| Re: Meaning of the assignment [message #93649 is a reply to message #93648] |
Wed, 21 September 2016 00:09   |
Helder Marchetto
Messages: 520
Registered: November 2011
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Senior Member |
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On Wednesday, September 21, 2016 at 8:12:54 AM UTC+2, Sanu wrote:
> On Wednesday, September 21, 2016 at 11:26:48 AM UTC+5:30, Sanu wrote:
>> exp = 'BYTE(BYTE(b1 LE 650 AND b2 GT 200)*1 + 0)'
>>
>> For this b< 650 and b2> 200 will be assigned as class 1.
>> What is the meaning of 0 which is added?????
>> Kindly help.
>
> Similar thing i found on thid
>
> exp = 'BYTE(BYTE(b1 GE 0.4)*21 OR BYTE((b2/b3) GE 4.0)*21)'
> What is the meaning of *21
Well, I also don't understand what you have. But you should maybe provide some more information: Normally a string is a string and will not be computed unless you use execute(). Is this what you are doing? Where is this exp being used?
If the assignment were not a string, then you can figure out things as they were described in the post from yesterday.
Take things apart, a bit like an onion... layer by layer.
Let's consider this expression:
BYTE(BYTE(b1 GE 0.4)*21 OR BYTE((b2/b3) GE 4.0)*21)
You have at the most inner parts expressions like: b1 GE 0.4
IDL evaluates if b1 is greater or equal to 0.4. If it is greater or equal, it will "substitute" in the expression this part with a 1. If not, it will put a 0. IDL returns from such an operation a byte value, so the conversion to byte using the function BYTE() is useless. The expression you gave can be simplified to:
BYTE((b1 GE 0.4)*21 OR ((b2/b3) GE 4.0)*21)
Now it gets a bit tricky.
on the left side of the OR you have
(b1 GE 0.4)*21
on the right side
((b2/b3) GE 4.0)*21
You have to evaluate these two separately. The first will give you:
1 * 21 ---> if b1 is greater or equal than 0.4
0 * 21 ---> if b1 is smaller than 0.4
The second will give you
1 * 21 ---> if b2/b3 is greater or equal than 4.0
0 * 21 ---> if b2/b3 is smaller than 4.0
The result of each operation is an integer. Either 0 or 21.
The OR operation will return either 0 or 21 depending on the input:
0 OR 0 ---> will give 0
21 OR 0 ---> will give 21
0 OR 21 ---> will give 21
21 OR 21 ---> will give 21
The outer shell of the onion is a byte function that converts the previous result to a byte value.
So depending on the last operation, you will either get a 0 or a 21 as a byte instead of an integer.
You should be able to figure out the rest alone. If not, I seriously recommend some basic programming reference book.
Helder
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