1 ## Copyright (C) 2012 Rik Wehbring
2 ## Copyright (C) 1995-2012 Kurt Hornik
4 ## This file is part of Octave.
6 ## Octave is free software; you can redistribute it and/or modify it
7 ## under the terms of the GNU General Public License as published by
8 ## the Free Software Foundation; either version 3 of the License, or (at
9 ## your option) any later version.
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14 ## General Public License for more details.
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21 ## @deftypefn {Function File} {} gaminv (@var{x}, @var{a}, @var{b})
22 ## For each element of @var{x}, compute the quantile (the inverse of
23 ## the CDF) at @var{x} of the Gamma distribution with shape parameter
24 ## @var{a} and scale @var{b}.
27 ## Author: KH <Kurt.Hornik@wu-wien.ac.at>
28 ## Description: Quantile function of the Gamma distribution
30 function inv = gaminv (x, a, b)
36 if (!isscalar (a) || !isscalar (b))
37 [retval, x, a, b] = common_size (x, a, b);
39 error ("gaminv: X, A, and B must be of common size or scalars");
43 if (iscomplex (x) || iscomplex (a) || iscomplex (b))
44 error ("gaminv: X, A, and B must not be complex");
47 if (isa (x, "single") || isa (a, "single") || isa (b, "single"))
48 inv = zeros (size (x), "single");
50 inv = zeros (size (x));
53 k = ((x < 0) | (x > 1) | isnan (x)
54 | !(a > 0) | !(a < Inf) | !(b > 0) | !(b < Inf));
57 k = (x == 1) & (a > 0) & (a < Inf) & (b > 0) & (b < Inf);
60 k = find ((x > 0) & (x < 1) & (a > 0) & (a < Inf) & (b > 0) & (b < Inf));
62 if (!isscalar (a) || !isscalar (b))
67 y = a * b * ones (size (k));
71 if (isa (x, "single"))
72 myeps = eps ("single");
79 y(l) = sqrt (myeps) * ones (length (l), 1);
84 h = (gamcdf (y_old, a, b) - x) ./ gampdf (y_old, a, b);
86 ind = find (y_new <= myeps);
88 y_new (ind) = y_old (ind) / 10;
91 if (max (abs (h)) < sqrt (myeps))
104 %! x = [-1 0 0.63212055882855778 1 2];
105 %!assert(gaminv (x, ones(1,5), ones(1,5)), [NaN 0 1 Inf NaN], eps);
106 %!assert(gaminv (x, 1, ones(1,5)), [NaN 0 1 Inf NaN], eps);
107 %!assert(gaminv (x, ones(1,5), 1), [NaN 0 1 Inf NaN], eps);
108 %!assert(gaminv (x, [1 -Inf NaN Inf 1], 1), [NaN NaN NaN NaN NaN]);
109 %!assert(gaminv (x, 1, [1 -Inf NaN Inf 1]), [NaN NaN NaN NaN NaN]);
110 %!assert(gaminv ([x(1:2) NaN x(4:5)], 1, 1), [NaN 0 NaN Inf NaN]);
112 %% Test class of input preserved
113 %!assert(gaminv ([x, NaN], 1, 1), [NaN 0 1 Inf NaN NaN], eps);
114 %!assert(gaminv (single([x, NaN]), 1, 1), single([NaN 0 1 Inf NaN NaN]), eps("single"));
115 %!assert(gaminv ([x, NaN], single(1), 1), single([NaN 0 1 Inf NaN NaN]), eps("single"));
116 %!assert(gaminv ([x, NaN], 1, single(1)), single([NaN 0 1 Inf NaN NaN]), eps("single"));
118 %% Test input validation
122 %!error gaminv (1,2,3,4)
123 %!error gaminv (ones(3),ones(2),ones(2))
124 %!error gaminv (ones(2),ones(3),ones(2))
125 %!error gaminv (ones(2),ones(2),ones(3))
126 %!error gaminv (i, 2, 2)
127 %!error gaminv (2, i, 2)
128 %!error gaminv (2, 2, i)