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1 ## Copyright (C) 1996, 1997 John W. Eaton |
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2 ## |
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3 ## This file is part of Octave. |
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4 ## |
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5 ## Octave is free software; you can redistribute it and/or modify it |
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6 ## under the terms of the GNU General Public License as published by |
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7 ## the Free Software Foundation; either version 2, or (at your option) |
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8 ## any later version. |
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9 ## |
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10 ## Octave is distributed in the hope that it will be useful, but |
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11 ## WITHOUT ANY WARRANTY; without even the implied warranty of |
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12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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13 ## General Public License for more details. |
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14 ## |
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15 ## You should have received a copy of the GNU General Public License |
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16 ## along with Octave; see the file COPYING. If not, write to the Free |
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17 ## Software Foundation, 59 Temple Place - Suite 330, Boston, MA |
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18 ## 02111-1307, USA. |
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19 |
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20 ## -*- texinfo -*- |
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21 ## @deftypefn {Function File} {} tril (@var{a}, @var{k}) |
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22 ## @deftypefnx {Function File} {} triu (@var{a}, @var{k}) |
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23 ## Return a new matrix formed by extracting extract the lower (@code{tril}) |
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24 ## or upper (@code{triu}) triangular part of the matrix @var{a}, and |
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25 ## setting all other elements to zero. The second argument is optional, |
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26 ## and specifies how many diagonals above or below the main diagonal should |
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27 ## also be set to zero. |
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28 ## |
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29 ## The default value of @var{k} is zero, so that @code{triu} and |
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30 ## @code{tril} normally include the main diagonal as part of the result |
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31 ## matrix. |
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32 ## |
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33 ## If the value of @var{k} is negative, additional elements above (for |
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34 ## @code{tril}) or below (for @code{triu}) the main diagonal are also |
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35 ## selected. |
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36 ## |
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37 ## The absolute value of @var{k} must not be greater than the number of |
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38 ## sub- or super-diagonals. |
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39 ## |
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40 ## For example, |
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41 ## |
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42 ## @example |
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43 ## @group |
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44 ## tril (ones (3), -1) |
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45 ## @result{} 0 0 0 |
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46 ## 1 0 0 |
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47 ## 1 1 0 |
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48 ## @end group |
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49 ## @end example |
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50 ## |
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51 ## @noindent |
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52 ## and |
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53 ## |
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54 ## @example |
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55 ## @group |
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56 ## tril (ones (3), 1) |
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57 ## @result{} 1 1 0 |
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58 ## 1 1 1 |
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59 ## 1 1 1 |
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60 ## @end group |
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61 ## @end example |
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62 ## @end deftypefn |
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63 ## @seealso{triu and diag} |
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64 |
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65 ## Author: jwe |
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66 |
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67 function retval = tril (x, k) |
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68 |
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69 if (nargin > 0) |
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70 [nr, nc] = size (x); |
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71 retval = zeros (nr, nc); |
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72 endif |
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73 |
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74 if (nargin == 1) |
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75 k = 0; |
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76 elseif (nargin == 2) |
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77 if ((k > 0 && k > nc) || (k < 0 && k < -nr)) |
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78 error ("tril: requested diagonal out of range"); |
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79 endif |
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80 else |
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81 usage ("tril (x, k)"); |
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82 endif |
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83 |
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84 for j = 1 : min (nc, nr+k) |
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85 nr_limit = max (1, j-k); |
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86 retval (nr_limit:nr, j) = x (nr_limit:nr, j); |
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87 endfor |
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88 |
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89 endfunction |
