| Re: more bugs in envi !!! [message #35808 is a reply to message #35725] |
Mon, 14 July 2003 09:30   |
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marc schellens[1]
Messages: 183
Registered: January 2000
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Senior Member |
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Would be helpful if you post WHAT they output also.
And then of course the context from which you call them...
(Two functions with the same name cannot live in one (IDL-) program)
Gaurav wrote:
> Hi all!
> Thanx for the help...well u both were right...i was under the wrong
> impression that i had defined it as a long integer...but i hadn't. But
> i still cant find out why these 2 functions are producing different
> outputs :
> Please go through the 2 functions:
>
>>> 1.function example,b1
>>> help,b1
>>> end
>>> 2.function example,b1,b2
>>> help,b1
>>> end
>>>
>>> Now could anyone tell me why these 2 codes are producing different
>>> final answers??? I just cant find any reason for it.
>>
>
> Gaurav Jain
> ENST-Bretagne
>
>
> Marc Schellens <m_schellens@hotmail.com> wrote in message news:<3F115ED4.6040200@hotmail.com>...
>
>>> Hi folks!
>>> Sorry to trouble u again....but envi seems to be just going above my
>>> head...there is another problem that i am facing:
>>> Please go through the 2 functions:
>>>
>>> 1.function example,b1
>>> help,b1
>>> end
>>> 2.function example,b1,b2
>>> help,b1
>>> end
>>>
>>> Now could anyone tell me why these 2 codes are producing different
>>> final answers??? I just cant find any reason for it.
>>>
>>> One more problem :
>>>
>>> 'b1' is a positive array and 'avgb1' is a positive number but when i
>>> use the formula : result=b1*100/avgb1
>>> print, result
>>> , the answer that i get is a negative array.
>>>
>>> Whereas if i use the formula : result=b1/avgb1
>>> print, result * 100
>>> , then i get a positive array
>>>
>>
>> Even without knowing ENVI, the second problem isn't a bug in ENVI.
>>
>> A integer occupies (in IDL) 16-bit, and therfore covers a range form
>> -32768 to
>> 32767
>>
>> anything larger (or smaller) cannot be represented and therefore you
>> observe an 'overflow', resulting in a negative number.
>> IDL evalutates your expressinon from left to right, the
>> overflow occurs after the multiplication with 100.
>> In the second case the division is done first and thus the value seems
>> to be small enough not to overflow in the later multiplication.
>>
>> The solution is to convert your b1 array to LONG.
>> But did you notice that you do an integer division?
>> 1/2 equals 0.
>> Maybe you want to use FLOAT or DOUBLE.
>>
>> hdh,
>> marc
>
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