How do I update an element in a this vector? - ada

Question

I am trying to update an element in a vector. I have quite a few issues that I don't know how to solve. When I use Replace_Element, everything works, but I want to use the proper procedure.
This is my code:
with Ada.Containers.Vectors;
procedure Test_Update is
type Node is record
Parent : Integer := -1;
Size : Integer := -1;
end record;
function TestUpdate(n : Node; val : Integer) return Node is
begin
n.Size := n.Size + val;
return n;
end TestUpdate;
package NodeVector is new
Ada.Containers.Vectors
(
Index_Type => Natural,
Element_Type => Node
);
Nodes : NodeVector.vector;
Current_Node : Node;
begin
Current_Node.Size := 10;
Nodes.Append(Current_Node);
NodeVector.Update_Element(Nodes, 0, TestUpdate'Access(5));
--NodeVector.Update_Element(Nodes, 0, TestUpdate'Access);
end Test_Update;
These are the errors that I am getting, and I know what is causing them, but no idea how to fix them:
test_update.adb:11:09: error: assignment to "in" mode parameter not allowed
test_update.adb:28:59: error: unexpected argument for "Access" attribute
When I use the code in the comments, and remove the second parameter for the function, it still doesn't work.

Answer 1

You have a lot of things mixed up here. First if you look at Zerte's answer more carefully than before, you'll notice that your TestUpdate operation doesn't match that Process argument at all. It expects a procedure (you have a function) and the procedure's parameter is "in out" while you are using "in" and you have an additional parameter. You need to fix this first
If you want to use Update_Element (I don't recommend it for your specific case) then you need to look into "Nested Subprograms". Here is an example of how to change your TestUpdate operation to work with Update_Element:
procedure TestUpdate(V : in out NodeVector.Vector; val : Integer) is
procedure Actual_Update(N : in out Node) is
begin
n.Size := n.Size + val;
end Actual_Update;
begin
V.Update_Element(V.Last,Actual_Update'Access);
end TestUpdate;
Notice how the Actual_Update procedure actually matches the process argument of the Update_Element operation contract. Additionally, since Actual_Update is nested, it has access to the Val argument from TestUpdate.
Full example:
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Containers.Vectors;
procedure Test_Update is
type Node is record
Parent : Integer := -1;
Size : Integer := -1;
end record;
package NodeVector is new
Ada.Containers.Vectors
(
Index_Type => Natural,
Element_Type => Node
);
Nodes : NodeVector.vector;
Current_Node : Node;
procedure TestUpdate(V : in out NodeVector.Vector; val : Integer) is
procedure Actual_Update(N : in out Node) is
begin
n.Size := n.Size + val;
end Actual_Update;
begin
V.Update_Element(V.Last,Actual_Update'Access);
end TestUpdate;
begin
Current_Node.Size := 10;
Nodes.Append(Current_Node);
TestUpdate(Nodes, 5);
end Test_Update;
With Vectors, you can also just index things directly. After you append an item, you can use <vector_name>.Last to get the cursor (index) for the last element...the one you just added. Example:
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Containers.Vectors;
procedure Test_Update is
type Node is record
Parent : Integer := -1;
Size : Integer := -1;
end record;
package NodeVector is new
Ada.Containers.Vectors
(
Index_Type => Natural,
Element_Type => Node
);
Nodes : NodeVector.vector;
Current_Node : Node;
procedure Using_Cursors
(V : in out NodeVector.Vector;
N : Node;
Value : Integer)
is begin
Nodes.Append(N);
Nodes(Nodes.Last).Size := Nodes(Nodes.Last).Size + Value;
end Using_Cursors;
begin
Current_Node.Size := 10;
Using_Cursors(Nodes,Current_Node,5);
end Test_Update;

Answer 2

A quick Web search will lead you to...
A.18.2 The Package Containers.Vectors
(or, if you use GNAT Studio: right-click on "Vectors" in "Ada.Containers.Vectors", then choose Go To Declaration).
Then you will find this:
procedure Update_Element
(Container : in out Vector;
Index : in Index_Type;
Process : not null access procedure
(Element : in out Element_Type));