Re: bitwise operators in IDL?

[thompson(at)orpheus.nascom.nasa.gov (William Thompson)]

Craig Markwardt <craigmnet@cow.physics.wisc.edu> writes:


>thompson@orpheus.nascom.nasa.gov (William Thompson) writes:

>> "Rick Towler" <rtowler@u.washington.edu> writes:
>> 
>> >Is there a built in function in IDL for the c++ bitwise operator "&" or is
>> >this going to be the first DLM i write?
>> 
>> >Rick Towler
>> 
>> 
>> AND, OR, and NOT are bitwise operators.
>> 
>> William Thompson

>Which leads to some interesting confusion sometimes when they are used
>as logical operators.  Consider that:

>  255 AND 'fe'xl    is false, and
>  NOT 2             is true

It's not the operators which are confusing here.  They are doing exactly what
they should.  Consider the following:

	IDL> if 255 then print,'true' else print,'false'
	true
	IDL> if 'fe'xl then print,'true' else print,'false'
	false
	IDL> if 255 and 'fe'xl then print,'true' else print,'false'
	false

	IDL> if 2 then print,'true' else print,'false'     
	false
	IDL> if not 2 then print,'true' else print,'false'
	true

So, even in a boolean sense, the operators are working correctly.

What is confusing is that sometimes IDL considers all even numbers to be false,
while other times only 0 is false.  Generally, this depends on whether the
number is an integer or floating point; integers use even/odd logic, while
floating point numbers use zero/nonzero logic.  For example, the result for the
statement "if 2 then ..." is completely the opposite of "if 2.0 then ...".  To
mess things up even further, the function KEYWORD_SET() uses zero/nonzero logic
even if the input is integer, and thus has the potential of changing the
meaning of a boolean expression.  For example, consider the result of
KEYWORD_SET(NOT 1).

The behavior for integers is necessary because of the bitwise nature of the
operators, while floating point numbers are too complicated to permit such
bitwise treatment.  Thus, these operators are only bitwise for integers.

It would be nice if IDL had a boolean type that could only take the values
True and False.  Alternatively, one could define system variables !true and
!false,

	DEFSYSV, '!TRUE', -1B
	DEFSYSV, '!FALSE', 0B

and use those when setting variables meant to be boolean.  (The -1B is the
bitwise opposite of 0B.)

William Thompson