function [ x, seed ] = r8_normal_01 ( seed )

%*****************************************************************************80
%
%% R8_NORMAL_01 samples the standard normal probability distribution.
%
%  Discussion:
%
%    The standard normal probability distribution function (PDF) has
%    mean 0 and standard deviation 1.
%
%    The Box-Muller method is used.
%
%  Licensing:
%
%    This code is distributed under the MIT license. 
%
%  Modified:
%
%    06 August 2013
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    integer SEED, a seed for the random number generator.
%
%  Output:
%
%    real X, a sample of the standard normal PDF.
%
%    integer SEED, an updated seed for the random number generator.
%
  [ r1, seed ] = r8_uniform_01 ( seed );
  [ r2, seed ] = r8_uniform_01 ( seed );

  x = sqrt ( -2.0 * log ( r1 ) ) * cos ( 2.0 * pi * r2 );

  return
end
