| Re: bits into bytes [message #81031] |
Wed, 01 August 2012 20:19 |
wlandsman
Messages: 743
Registered: June 2000
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Senior Member |
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On Wednesday, August 1, 2012 7:55:16 PM UTC-4, Heinz Stege wrote:
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> processor. However the /PRESERVE_TYPE option of the TOTAL function
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> should speed up the calculation.
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Heinz,
Thanks for reminding me of the ISHFT() function and the /PRESERVE_TYPE keyword to TOTAL(). Actually, rather than use TOTAL(), I found the fastest method when dealing with millions of values is to use matrix multiply, as in the following example:
;Create an 8 x n byte array of random 0s and 1s. This will be compressed by a factor of 8 by storing ;each value in a bit rather than a byte
n = 1000000
x = byte(round(randomu(seed,8,n)))
yy = [128b,64b,32b,16b,8b,4b,2b,1b]
return, byte(yy#x)
The above code would be slightly faster if there were the equivalent of /PRESERVE_TYPE for matrix multiplication. Right now -- rather surprisingly, I think -- matrix multiplication of two byte arrays yields a long array. But the code is very fast anyway. --Wayne
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| Re: bits into bytes [message #81032 is a reply to message #81031] |
Wed, 01 August 2012 16:55  |
Heinz Stege
Messages: 189
Registered: January 2003
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Senior Member |
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On Wed, 1 Aug 2012 15:49:21 -0700 (PDT), wlandsman wrote:
> I have to write an array of "on-off" states in which the values are stored as bits. So if I have
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> x = [1b,0b,1b,1b,1b,0b,1b,1b]
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> i want to convert this to a single byte value. (This is the inverse of David Fanning's bitget.pro function.)
> One way to to do this is
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> IDL> yy = [128b,64b,32b,16b,8b,4b,2b,1b]
> IDL> print,byte( total(yy*x))
> 187b
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> But there must be a way to do this using masking rather than multiplication. I just can't figure it out right now. (The conversion will be done for millions of bits). Thanks, --Wayne
You can use ISHFT instead of the multiplication:
print,total(ishft(x,7b-bindgen(8)),/preserve_type)
But I don't expect any advantages on use of a floating point
processor. However the /PRESERVE_TYPE option of the TOTAL function
should speed up the calculation.
Cheers, Heinz
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