| Re: multi conditional for loops [message #20378] |
Wed, 21 June 2000 00:00 |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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K Mankoff <mankoff@colorado.edu> writes:
> 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
Your IDL snippet does not correspond to the C code at all. As already
mentioned, in a FOR statement the two arguments are the first and last
values of the iteration variable. The IDL FOR loop is not nearly as
sophisticated as the C version. However, you can achieve the same
thing with a WHILE loop:
j = 0
i = 0
while (j LE 4) AND (i LT 10) do begin
j = j + 2
print, i, j
endwhile
Also note that the expression you had, ((j LE 4) AND 10), is
completely different than the expression (j LE 4) AND (i LT 10), even
in C.
Finally, IDL does not distinguish between *logical* boolean operators
and *bitwise* boolean operators, whereas C does. For example C has &&
for logical AND and & for bitwise AND. In IDL it is always bitwise.
This means that an IDL expression like (i AND j) will fail for
(i=1,j=2) while for C, (i && j) will succeed.
Craig
--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
------------------------------------------------------------ --------------
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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 |
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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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