doc: Ensure documentation lists output argument when it exists for all m-files. For new users of Octave it is best to show explicit calling forms in the documentation and to show a return argument when it exists. * bp-table.cc, shift.m, accumarray.m, accumdim.m, bincoeff.m, bitcmp.m, bitget.m, bitset.m, blkdiag.m, celldisp.m, cplxpair.m, dblquad.m, flip.m, fliplr.m, flipud.m, idivide.m, int2str.m, interpft.m, logspace.m, num2str.m, polyarea.m, postpad.m, prepad.m, randi.m, repmat.m, rng.m, rot90.m, rotdim.m, structfun.m, triplequad.m, uibuttongroup.m, uicontrol.m, uipanel.m, uipushtool.m, uitoggletool.m, uitoolbar.m, waitforbuttonpress.m, help.m, __additional_help_message__.m, hsv.m, im2double.m, im2frame.m, javachk.m, usejava.m, argnames.m, char.m, formula.m, inline.m, __vectorize__.m, findstr.m, flipdim.m, strmatch.m, vectorize.m, commutation_matrix.m, cond.m, cross.m, duplication_matrix.m, expm.m, orth.m, rank.m, rref.m, trace.m, vech.m, cast.m, compare_versions.m, delete.m, dir.m, fileattrib.m, grabcode.m, gunzip.m, inputname.m, license.m, list_primes.m, ls.m, mexext.m, movefile.m, namelengthmax.m, nargoutchk.m, nthargout.m, substruct.m, swapbytes.m, ver.m, verLessThan.m, what.m, fminunc.m, fsolve.m, fzero.m, optimget.m, __fdjac__.m, matlabroot.m, savepath.m, campos.m, camroll.m, camtarget.m, camup.m, camva.m, camzoom.m, clabel.m, diffuse.m, legend.m, orient.m, rticks.m, specular.m, thetaticks.m, xlim.m, xtickangle.m, xticklabels.m, xticks.m, ylim.m, ytickangle.m, yticklabels.m, yticks.m, zlim.m, ztickangle.m, zticklabels.m, zticks.m, ellipsoid.m, isocolors.m, isonormals.m, stairs.m, surfnorm.m, __actual_axis_position__.m, __pltopt__.m, close.m, graphics_toolkit.m, pan.m, print.m, printd.m, __ghostscript__.m, __gnuplot_print__.m, __opengl_print__.m, rotate3d.m, subplot.m, zoom.m, compan.m, conv.m, poly.m, polyaffine.m, polyder.m, polyint.m, polyout.m, polyreduce.m, polyvalm.m, roots.m, prefdir.m, prefsfile.m, profexplore.m, profexport.m, profshow.m, powerset.m, unique.m, arch_rnd.m, arma_rnd.m, autoreg_matrix.m, bartlett.m, blackman.m, detrend.m, durbinlevinson.m, fftconv.m, fftfilt.m, fftshift.m, fractdiff.m, hamming.m, hanning.m, hurst.m, ifftshift.m, rectangle_lw.m, rectangle_sw.m, triangle_lw.m, sinc.m, sinetone.m, sinewave.m, spectral_adf.m, spectral_xdf.m, spencer.m, ilu.m, __sprand__.m, sprand.m, sprandn.m, sprandsym.m, treelayout.m, beta.m, betainc.m, betaincinv.m, betaln.m, cosint.m, expint.m, factorial.m, gammainc.m, gammaincinv.m, lcm.m, nthroot.m, perms.m, reallog.m, realpow.m, realsqrt.m, sinint.m, hadamard.m, hankel.m, hilb.m, invhilb.m, magic.m, pascal.m, rosser.m, toeplitz.m, vander.m, wilkinson.m, center.m, corr.m, cov.m, discrete_cdf.m, discrete_inv.m, discrete_pdf.m, discrete_rnd.m, empirical_cdf.m, empirical_inv.m, empirical_pdf.m, empirical_rnd.m, kendall.m, kurtosis.m, mad.m, mean.m, meansq.m, median.m, mode.m, moment.m, range.m, ranks.m, run_count.m, skewness.m, spearman.m, statistics.m, std.m, base2dec.m, bin2dec.m, blanks.m, cstrcat.m, deblank.m, dec2base.m, dec2bin.m, dec2hex.m, hex2dec.m, index.m, regexptranslate.m, rindex.m, strcat.m, strjust.m, strtrim.m, strtrunc.m, substr.m, untabify.m, __have_feature__.m, __prog_output_assert__.m, __run_test_suite__.m, example.m, fail.m, asctime.m, calendar.m, ctime.m, date.m, etime.m: Add return arguments to @deftypefn macros where they were missing. Rename variables in functions (particularly generic "retval") to match documentation. Rename some return variables for (hopefully) better clarity (e.g., 'ax' to 'hax' to indicate it is a graphics handle to an axes object).

