Ada 2012 Multiway Tree, Create Root Node

Question

I'm using the bounded version of the Multiway Tree. I can create my element type and instantiate a tree of my type but how do I create the root? I see several forms of Insert_Child. All my attempts at using Insert_Child fail because I use No_Element for the Parent or Before argument.

Does anyone have an example of using the multiway tree package they can point me to?

I'm using the free Gnat version of Ada if it matters.

Answer 1

Create a variable of type Tree; there's a hint in the ARM,

There is a special node, the root, which is always present and has neither an associated element value nor any parent node. The root node provides a place to add nodes to an otherwise empty tree and represents the base of the tree.

Something like

with Ada.Containers.Multiway_Trees;
with Ada.Text_IO; use Ada.Text_IO;
procedure Mway is
   package Trees is new Ada.Containers.Multiway_Trees (Integer);
   T : Trees.Tree;
   C : Trees.Cursor;
begin
   C := T.Root;
   T.Append_Child (Parent => C, New_Item => 1);
   T.Append_Child (Parent => C, New_Item => 2);
   for E of T loop
      Put_Line (E'Img);
   end loop;
end Mway;

the output of which is

1
2

Answer 2

Thank you for the answer but your description of the root not having an element may cause me to change my design. I anticipated building a tree of components made of subcomponents and so forth. I wanted to roll up the property, say mass or price, of each element into the total for the assembly and store it in the root.

I guess I can have the first node I create be the placeholder for the assembly total, then have the real tree branch off of that. Seems clumsy but workable.

/s/ Bob

Answer 3

Interestingly, when I try the example code I get a STORAGE_ERROR at the tree iterator loop.

I'm running Debian under the Windows Subsystem for Linux. WSL1 does not allow you to adjust the stack size via ulimit.

The fix appears to be to turn on the compiler optimisation:

ebolar@Beowulf:~/Build/Ada/src$ gnatmake -s -g simpletree
gcc-6 -c -g simpletree.adb
simpletree.adb:3:11: warning: file name does not match unit name, should be "mway.adb"
gnatbind-6 -x simpletree.ali
gnatlink-6 simpletree.ali -g
ebolar@Beowulf:~/Build/Ada/src$ ./simpletree
Load up the tree
Check out the contents

raised STORAGE_ERROR : stack overflow or erroneous memory access


ebolar@Beowulf:~/Build/Ada/src$ gnatmake -s -g -O simpletree
gcc-6 -c -g -O simpletree.adb
simpletree.adb:3:11: warning: file name does not match unit name, should be "mway.adb"
gnatbind-6 -x simpletree.ali
gnatlink-6 simpletree.ali -g -O
ebolar@Beowulf:~/Build/Ada/src$ ./simpletree
Load up the tree
Check out the contents
 1
 2