>>>> > "dean" == dean <dean@phobos.cira.colostate.edu> writes:
dean> We are set up to routinely collect GOES-8 GVAR data. Like with
dean> most data from satellites, they put information in the
dean> individual bits. Below is a C structure that extracts the
dean> individual bits from part of the data header which contains some
dean> time information. Extracting individual bits from data with IDL
dean> is difficult. At first I consider building an IDL structure, but
dean> IDL is unable to break down to bits.
dean> The information can fit into LONARR(2). If anyone has any
dean> suggestion that would allow IDL to extract the information from
dean> these LONARR(2), please forward your comments to me or post
dean> them. I hope to use this information to build a widget to
dean> navigate through this new and fine data set coming from GOES-8.
There was a discussion in this group some time ago about obtaining bit
information in IDL. I wrote a very general function for arbitrary
base decoding called decode.pro which I include for your use below.
One of its applications can be to decode an array of scalars into base
2. For example, with geos = lonarr(2), decode(geos,2) returns an
array containing the binary expansion for geos which you can then
test.
I am not exactly sure which bits of your LONARR(2) correspond to your
C struct definition because the C ANSI standard says that bit-fields
are implementation dependent, i.e., non-portable. For example,
whether fields are assigned left to right or right to left and/or can
overlap word boundaries can vary between machines.
Good luck,
Chris
--- begin included file ---
FUNCTION Decode, scl, dim, help=help
;+
; $Id: decode.pro,v 1.1 1994/07/18 16:05:32 chase Exp $
;
; NAME:
;
; DECODE
;
; PURPOSE:
;
; Decode a vector of scalars into the dimensions d1,...,dn
; in order of increasing significance.
;
; CATEGORY:
;
; Mathematical Functions
;
; CALLING SEQUENCE:
;
; Result = DECODE(Scl, Dim)
;
; INPUTS:
;
; Scl - Vector of scalars to be decoded. Will be converted to
; integers first, truncating if necessary.
;
; Dim - If a scalar, then it is converted to an integer base for the
; decoding, i.e., D1,...,DN=Dim.
; If a 1 dimensional vector, then it is converted to the
; integer dimensions D1,...,DN.
; If > 1 dimensional array, then the dimensions of the array
; are used for D1,...,DN.
; The dimensions increase in significance, i.e., the first
; dimension is the least significant and the last dimension is
; the most significant.
;
; KEYWORD PARAMETERS:
;
; HELP - Provide help (this information). No other action is performed.
;
; OUTPUTS:
;
; Result - Array of size NxM where M is dimension of Scl.
; Result(*,i) is the decoding of the scalar Scl(i). If
; Scl(i) is larger then the dimensioned size,
; i.e. D1x...xDN, then the modulus, Scl(i) mod D1x...xDN,
; is decoded. Result(j-1,i) corresponds to dimension Dj
; with Result(N-1,i) the most significant digit of the
; decoding.
;
; PROCEDURE:
;
; Let b1,...,bN be the decoding. Then Scl can be represented as:
;
; Scl = D1(D2(...(D[N-1]*bN + b[N-1])...+ b3 ) + b2) + b1
;
; with 0 <= bi < Di
;
; EXAMPLE:
;
; scl = [20,63]
; ; Conversion to base 16
; print,decode(scl,16)
;
; ; Conversion to binary (base 2)
; print,decode(scl,2)
; ; Invert the decoding
; print,2^indgen(5)#decode(scl,2)
;
; ; Arbitrary decoding. Generates a warning for decoding 63 in
; ; which case (63 mod 3*4*5) = 3 is decoded.
; print,decode(scl,[3,4,5])
; print,[1,3,3*4]#decode(scl,[3,4,5])
; print,[1,3,3*4,3*4*5]#decode(scl,[3,4,5,6])
;
; ; Convert 1D index into a multi-dimensional index
; w=dist(20,20)
; a=max(w,i)
; ; Get 2D index for max
; print,decode(i,w)
;
; MODIFICATION HISTORY:
;
; Mon Jul 18 15:58:18 1994, Chris Chase S1A <chase@jackson>
; Fixed/cleaned up.
;
; Mon Jul 26 12:17:56 1993, Chris Chase <chase@aphill>
; Created. Named for similar APL function.
;
;-
if keyword_set(help) then begin
doc_library, 'decode'
return, ''
endif
s = size(dim)
if (s(0) eq 0) then begin
d = replicate(long(dim), long(alog(max(scl))/alog(dim)) + 1)
endif else begin
if (s(0) gt 1) then d = s(1:s(0)) $
else d = dim
endelse
d = long(d)
nd = n_elements(d)
v = long(scl)
index = lonarr(nd, n_elements(v))
for i = 0, nd-1 do begin
f = v/d(i)
index(i, *) = v-f*d(i)
v = f
endfor
if (max(v) ne 0) then begin
print, "Warning - function DECODE: scalar outside dimension " + $
"bounds, decode of modulus returned."
endif
return, index
end
--
===============================
Bldg 24-E188
The Applied Physics Laboratory
The Johns Hopkins University
(301)953-6000 x8529
chris_chase@jhuapl.edu
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