function u = pressure_grad(xx,h,x0,k,N,v,x)
%
% function u = pressure_grad(xx,h,x0,k,N,v,x)
%
% This function evaluates the gradient of the representation formula (2.32) in the notes by
% a simple boundary element discretisation, as described in question 5 of
% the problem sheet.
%
% On entry:
%
% xx is an N x 2 matrix, the jth row of xx containing the position
% vector of the collocation point in the jth element
% h is an N x 1 vector, the jth entry of h the length of the jth
% element
% x0 is a 1x2 vector, the position vector of the point source of sound
% k is the wavenumber
% N is the number of boundary elements
% v is an N x 1 (complex) vector, the jth element of v an approximation to the normal
% derivative of the pressure (in the direction pointing into the domain) on
% element j
% x is a 2x1 vector, the position vector of the point at which the
% pressure is required
%
% On exit:
%
% u is an approximation to the gradient of the pressure at x
%
u = G_grad(x,x0,k);
for j = 1:N
u = u + v(j)*h(j)*G_grad(x,xx(j,:),k);
end