elctricfieldelectricalfunctionMATLABvector

Hello everybody, I'm doing a physics project with Matlab. There are some problems and I hope you and everyone here can help me.
The project is:
Draw the electric field and the vector of electric field intensity.The requirements are: • Initialize variables (e.g., potential V(x,y), graphics).• Evaluate electric field as Ex = – dV/ x and Ey = – dV/ y.• Loop over all grid points and evaluate V(x,y) and E(x,y) on grid.• Compute potential at the grid point.• Compute components of the electric field. • Normalize E-field vectors to unit length.• Plot contours of constant electric potential.• Add electric field direction to potential contour plot.
I have found a code for reference but I don't understand it and when I tested with Matlab, the code had a problem. Here's the code:
    if true      % code  % 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    end
Here's the error:
??? Undefined function or method 'symop' for input arguments oftype 'sym'.Error in ==> Untitled at 17Ex = symop( '-1', '*', diff(V,'x') );
Can anyone explain the code and the error for me? For the code, I don't understand the thing:
    if true      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') );    endand     if true      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 );    end
Thank you so much. Your help will be really appreciated.

N=100; % numbef of pointsl=linspace(0,1,N); % linear spacing[x,y]=meshgrid(l); % a plate of length 1m as exampleV=100*sin(x.*y); %  example of potential distriibution.figure(1),surf(x,y,V), title(' Potential distribution' );[Ex,Ey]=gradient(V);Ex=Ex*(-1);Ey=Ey*(-1);% Modulus E=sqrt(Ex^2+Ey^2);E=E/norm(E); %  Normalization so that ||E(x,y)||=1.figure(2),contour(E); figure(3)contour(x,y,V);hold on,quiver(x,y,Ex,Ey)