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## Copyright (C) 2001, 2006, 2007 Paul Kienzle
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## This file is part of Octave.
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## Octave is free software; you can redistribute it and/or modify it
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## under the terms of the GNU General Public License as published by
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## the Free Software Foundation; either version 3 of the License, or (at
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## your option) any later version.
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## Octave is distributed in the hope that it will be useful, but
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## WITHOUT ANY WARRANTY; without even the implied warranty of
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## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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## General Public License for more details.
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## You should have received a copy of the GNU General Public License
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## along with Octave; see the file COPYING. If not, see
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## <http://www.gnu.org/licenses/>.
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## @deftypefn {Function File} {} interpft (@var{x}, @var{n})
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## @deftypefnx {Function File} {} interpft (@var{x}, @var{n}, @var{dim})
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## Fourier interpolation. If @var{x} is a vector, then @var{x} is
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## resampled with @var{n} points. The data in @var{x} is assumed to be
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## equispaced. If @var{x} is an array, then operate along each column of
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## the array separately. If @var{dim} is specified, then interpolate
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## along the dimension @var{dim}.
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## @code{interpft} assumes that the interpolated function is periodic,
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## and so assumptions are made about the end points of the interpolation.
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## Author: Paul Kienzle
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## * added code to work on matrices as well
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## 2006-05-25 dbateman
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## * Make it matlab compatiable, cutting out the 2-D interpolation
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function z = interpft (x, n, dim)
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if (nargin < 2 || nargin > 3)
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if (isvector (x) && size (x, 1) == 1)
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error ("interpft: n must be an integer scalar");
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if (dim < 1 || dim > nd)
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error ("interpft: integrating over invalid dimension");
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perm = [dim:nd, 1:(dim-1)];
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x = permute (x, perm);
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z = cat (1, y(idx{:}), zeros (sz));
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z = cat (1, z, y(idx{:}));
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z = inc * reshape (z(1:inc:end), sz);
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z = ipermute (z, perm);
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%! t = 0 : 0.3 : pi; dt = t(2)-t(1);
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%! n = length (t); k = 100;
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%! ti = t(1) + [0 : k-1]*dt*n/k;
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%! y = sin (4*t + 0.3) .* cos (3*t - 0.1);
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%! yp = sin (4*ti + 0.3) .* cos (3*ti - 0.1);
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%! plot (ti, yp, 'g', ti, interp1(t, y, ti, 'spline'), 'b', ...
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%! ti, interpft (y, k), 'c', t, y, 'r+');
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%! legend ('sin(4t+0.3)cos(3t-0.1','spline','interpft','data');
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%! x = [0:10]'; y = sin(x); n = length (x);
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%!assert (interpft(y, n), y, eps);
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%!assert (interpft(y', n), y', eps);
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%!assert (interpft([y,y],n), [y,y], eps);
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%!error (interpft(y,n,0))
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%!error (interpft(y,[n,n]))