Could anyone explain this code for me please?

%A star algorithm

tic

%Initialize

%Node from which the good neighbour is reached
came_fromx=zeros(size(E));        %???!!
came_fromy=zeros(size(E));

%Nodes already evaluated
closed=[];

%Nodes to be evaluated
open=[y0,x0];

%Cost of moving from start point to the node
G=elev*ones(size(E));
G(y0,x0)=0;

%Cost of moving from the node to the arrival point
H=elev*ones(size(E));
H(y0,x0)=sqrt((xend-x0)^2+(yend-y0)^2);
% H(y0,x0)=octile_distance(x0,y0,xend,yend);

%Initial total cost
F=G+H;

%Cost weight
kg=0.5;
kh=0.8;
ke=0.5;


%Initialize
exit=0;
k=1;

%While open is not empty
while isempty(open)==0 && exit==0    
    
    %Find node with minimum f in open
    
    %Initialize
    f_open=zeros(size(open,1),1);
    
    %Evaluate f for the nodes in open
    for i=1:size(open,1)
        f_open(i,:)=F(open(i,1),open(i,2));
    end

    %Find the index location in open for the node with minimum f
    [~,i_f_open_min]=min(f_open);
    
    %Location of node with minimum f in open
    ycurrent=open(i_f_open_min,1);
    xcurrent=open(i_f_open_min,2);    
    
%     %Path search video
%     x=1:d_grid:x_size;
%     y=1:d_grid:y_size;
%     ax=figure(99);
%     surf(x,y,E)
%     hold on
%     plot3(xcurrent,ycurrent,elev,'g*')
%     plot3(x0,y0,elev,'go')
%     plot3(xend,yend,elev,'ro')  
%     frames(k) = getframe(ax);
%     axis tight
%     view(0, 90);   
%     k=k+1;
    
    
    %Check if the arrival node is reached
    if xcurrent==xend && ycurrent==yend
        
        %Arrival node reached, exit and generate path
        exit=1; 
        
    else
        
        %Add the node to closed
        closed(size(closed,1)+1,:)=[ycurrent, xcurrent];
        
        %Remove the node from open
        i_open_keep=horzcat(1:i_f_open_min-1,i_f_open_min+1:size(open,1));
        open=open(i_open_keep,:);  
        
        %Check neighbour nodes
        for i=-1:1
            for j=-1:1                
                
                %If the neighbour node is within grid 
                if xcurrent+i>0 && ycurrent+j>0 && xcurrent+i<=x_size && ycurrent+j<=y_size 
                                        
                    %If the neighbour node does not belong to open and to closed
                    if sum(xcurrent+i==open(ycurrent+j==open(:,1),2))==0 && sum(xcurrent+i==closed(ycurrent+j==closed(:,1),2))==0
                                              
                        %Add the neighbour node to open
                        open(size(open,1)+1,:)=[ycurrent+j, xcurrent+i];
                        
                        %Evaluate the distance from start to the current node + from the current node to the neighbour node
                        g_try=G(ycurrent,xcurrent)+sqrt(i^2+j^2);
                        
                        %If this distance is smaller than the neighbour distance
                        if g_try<G(ycurrent+j,xcurrent+i)
                                                                                                         
                            %We are on the good path, save information
                            
                            %Record from which node the good neighbour is reached
                            came_fromy(ycurrent+j,xcurrent+i)=ycurrent;
                            came_fromx(ycurrent+j,xcurrent+i)=xcurrent;
                            
                            %Evaluate the cost function
                            G(ycurrent+j,xcurrent+i)=g_try;
                            H(ycurrent+j,xcurrent+i)=sqrt((xend-(xcurrent+i))^2+(yend-(ycurrent+j))^2);
                            % H(ycurrent+j,xcurrent+i)=octile_distance(xcurrent+i,ycurrent+j,xend,yend);
                            F(ycurrent+j,xcurrent+i)=kg*G(ycurrent+j,xcurrent+i)+kh*H(ycurrent+j,xcurrent+i)+ke*E(ycurrent+j,xcurrent+i);
                            
