Re: multi conditional for loops

[Craig Markwardt <craigmnet(at)cow.physics.wisc.edu>]

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

-- 
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Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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