Binary tree from unsorted array - data-structures

Question

I have an array of records from which I am trying to construct a binary tree.
The end result should be as follow. -1 represents null and if both the left and right pointer are -1, it is a leaf node.
| Index | Left Pointer | Data | Right Pointer |
|:-----:|:------------:|:----:|:-------------:|
| 0 | 1 | 17 | 4 |
| 1 | 2 | 8 | 3 |
| 2 | -1 | 4 | 7 |
| 3 | -1 | 12 | 6 |
| 4 | 5 | 22 | 8 |
| 5 | -1 | 19 | -1 |
| 6 | -1 | 14 | -1 |
| 7 | -1 | 5 | -1 |
| 8 | 9 | 30 | -1 |
| 9 | -1 | 25 | -1 |
This is my current code:
type node = record
data : string;
left : integer;
right : integer;
end;
var
binaryTree : array[0..9] of node;
procedure output;
var
i : integer;
begin
for i := 0 to 9 do
begin
writeln('Data: ' , binaryTree[i].data);
writeln('Left: ' , binaryTree[i].left);
writeln('Right: ' , binaryTree[i].right);
end;
end;
procedure takeInput;
var
i : integer;
begin
for i := 0 to 9 do
readln(binaryTree[i].data);
end;
procedure initialisePointers;
var
i : integer;
begin
// initialise to -1 for null
for i := 0 to 9 do
begin
binaryTree[i].left := -1;
binaryTree[i].right := -1;
end;
end;
procedure setPointers;
var
i : integer;
root, currentNode : node;
begin
// first value becomes root
root := binaryTree[0];
for i := 1 to 9 do
begin
currentNode := root;
while true do
begin
if binaryTree[i].data < currentNode.data then
begin
if currentNode.left = -1 then
begin
currentNode.left := i;
break;
end
else
currentNode := binaryTree[currentNode.left]
end
else
begin
if binaryTree[i].data >= currentNode.data then
begin
if currentNode.right = -1 then
begin
currentNode.right := i;
break;
end
else
currentNode := binaryTree[currentNode.right]
end;
end;
end;
end;
end;
begin
takeInput;
initialisePointers;
setPointers;
output;
end.
With the input shown in the table, I get an output of all pointers remaining at -1 as initialised. Any idea why this may be?

Answer 1

In Pascal, records are value types, so when you do e.g.
root := binaryTree[0];
then root is a copy of binaryTree[0].
When you set fields in root you are setting fields in the copy.
If you want chose changes reflected back in the binaryTree array, you have to assign the value back after use:
binaryTree[0] := root;
Or else, use pointers, but I am not sure you have learned about those yet.