cgrandomindices.pro
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; docformat = 'rst'
;
; NAME:
; cgRandomIndex
;
; PURPOSE:
; This function returns random indices without replacement. You can, for example,
; select 100 random, unique indices from a vector of 10000 random indices.
;
;******************************************************************************************;
; ;
; Copyright (c) 2010, by Fanning Software Consulting, Inc. All rights reserved. ;
; ;
; Redistribution and use in source and binary forms, with or without ;
; modification, are permitted provided that the following conditions are met: ;
; ;
; * Redistributions of source code must retain the above copyright ;
; notice, this list of conditions and the following disclaimer. ;
; * Redistributions in binary form must reproduce the above copyright ;
; notice, this list of conditions and the following disclaimer in the ;
; documentation and/or other materials provided with the distribution. ;
; * Neither the name of Fanning Software Consulting, Inc. nor the names of its ;
; contributors may be used to endorse or promote products derived from this ;
; software without specific prior written permission. ;
; ;
; THIS SOFTWARE IS PROVIDED BY FANNING SOFTWARE CONSULTING, INC. ''AS IS'' AND ANY ;
; EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES ;
; OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT ;
; SHALL FANNING SOFTWARE CONSULTING, INC. BE LIABLE FOR ANY DIRECT, INDIRECT, ;
; INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED ;
; TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; ;
; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ;
; ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT ;
; (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS ;
; SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. ;
;******************************************************************************************;
;+
; This function returns random indices without replacement. You can, for example,
; select 100 random, unique indices from a vector of 10000 random indices.
;
; The program uses an `algorithm offered by JD Smith <http://www.idlcoyote.com/code_tips/randomindex.html>`
; on the IDL newsgroup.
;
; :Categories:
; Utilities
;
; :Params:
; length: in, required, type=long
; The number of random values to be considered in the selection process. In
; other words, the length of the selection vector.
; number: in, required, type=long
; The number of unique, random indices you want the function to return out of
; the selection vector.
;
; :Keywords:
; seed: in, optional, type=long
; The seed for the random number generator, RandomU. This is also an output
; variable, which you should use as the input to the next cgRandomIndices call,
; if you need to do several random selections very quickly in a program.
;
; :Examples:
; To select 10 random indices from a list of 100::
; indices = cgRandomIndices(100, 10, SEED=seed)
; Print, indices
; 7 13 20 21 32 44 50 66 80 93
;
; :Author:
; FANNING SOFTWARE CONSULTING::
; David W. Fanning
; 1645 Sheely Drive
; Fort Collins, CO 80526 USA
; Phone: 970-221-0438
; E-mail: david@idlcoyote.com
; Coyote's Guide to IDL Programming: http://www.idlcoyote.com
;
; :History:
; Change History::
; Written, 22 December 2011 from an algorithm by JD Smith. David W. Fanning.
;
; :Copyright:
; Copyright (c) 2011, Fanning Software Consulting, Inc.
;-
FUNCTION cgRandomIndices, length, number, SEED=seed
Compile_Opt idl2
; Error handling.
Catch, theError
IF theError NE 0 THEN BEGIN
Catch, /CANCEL
void = cgErrorMsg()
RETURN, -1
ENDIF
; If number is greater than half the length, then instead of accumulating
; good indices, we will discard bad indices and use Histogram to generate
; the "real" indices list.
swap = number GT length/2
IF swap THEN n = length - number ELSE n = number
; Make an array of the proper length for the indices.
indices = LonArr(n, /NOZERO)
; Accumulate unique indices in the array.
M = n
WHILE n GT 0 DO BEGIN
indices[M-n] = Long( RandomU(seed, n) * length )
indices = indices[Sort(indices)]
u = Uniq(indices)
n = M - N_Elements(u)
indices[0] = indices[u]
ENDWHILE
; If you are discarding, then the indices are selected with HISTOGRAM.
IF swap THEN indices = Where(Histogram(indices, MIN=0, MAX=length-1) EQ 0)
RETURN, indices
END