Hi Christopher, here's a solution that does the job. If you can guarantee that you only have one crossing in your vector V, then you don't need the loop (or the break command).
V = [.8;1;.8;.3;-.3;-.8;-1];
x = [1;.8;.5;.4;.3;.2;.1];
y = [1;.5;.2;-.4;-.5;-.6;-.7];
xy = [x y]; % I concat x and y for convenience
while 1
% Find the first available zero-crossing index of V
idx = find(abs(diff(sign(V)))==2, 1);
if isempty(idx)
break
end
% Interpolate xy at that zero
newXY = interp1(V(idx:idx+1),xy(idx:idx+1,:),0);
% Concat the results into xy and V
xy = [xy(1:idx,:); newXY; xy(idx+1:end,:)];
V = [V(1:idx); 0; V(idx+1:end)];
end
