MATLAB: Not Enough Input Argument. .

Question

% Program to build the bus admittance
function[Y] = PSMv1(zdata)
linefrom = zdata(:,1); 
lineto = zdata(:,2); 
Rpu = zdata(:,3); 
Xpu = zdata(:,4);
nbr = length(zdata(:,1)); 
nbus = max(max(linefrom), max(lineto));
Z = Rpu + 1i*Xpu;           % Branch impedance
y = ones(nbr,1)./Z;          % Branch admittance
Y = zeros(nbus,nbus);       % Initialize Y to zero
for k = 1:nbr;                % Formation of the off-diagonal elements
      if linefrom(k) > 0 && lineto(k) >0
          Y(linefrom(k),lineto(k)) = Y(linefrom(k),lineto(k)) - y(k);
          Y(lineto(k),linefrom(k)) = Y(linefrom(k),lineto(k));
      end
  end
for n = 1:nbus;             % Formation of the diagonal elements
      for k = 1:nbr
          if linefrom(k) == n || lineto(k) == n
              Y(n,n) = Y(n,n) + y(k);
          else
          end
      end
  end
% Example 6.1 Hadi Saadat
%       From    To  R   X
zdata = [0      1   0   1.0
         0      2   0   0.8
         1      2   0   0.4
         1      3   0   0.2
         2      3   0   0.2
         3      4   0   0.08];
Y = ybus(zdata);                        % Bus admittance matrix
Ibus = [-1i*1.1; -1i*1.25; 0; 0];       % Vector of bus currents
Vbus = Y\Ibus;

Answer 1

At the command line, you must type the command

zdata = [0      1   0   1.0
       0      2   0   0.8
       1      2   0   0.4
       1      3   0   0.2
       2      3   0   0.2
       3      4   0   0.08];
PSMv(zdata)

Question: what is ybus() ? And why do you re-calculate the value of "Y" ?