| Wrong documentation for BitGet()? [message #93162] |
Tue, 03 May 2016 10:34  |
penteado
Messages: 866
Registered: February 2018
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Senior Member
Administrator |
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Hello,
I was reading the help on the BitGet method,
https://www.harrisgeospatial.com/docs/idl_integer.html#BitGe t
And it seems there is a typo. It says
Print the 3rd bit value of each element of an array:
num = INDGEN(8)
PRINT, num.BitGet( 3 )
IDL prints:
0 0 0 0 1 1 1 1
My experimenting indicates the argument of BitGet is the bit number, starting from zero, not from one as the help indicates. To get the 3rd bit, and obtain the result shown in the example, it seems it should be num.BitGet(2).
Is this correct or am I missing something?
Paulo
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| Re: Wrong documentation for BitGet()? [message #93163 is a reply to message #93162] |
Tue, 03 May 2016 11:05  |
wlandsman
Messages: 743
Registered: June 2000
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Senior Member |
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Yes, it looks like the documentation might be been written before the IDL code was written ;-)
The situation is even worse for the .bitset method. Again, the documentation says that the bit position is 1-based whereas it is actually 0-based. But the documentation is also incorrect about how a vector of positions is applied.
The example is
IDL> num = [0, 0, 0, 0]
IDL> PRINT, num.BitSet([1, 2, 3, 4])
1 2 4 8
whereas what IDL actually displays is
30 30 30 30
The position vector applies to *every* element of the input array, setting them all to 2^1 + 2^2 + 2^3 + 2^4 = 30
--Wayne
On Tuesday, May 3, 2016 at 1:34:02 PM UTC-4, Paulo Penteado wrote:
> Hello,
>
> I was reading the help on the BitGet method,
>
> https://www.harrisgeospatial.com/docs/idl_integer.html#BitGe t
>
> And it seems there is a typo. It says
>
> Print the 3rd bit value of each element of an array:
> num = INDGEN(8)
> PRINT, num.BitGet( 3 )
> IDL prints:
> 0 0 0 0 1 1 1 1
>
> My experimenting indicates the argument of BitGet is the bit number, starting from zero, not from one as the help indicates. To get the 3rd bit, and obtain the result shown in the example, it seems it should be num.BitGet(2).
>
> Is this correct or am I missing something?
>
> Paulo
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