Bowling pins:
Printing ----I---- is just: (I'm going to use dashes instead of spaces throughout for readability)
Put_Line (4 * "-" & "I" & 4 * "-");
And printing the whole bowling triangle could be:
with Ada.Strings.Fixed; use Ada.Strings.Fixed;
with Ada.Text_IO; use Ada.Text_IO;
procedure Main is
procedure Print_Bowling_Line (Dashes : Natural; Pins : Positive) is
Middle : constant String := (if Pins = 1 then "I"
else (Pins - 1) * "I-" & "I");
begin
Put_Line (Dashes * "-" & Middle & Dashes * "-");
end Print_Bowling_Line;
begin
Print_Bowling_Line (0, 5);
Print_Bowling_Line (1, 4);
Print_Bowling_Line (2, 3);
Print_Bowling_Line (3, 2);
Print_Bowling_Line (4, 1);
end Main;
Writing the five repeated lines as a loop is quite obvious. For recursion, there are two ways.
Tail recursion
Tail recursion is the more natural 'ask questions, then shoot' approach; first check the parameter for an end-condition, if not, do some things and finally call self.
procedure Tail_Recurse (Pins : Natural) is
begin
if Pins = 0 then
return;
end if;
Print_Bowling_Line (Total_Pins - Pins, Pins);
Tail_Recurse (Pins - 1);
end Tail_Recurse;
Head recursion
Head recursion is what mathematicians love; how do you construct a proof for N? Well, assuming you already have a proof for N-1, you just apply X and you're done. Again, we need to check the end condition before we go looking for a proof for N-1, otherwise we would endlessly recurse and get a stack overflow.
procedure Head_Recurse (Pins : Natural) is
begin
if Pins < Total_Pins then
Head_Recurse (Pins + 1); -- assuming N + 1 proof here
end if;
Print_Bowling_Line (Total_Pins - Pins, Pins);
end Head_Recurse;
For the full code, expand the following snippet:
with Ada.Strings.Fixed; use Ada.Strings.Fixed;
with Ada.Text_IO; use Ada.Text_IO;
procedure Main is
Total_Pins : Natural := 5;
procedure Print_Bowling_Line (Dashes : Natural; Pins : Positive) is
Middle : constant String := (if Pins = 1 then "I"
else (Pins - 1) * "I-" & "I");
begin
Put_Line (Dashes * "-" & Middle & Dashes * "-");
end Print_Bowling_Line;
procedure Tail_Recurse (Pins : Natural) is
begin
if Pins = 0 then
return;
end if;
Print_Bowling_Line (Total_Pins - Pins, Pins);
Tail_Recurse (Pins - 1);
end Tail_Recurse;
procedure Head_Recurse (Pins : Natural) is
begin
if Pins < Total_Pins then
Head_Recurse (Pins + 1); -- assuming N + 1 proof here
end if;
Print_Bowling_Line (Total_Pins - Pins, Pins);
end Head_Recurse;
begin
Total_Pins := 8;
Head_Recurse (1);
end Main;
For simplicity, I don't pass around the second number, that indicates the stopping condition, but rather set it once before running the whole.
I always find it unfortunate to try to learn a technique by applying it where it makes the code more complicated, rather than less. So I want to show you a problem where recursion really does shine. Write a program that prints a maze with exactly one path between every two points in the maze, using the following depth-first-search pseudo code:
start by 'visiting' any field (2,2 in this example)
(recursion starts with this:)
while there are any neighbours that are unvisited,
pick one at random and
connect the current field to that field and
run this procedure for the new field
As you can see in the animation below, this should meander randomly until it gets 'stuck' in the bottom left, after which it backtracks to a node that still has an unvisited neighbour. Finally, when everything is filled, all the function calls that are still active will return because for each node there will be no neighbours left to connect to.
