Add a useful package (from Source forge) for octave

[CreaPhase.git] / octave_packages / miscellaneous-1.1.0 / reduce.m

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+## Copyright (C) 2007 Muthiah Annamalai <muthiah.annamalai@mavs.uta.edu>
+##
+## This program is free software; you can redistribute it and/or modify it under
+## the terms of the GNU General Public License as published by the Free Software
+## Foundation; either version 3 of the License, or (at your option) any later
+## version.
+##
+## This program is distributed in the hope that it will be useful, but WITHOUT
+## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
+## details.
+##
+## You should have received a copy of the GNU General Public License along with
+## this program; if not, see <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {@var{x} =} reduce (@var{function}, @var{sequence},@var{initializer})
+## @deftypefnx {Function File} {@var{x} =} reduce (@var{function}, @var{sequence})
+## Implements the 'reduce' operator like in Lisp, or Python.
+## Apply function of two arguments cumulatively to the items of sequence, 
+## from left to right, so as to reduce the sequence to a single value. For example,
+## reduce(@@(x,y)(x+y), [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5).
+## The left argument, x, is the accumulated value and the right argument, y, is the 
+## update value from the sequence. If the optional initializer is present, it is 
+## placed before the items of the sequence in the calculation, and serves as
+## a default when the sequence is empty. If initializer is not given and sequence
+## contains only one item, the first item is returned.
+##
+## @example
+##  reduce(@@add,[1:10])
+##  @result{} 55
+##      reduce(@@(x,y)(x*y),[1:7]) 
+##  @result{} 5040  (actually, 7!)
+## @end example
+## @end deftypefn
+
+## Parts of documentation copied from the "Python Library Reference, v2.5"
+
+function rv = reduce (func, lst, init)
+  if (nargin < 2) || nargin > 3 || (class(func)!='function_handle') || (nargin == 2 && length(lst)<2)
+    print_usage();
+  end
+
+  l=length(lst);
+
+  if (l<2 && nargin==3)
+    if(l==0)
+      rv=init;
+    elseif (l==1)
+      rv=func(init,lst(1));
+    end
+    return;
+  end
+
+  if(nargin == 3)
+    rv=func(init,lst(1));
+    start=2;
+  else
+    rv=func(lst(1),lst(2));
+    start=3;
+  end
+
+  for i=start:l
+    rv=func(rv,lst(i));
+  end
+end
+
+%!assert(reduce(@(x,y)(x+y),[],-1),-1)
+%!assert(reduce(@(x,y)(x+y),[+1],-1),0)
+%!assert(reduce(@(x,y)(x+y),[-10:-1]),-55)
+%!assert(reduce(@(x,y)(x+y),[-10:-1],+55),0)
+%!assert(reduce(@(x,y)(y*x),[1:4],5),120)