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+++ b/scripts/statistics/distributions/expcdf.m
@@ -0,0 +1,74 @@
+## Copyright (C) 1995, 1996, 1997  Kurt Hornik
+##
+## 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 2, 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, write to the Free
+## Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+## 02110-1301, USA.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {} exponential_cdf (@var{x}, @var{lambda})
+## For each element of @var{x}, compute the cumulative distribution
+## function (CDF) at @var{x} of the exponential distribution with
+## parameter @var{lambda}.
+##
+## The arguments can be of common size or scalar.
+## @end deftypefn
+
+## Author: KH <Kurt.Hornik@ci.tuwien.ac.at>
+## Description: CDF of the exponential distribution
+
+function cdf = exponential_cdf (x, l)
+
+  if (nargin != 2)
+    usage ("exponential_cdf (x, lambda)");
+  endif
+
+  if (!isscalar (x) && !isscalar(l))
+    [retval, x, l] = common_size (x, l);
+    if (retval > 0)
+      error ("exponential_cdf: x and lambda must be of common size or scalar");
+    endif
+  endif
+
+  if (isscalar (x))
+    sz = size (l);
+  else
+    sz = size (x);
+  endif
+
+  cdf = zeros (sz);
+
+  k = find (isnan (x) | !(l > 0));
+  if (any (k))
+    cdf(k) = NaN;
+  endif
+
+  k = find ((x == Inf) & (l > 0));
+  if (any (k))
+    cdf(k) = 1;
+  endif
+
+  k = find ((x > 0) & (x < Inf) & (l > 0));
+  if (any (k))
+    if isscalar (l)
+      cdf (k) = 1 - exp (- l .* x(k));
+    elseif isscalar (x)
+      cdf (k) = 1 - exp (- l(k) .* x);
+    else
+      cdf (k) = 1 - exp (- l(k) .* x(k));
+    endif
+  endif
+
+endfunction