![Maze generation using Depth First Search (backtracking)]()
You can use the skeleton code in the snippet below. The answer should only modify the Depth_First_Make_Maze procedure. It need be no longer than 15 lines, calling Get_Unvisited_Neighbours, Is_Empty on the result, Get_Random_Neighbour and Connect (and itself, of course).
with Ada.Strings.Fixed; use Ada.Strings.Fixed;
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Containers; use Ada.Containers;
with Ada.Containers.Vectors;
with Ada.Numerics.Discrete_Random;
procedure Main is
N : Positive := 11; -- should be X*2 + 1 for some X >= 1
type Cell_Status is (Filled, Empty);
Maze : array (1 .. N, 1 .. N) of Cell_Status := (others => (others => Filled));
procedure Print_Maze is
begin
for Y in 1 .. N loop
for X in 1 .. N loop
declare
C : String := (case Maze (X, Y) is
--when Filled => "X", -- for legibility,
when Filled => "█", -- unicode block 0x2588 for fun
when Empty => " ");
begin
Put (C);
end;
end loop;
Put_Line ("");
end loop;
end Print_Maze;
type Cell_Address is record
X : Positive;
Y : Positive;
end record;
procedure Empty (Address : Cell_Address) is
begin
Maze (Address.X, Address.Y) := Empty;
end Empty;
procedure Connect (Address1 : Cell_Address; Address2 : Cell_Address) is
Middle_X : Positive := (Address1.X + Address2.X) / 2;
Middle_Y : Positive := (Address1.Y + Address2.Y) / 2;
begin
Empty (Address1);
Empty (Address2);
Empty ((Middle_X, Middle_Y));
end Connect;
function Cell_At (Address : Cell_Address) return Cell_Status is (Maze (Address.X, Address.Y));
function Left (Address : Cell_Address) return Cell_Address is (Address.X - 2, Address.Y);
function Right (Address : Cell_Address) return Cell_Address is (Address.X + 2, Address.Y);
function Up (Address : Cell_Address) return Cell_Address is (Address.X, Address.Y - 2);
function Down (Address : Cell_Address) return Cell_Address is (Address.X, Address.Y + 2);
type Neighbour_Count is new Integer range 0 .. 4;
package Neighbours_Package is new Ada.Containers.Vectors (Index_Type => Natural, Element_Type => Cell_Address);
use Neighbours_Package;
function Get_Unvisited_Neighbours (Address : Cell_Address) return Neighbours_Package.Vector is
NeighbourList : Neighbours_Package.Vector;
begin
NeighbourList.Reserve_Capacity (4);
if Address.X >= 4 then
declare
L : Cell_Address := Left (Address);
begin
if Cell_At (L) = Filled then
NeighbourList.Append (L);
end if;
end;
end if;
if Address.Y >= 4 then
declare
U : Cell_Address := Up (Address);
begin
if Cell_At (U) = Filled then
NeighbourList.Append (U);
end if;
end;
end if;
if Address.X <= (N - 3) then
declare
R : Cell_Address := Right (Address);
begin
if Cell_At (R) = Filled then
NeighbourList.Append (R);
end if;
end;
end if;
if Address.Y <= (N - 3) then
declare
D : Cell_Address := Down (Address);
begin
if Cell_At (D) = Filled then
NeighbourList.Append (D);
end if;
end;
end if;
return NeighbourList;
end Get_Unvisited_Neighbours;
package Rnd is new Ada.Numerics.Discrete_Random (Natural);
Gen : Rnd.Generator;
function Get_Random_Neighbour (List : Neighbours_Package.Vector) return Cell_Address is
Random : Natural := Rnd.Random (Gen);
begin
if Is_Empty (List) then
raise Program_Error with "Cannot select any item from an empty list";
end if;
declare
Index : Natural := Random mod Natural (List.Length);
begin
return List (Index);
end;
end Get_Random_Neighbour;
procedure Depth_First_Make_Maze (Address : Cell_Address) is
begin
null;
end Depth_First_Make_Maze;
begin
Rnd.Reset (Gen);
Maze (1, 2) := Empty; -- entrance
Maze (N, N - 1) := Empty; -- exit
Depth_First_Make_Maze ((2, 2));
Print_Maze;
end Main;
To see the answer, expand the following snippet.
procedure Depth_First_Make_Maze (Address : Cell_Address) is
begin
loop
declare
Neighbours : Neighbours_Package.Vector := Get_Unvisited_Neighbours (Address);
begin
exit when Is_Empty (Neighbours);
declare
Next_Node : Cell_Address := Get_Random_Neighbour (Neighbours);
begin
Connect (Address, Next_Node);
Depth_First_Make_Maze (Next_Node);
end;
end;
end loop;
end Depth_First_Make_Maze;
Converting recursion to loop
Consider how a function call works; we take the actual parameters and put them on the call stack along with the function's address. When the function completes, we take those values off the stack again and put back the return value.
