MATLAB: Help me this. Why it doesnt work

error line 14

% e_and_v - Compute electric field from potential
%           and graph potential contours and E-field direction
clear all; help e_and_v;  % Clear memory; print header
%@ Initialize variables (e.g., potential V(x,y), graphics)
fprintf('Enter potential V(x,y) as an equation \n');
fprintf('For example:  log(x^2 + y^2) \n');
V = input(': ','s');   % Read in V(x,y) as text string
NGrid = 20;            % Number of grid points for plots
xMax = 5;              % Values plotted from x= -xMax to x= xMax
yMax = xMax;           % Values plotted from y= -yMax to y= yMax
for i=1:NGrid
  xPlot(i) = -xMax + (i-1)/(NGrid-1)*(2*xMax); % x values to plot
  yPlot(i) = -yMax + (i-1)/(NGrid-1)*(2*yMax); % y values to plot
end
%@ Evaluate electric field as Ex = (-1)*dV/dx and Ey = (-1)*dV/dy
%  Note use of symop command to perform symbolic multiplication by -1
Ex = symop( '-1', '*', diff(V,'x') );
Ey = symop( '-1', '*', diff(V,'y') );
fprintf('Electric field components are \n');
disp(['x component : ', Ex]);
disp(['y component : ', Ey]);
%@ Loop over all grid points and evaluate V(x,y) and E(x,y) on grid
for i=1:NGrid
 y = yPlot(i);
 for j=1:NGrid
   x = xPlot(j);   
     %@ Compute potential at the grid point
     VPlot(i,j) = eval( V );    % Potential V(x,y)
     %@ Compute components of the electric field
     ExPlot(i,j) = eval( Ex );
     EyPlot(i,j) = eval( Ey );
     %@ Normalize E-field vectors to unit length
     MagnitudeE = sqrt( ExPlot(i,j)^2 + EyPlot(i,j)^2 );
     ExPlot(i,j) = ExPlot(i,j)/MagnitudeE;
     EyPlot(i,j) = EyPlot(i,j)/MagnitudeE;
   end
  end
%@ Plot contours of constant electric potential
clf;  figure(gcf);    % Clear figure; bring figure window forward
meshc(xPlot,yPlot,VPlot);  % Plot potential in contour/mesh plot
xlabel('x');  ylabel('y'); zlabel('Potential');
title('Strike any key to continue ...');
pause;
% Specify contour levels used in contour plot
axis([-xMax xMax -yMax yMax]);    % Fix the min,max for x,y axes
cs = contour(xPlot,yPlot,VPlot); % Draw contour plot
clabel(cs);           % Place contour labels on contour levels
%@ Add electric field direction to potential contour plot
hold on;
quiver(xPlot,yPlot,ExPlot,EyPlot);  % Draw arrows for E field
title('Potential contours and electric field direction');
xlabel('x');  ylabel('y');
hold off;

Best Answer

symop() is very old Symbolic Toolbox, before R14. It was part of version 1 of the Symbolic Toolbox, and was replaced by Version 2. You will need to rewrite the code.
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