                        end
                    end
                end
            end            
        end              
    end    
end


%Reconstruct path backwards knowing from which node the good neighbour is reached

%First element is the arrival point
path_backwards=[ycurrent,xcurrent];

%Initialize
i=2;

%While the start point is not reached
while xcurrent~=x0 || ycurrent~=y0
    
    path_backwards(i,:)=[came_fromy(ycurrent,xcurrent) came_fromx(ycurrent,xcurrent)];
    ycurrent=path_backwards(i,1);
    xcurrent=path_backwards(i,2);    
    i=i+1;
    
end
clear i

t_path=toc


%Number of waypoints
n_points=size(path_backwards,1);

%Initialize
path=zeros(n_points,2);
path_distance=zeros(n_points,1);
theta=zeros(n_points-1,1);

for i=1:n_points
    
    %Reverse path sequence
    path(i,:)=path_backwards(n_points+1-i,:);
    
    if i>=2
        
        path_distance(i)=path_distance(i-1)+sqrt((path(i,2)-path(i-1,2))^2+(path(i,1)-path(i-1,1))^2);
        theta(i-1)=atan2(path(i,1)-path(i-1,1),path(i,2)-path(i-1,2));
        
    end
    
end
 
%Angle variations
d_theta=diff(theta);
for i=1:size(d_theta,1)
    if d_theta(i)<-pi
        d_theta(i)=d_theta(i)+2*pi;
    elseif d_theta(i)>pi
        d_theta(i)=d_theta(i)-2*pi;
    end
end

%Cumulative angle variations
d_theta_sum=sum(abs(d_theta));

%Segment length
distance_segment=diff(path_distance);

%Distance of each segment with respect to all path [%]
fraction_segment_perc=distance_segment/path_distance(end)*100;

%Ratio between path lenght and direct line distance start-end
distance_ratio=path_distance(end)/line_distance;



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Plot

%Grid side vectors
x=1:d_grid:x_size;
y=1:d_grid:y_size;

%Path elevation
h_vect=h*ones(size(path,1),1);
%Open elevation
h_vect_open=h*ones(size(open,1),1);
%Closed elevation
h_vect_closed=h*ones(size(closed,1),1);


figure(1)
surf(x,y,E)
hold on
plot3(x0,y0,h,'go')
plot3(xend,yend,h,'ro')    
plot3(path(:,2),path(:,1),h_vect,'c+')
plot3(path(:,2),path(:,1),h_vect,'g')
axis tight
view(0, 90);
colorbar

figure(2)
contourf(x,y,E,20)
hold on
plot3(x0,y0,h,'go')
plot3(xend,yend,h,'ro')    
plot3(path(:,2),path(:,1),h_vect,'c+')
plot3(path(:,2),path(:,1),h_vect,'g')
axis tight
colorbar

figure(3)
surf(x,y,E)
hold on
plot3(x0,y0,h,'go')
plot3(xend,yend,h,'ro')  
plot3(open(:,2),open(:,1),h_vect_open,'mx')
plot3(closed(:,2),closed(:,1),h_vect_closed,'cx')
axis tight
view(0, 90);
colorbar

figure(4)
plot(path_distance)
hold on
grid on
plot(line_distance*ones(size(path_distance,1),1))
legend('Path distance','Line distance')

[John D'Errico]

Contact the author. Wanting an explanation of a lengthy code that does something we are not even told is virtually impossible. That would mean we would need to write a complete MATLAB manual for you. And the nice thing is? The complete set of documentiion is already written. READ THE MANUAL. Learn MATLAB.

If you have a specfific question, about a specific line? Ask it.

[Walter Roberson]

The initial comment says it is the A* algorithm. Read about it https://en.wikipedia.org/wiki/A*_search_algorithm