33 lines
634 B
Matlab
33 lines
634 B
Matlab
function x = qbeta(p,a,b)
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%QBETA The beta inverse distribution function
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%
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% x = qbeta(p,a,b)
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% Anders Holtsberg, 27-07-95
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% Copyright (c) Anders Holtsberg
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if any(any((a<=0)|(b<=0)))
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error('Parameter a or b is nonpositive')
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end
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if any(any(abs(2*p-1)>1))
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error('A probability should be 0<=p<=1, please!')
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end
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if exist('OCTAVE_VERSION')
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x = betainv(p, a, b);
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return
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end
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b = min(b,100000);
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x = a ./ (a+b);
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dx = 1;
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while any(any(abs(dx)>256*eps*max(x,1)))
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dx = (betainc(x,a,b) - p) ./ dbeta(x,a,b);
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x = x - dx;
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x = x + (dx - x) / 2 .* (x<0);
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x = x + (1 + (dx - x)) / 2 .* (x>1);
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end
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