We can convert a recursive function by replacing the implicit callstack containing the parameters with an explicit stack. I.e. instead of:
procedure Foo (I : Integer) is
begin
Foo (I + 1);
end Foo;
We would put I onto the stack, and as long as there are values on the stack, peek at the top value and do the body of the Foo procedure using that value. When there is a call to Foo within that body, push the value you would call the procedure with instead, and restart the loop so that we immediately start processing the new value. If there is no call to self in this case, we discard the top value on the stack.
Restructuring a recursive procedure in this way will give you an insight into how it works, especially since pushing on to the stack is now separated from 'calling' that function since you explicitly take an item from the stack and do something with it.
You will need a stack implementation, here is one that is suitable:
Bounded_Stack.ads
generic
max_stack_size : Natural;
type Element_Type is private;
package Bounded_Stack is
type Stack is private;
function Create return Stack;
procedure Push (Onto : in out Stack; Item : Element_Type);
function Pop (From : in out Stack) return Element_Type;
function Top (From : in out Stack) return Element_Type;
procedure Discard (From : in out Stack);
function Is_Empty (S : in Stack) return Boolean;
Stack_Empty_Error : exception;
Stack_Full_Error : exception;
private
type Element_List is array (1 .. max_stack_size) of Element_Type;
type Stack is
record
list : Element_List;
top_index : Natural := 0;
end record;
end Bounded_Stack;
Bounded_Stack.adb
package body Bounded_Stack is
function Create return Stack is
begin
return (top_index => 0, list => <>);
end Create;
procedure Push (Onto : in out Stack; Item : Element_Type) is
begin
if Onto.top_index = max_stack_size then
raise Stack_Full_Error;
end if;
Onto.top_index := Onto.top_index + 1;
Onto.list (Onto.top_index) := Item;
end Push;
function Pop (From : in out Stack) return Element_Type is
Top_Value : Element_Type := Top (From);
begin
From.top_index := From.top_index - 1;
return Top_Value;
end Pop;
function Top (From : in out Stack) return Element_Type is
begin
if From.top_index = 0 then
raise Stack_Empty_Error;
end if;
return From.list (From.top_index);
end Top;
procedure Discard (From : in out Stack) is
begin
if From.top_index = 0 then
raise Stack_Empty_Error;
end if;
From.top_index := From.top_index - 1;
end Discard;
function Is_Empty (S : in Stack) return Boolean is (S.top_index = 0);
end Bounded_Stack;
It can be instantiated with a maximum stack size of Width*Height, since the worst case scenario is when you happen to choose a non-forking path that visits each cell once:
N_As_Cell_Size : Natural := (N - 1) / 2;
package Cell_Stack is new Bounded_Stack(max_stack_size => N_As_Cell_Size * N_As_Cell_Size, Element_Type => Cell_Address);
Take your answer to the previous assignment, and rewrite it without recursion, using the stack above instead.
To see the answer, expand the following snippet.
procedure Depth_First_Make_Maze (Address : Cell_Address) is
Stack : Cell_Stack.Stack := Cell_Stack.Create;
use Cell_Stack;
begin
Push (Stack, Address);
loop
exit when Is_Empty (Stack);
declare
-- this shadows the parameter, which we shouldn't refer to directly anyway
Address : Cell_Address := Top (Stack);
Neighbours : Neighbours_Package.Vector := Get_Unvisited_Neighbours (Address);
begin
if Is_Empty (Neighbours) then
Discard (Stack); -- equivalent to returning from the function in the recursive version
else
declare
Next_Cell : Cell_Address := Get_Random_Neighbour (Neighbours);
begin
Connect (Address, Next_Cell);
Push (Stack, Next_Cell); -- equivalent to calling self in the recursive version
end;
end if;
end;
end loop;
end Depth_First_Make_Maze;
