| Re: Most Common IDL Programming Errors [message #59726 is a reply to message #59724] |
Wed, 09 April 2008 08:26   |
rigby
Messages: 16
Registered: September 1995
|
Junior Member |
|
|
> What would be on your list?
These are my top gotchas:
1) IDL silently truncates array operations to the shorter array.
help, [1,2,3,4] - [1,2]
[0, 0]
(I wish there were a compile_opt switch to prohibit this!0
2) Where() returns a one-element array when there is only one match.
help, where([1,2,3,4] EQ 4)
<Expression> LONG = ARRAY[1]
The combination (1) and (2) is deadly:
array = [1, 2, 3, 4]
j = where(array EQ max(array))
array -= array[j]
print, array
-3
My solution is to use a wrapper around where() which will return a
scalar.
3) Operations which mix signed and unsigned integers can't be trusted.
maxuint = 'ffff'xu
help, maxuint
MAXUINT UINT = 65535
print, fix(1) GT maxuint
1
print, long(1) GT maxuint
0
These results can be understood using IDL's promotion rules for mixed
expressions,
but it's just not worth it. My solution: Never use unsigned integers.
(Why would one use unsigned integers, you might ask? I've done a few
bit-level
simulations of digital hardware using IDL. It would be conceptually
simpler to
use unsigned integer variables to represent unsigned quantities or
signed quantities
where the sign bit is not the left-most bit.)
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