########################################################################
##
## Copyright (C) 1994-2022 The Octave Project Developers
##
## See the file COPYRIGHT.md in the top-level directory of this
## distribution or <https://octave.org/copyright/>.
##
## This file is part of Octave.
##
## Octave 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.
##
## Octave 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 Octave; see the file COPYING.  If not, see
## <https://www.gnu.org/licenses/>.
##
########################################################################

## -*- texinfo -*-
## @deftypefn  {} {@var{c} =} fftconv (@var{x}, @var{y})
## @deftypefnx {} {@var{c} =} fftconv (@var{x}, @var{y}, @var{n})
## Convolve two vectors using the FFT for computation.
##
## @code{c = fftconv (@var{x}, @var{y})} returns a vector of length equal to
## @code{length (@var{x}) + length (@var{y}) - 1}.  If @var{x} and @var{y}
## are the coefficient vectors of two polynomials, the returned value is the
## coefficient vector of the product polynomial.
##
## The computation uses the FFT by calling the function @code{fftfilt}.  If
## the optional argument @var{n} is specified, an N-point FFT is used.
## @seealso{deconv, conv, conv2}
## @end deftypefn

function c = fftconv (x, y, n)

  if (nargin < 2)
    print_usage ();
  endif

  if (! (isvector (x) && isvector (y)))
    error ("fftconv: both A and B must be vectors");
  endif
  la = length (x);
  lb = length (y);
  if ((la == 1) || (lb == 1))
    c = x * y;
  else
    lc = la + lb - 1;
    x(lc) = 0;
    y(lc) = 0;
    if (nargin == 2)
      c = fftfilt (x, y);
    else
      if (! isscalar (n))
        error ("fftconv: N must be a scalar");
      endif
      c = fftfilt (x, y, n);
    endif
  endif

endfunction


## FIXME: Borrow tests from conv.m.  May need tolerance on the assert stmt.
%!testif HAVE_FFTW
%! x = ones (3,1);
%! y = ones (1,3);
%! b = 2;
%! c = 3;
%! assert (fftconv (x, x), [1; 2; 3; 2; 1], 5*eps);
%! assert (fftconv (y, y), [1, 2, 3, 2, 1], 5*eps);
%! assert (fftconv (x, y), [1, 2, 3, 2, 1], 5*eps);
%! assert (fftconv (y, x), [1; 2; 3; 2; 1], 5*eps);
%! assert (fftconv (c, x), [3; 3; 3], 5*eps);
%! assert (fftconv (c, y), [3, 3, 3], 5*eps);
%! assert (fftconv (x, c), [3; 3; 3], 5*eps);
%! assert (fftconv (y, c), [3, 3, 3], 5*eps);
%! assert (fftconv (b, c), 6, 5*eps);

%!test
%! a = 1:10;
%! b = 1:3;
%! assert (size (conv (a,b)), [1, numel(a)+numel(b)-1]);
%! assert (size (conv (b,a)), [1, numel(a)+numel(b)-1]);

%! a = (1:10).';
%! b = 1:3;
%! assert (size (conv (a,b)), [numel(a)+numel(b)-1, 1]);
%! assert (size (conv (b,a)), [numel(a)+numel(b)-1, 1]);

%!test
%! a = 1:10;
%! b = (1:3).';
%! assert (size (conv (a,b)), [1, numel(a)+numel(b)-1]);
%! assert (size (conv (b,a)), [1, numel(a)+numel(b)-1]);

## Test input validation
%!error <Invalid call> fftconv (1)
%!error fftconv ([1, 2; 3, 4], 3)
%!error fftconv (2, [])
%!error fftconv ([1,1], [2,2] , [3, 4])