在Erlang中重叠2D形状

Question

Hello following my question before i have done my research and came up with this,

Basically i am trying to find whether a circle,circle...rectangle,rectangle or circle,rectangle overlap

Mathematically i have my 3 cases as pseudocoded algorithms,what i need help with is to implement this in erlang but i will only ask for circle,circle since i can just figure out the rest syntaxically.

perim({circle, {X,Y}, R}) -> 
math:pi()*(R * 2);
perim({rectangle, {X,Y}, H, W})-> 
  (H + W) * 2.

overlap({_,{_,_},_,_},{_,{_,_},_,_}) ->
overlapping({_,{_,_},_,_},{_,{_,_},_,_}).


%overlap cases

%Circle,Circle
overlapping({circle, {X,Y}, R},{circle2, {X2,Y2}, R2}) ->
   circle1 = {{X,Y},R},
   circle2 = {{X2,Y2},R2},
   case (math:sqrt(abs(X)-abs(X2)) + math:sqrt(abs(Y)-abs(Y2))) < math:sqrt(abs(R)+abs(R2)) of
           true -> true;
           false -> false
    end.    

Thanks in advance

Answer 1

Two things:

• Since you're representing circles as tuples of the form {circle, {X,Y}, R} , both arguments to the overlapping function should say circle . If you put circle2 for the second argument, it won't match your circle tuples, and you'll get a function_clause error.

• Since you're already matching out the variables X , Y , R , X2 , Y2 and R2 in the function head, there is no need for the two lines below that. (In fact, those will always fail, since any word starting with a lower-case letter is an atom, and you're trying to match an atom against a tuple: that will give you a badmatch error.)

So that would be:

overlapping({circle, {X,Y}, R},{circle, {X2,Y2}, R2}) ->
   case (math:sqrt(abs(X)-abs(X2)) + math:sqrt(abs(Y)-abs(Y2))) < math:sqrt(abs(R)+abs(R2)) of
           true -> true;
           false -> false
    end.    

Also, since all that case is doing is checking for true or false and returning the same value, you can simplify your code by removing it:

overlapping({circle, {X,Y}, R},{circle, {X2,Y2}, R2}) ->
   (math:sqrt(abs(X)-abs(X2)) + math:sqrt(abs(Y)-abs(Y2))) < math:sqrt(abs(R)+abs(R2)).    

---

To learn more about pattern matching, you might find this chapter from "Learn You Some Erlang" useful.

Answer 2

Can't properly format your code snippet in the comment. Try this (with ';' after each case statement):

overlap({{circle,{X,Y},R},{rectangle,{X2,Y2},H,W}}) ->
   X_Axis = X - X2,
   Y_Axis = Y - Y2,
      case abs(X_Axis) >= (0.5*abs(W) + R) orelse abs(Y_Axis) >= (0.5*abs(H) + R) of
        true -> ((X == X2) andalso (Y-Y2));
        true -> ((abs(X_Axis) < 0.5*W) andalso (abs(Y_Axis) < (0.5*H + R)));
        true -> ((abs(X_Axis) < 0.5*W + R) andalso (abs(Y_Axis) < 0.5*H));
        false -> false  
      end.