function value = norm_l1 ( f_string, a, b )

%*****************************************************************************80
%
%% norm_l1 interactively estimates the L1 norm of a function.
%
%  Discussion:
%
%    l1_norm(f,a,b) = integral ( a <= x <= b ) |f(x)| dx )
%
%    The norm is estimated by calling the MATLAB function integral().
%
%    The user enters a formula for f(x), and the values of a and b.
%
%    The program can be invoked by a function call, in which case the
%    string specifying f(x) must be quoted:
%      value = norm_l1 ( 'x^2', -1, 3 )
%    or, if called with no arguments, it will request them:
%      value = norm_l1 ( );
%      Enter function formula, like x^2: x^2
%      Enter left limit, a: -1
%      Enter right limit, b: 3
%
%    The function is specified as a string which is either:
%    * a MATLAB expression using the argument 'x', 
%    * the name of an M-file followed by the argument '(x)'.
%
%    The string should not contain any spaces between symbols, except
%    when it is passed as a function argument in quotes.
%
%    It is not necessary to use the "dot" notation for expressions involving
%    '*', '/', or '^', but it doesn't hurt either.
%
%    Examples of function specifications:
%
%      x^2
%      x.^2
%      3/(x^4+5*x-6)
%      sin(7*x)*sqrt(x)/8
%      wiggle(x)     <-- where "wiggle.m" is a user-provided M file.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    01 July 2019
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    string f_string: the function to be measured.
%
%    real a, b: the interval.
%
%  Output:
%
%    real value: the norm.
%

%
%  Get F, A, B.
%
  if ( nargin < 1 )
    f_string = input ( 'Enter function formula, like x^2: ', 's' );
  end
  f_element = vector_to_element ( f_string );
  f = str2func ( [ '@(x)', 'abs ( ', f_element, ' )' ] );

  if ( nargin < 2 )
    a = input ( 'Enter left limit a: ' );
  elseif ( ischar ( a ) )
    a = str2num ( a );
  end

  if ( nargin < 3 )
    b = input ( 'Enter right limit b: ' );
  elseif ( ischar ( b ) )
    b = str2num ( b );
  end

  value = integral ( f, a, b, 'AbsTol', 1.0e-14, 'RelTol', 1.0e-12 );

  return
end
function f_element = vector_to_element ( f_vector )

%*****************************************************************************80
%
%% vector_to_element rewrites a MATLAB vector function elementwise.
%
%  Discussion:
%
%    In MATLAB, by default, the operators '*', '/', and '^' are interpreted
%    as linear algebraic operations.  In many cases, a user simply desired
%    the elementwise operation instead, which MATLAB indicates using a 
%    dotted notation: '.*', './', and '.^'.
%
%    This function converts a string to a form that uses dotted operators.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    29 June 2019
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    string f_vector, a string presumably representing a MATLAB expression.
%
%  Output:
%
%    string f_element, a copy of f_vector in which all operators have been
%    replaced by their dotted (elementwise) form.
%
  n_vector = length ( f_vector );
  f_element = [''];

  c_prev = '';
  for iv = 1 : n_vector
    c = f_vector(iv);
    if ( ( c == '*' || c == '/' || c == '^' ) && ( c_prev ~= '.' ) )
      f_element = [ f_element, ' .', c, ' ' ];
    else
      f_element = [ f_element, c ];
    end
    c_prev = c;
  end

  return
end

