => 0, so the terminating expression is 0 and
the loop should execute once (FOR i=0, 0 DO...). The first and second
expressions are only evaluated before the loop begins, not during its
execution, so even if the condition changes the value used as the limit
will not.

What you need to use is a while loop.

--
-----------------------------------------------------------
Nigel Wade, System Administrator, Space Plasma Physics Group,
University of Leicester, Leicester, LE1 7RH, UK
E-mail : nmw@ion.le.ac.uk
Phone : +44 (0)116 2523568, Fax : +44 (0)116 2523555

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		Re: multi conditional for loops By: Craig Markwardt on Wed, 21 June 2000 00:00
		Re: multi conditional for loops By: Nigel Wade on Wed, 21 June 2000 00:00

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