index out of bounds because numel(xcorn)=1

 I try to run this code, but this message appear in the screen,

"??? Attempted to access xcorn(2); index out of bounds because numel(xcorn)=1.
Error in ==> blakely at 31
  x2=xcorn(n2)-x0;

 " the code is the "talwani code"

function g=blakely(x0,z0,xcorn,zcorn,ncorn,rho)
% g=gpoly(x0,z0,xcorn,zcorn,ncorn,rho)
% computes the vertical attraction of a 2-dimensional body with polygonal
% cross section. Axis are right-handed system with y-axis parallel to long
% direction of body and z-axis vertical, positive downward.
% after Blakely, 
% Potential Theory in Gravity and Magnetic Applications, 1995, C.U.P., p 378
% Observation point is (x0,z0). Arrays xcorn & zcorn (each of length ncorn)
% contain the coordinates of the polygon corners, arranged in clockwise
% order when viewed with x-axis to right. Density of body is rho.
% all distance parameters in units of km, rho in units of kg/m^3
% Output = vertical attraction of gravity g, in mgal

% parameters
gamma=6.673e-11;
si2mg=1e5;
km2m=1e3;
% run
sum=0;
for n=1:ncorn
  if n==ncorn
    n2=1;
  else
    n2=n+1;
  end
  x1=xcorn(n)-x0;
  z1=zcorn(n)-z0;
  x2=xcorn(n2)-x0;
  z2=zcorn(n2)-z0;
  r1sq=x1*x1+z1*z1;
  r2sq=x2*x2+z2*z2;
  if r1sq==0
    disp('Field point on corner')
    break
  elseif r2sq==0
    disp('Field point on corner')
    break
  end
  denom=z2-z1;
  if denom==0
    denom=1E-6;
  end
  alpha=(x2-x1)/denom;
  beta=(x1*z2-x2*z1)/denom;
  factor=beta/(1+alpha*alpha);
  term1=0.5*(log(r2sq)-log(r1sq));
  term2=atan2(z2,x2)-atan2(z1,x1);
  sum=sum+factor*(term1-alpha*term2);
end
g=2*rho*gamma*sum*si2mg*km2m;

Answer 1

[dpb]

You called it with a single valued array (vector) apparently with the argument ncorn having a value greater than 1 (one).

As the input description says, the function requires "Arrays xcorn & zcorn (each of length ncorn)"