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   ► KBTo/From GuidesDelphiData Structures  Print This     

Cross Ref > Data Structures

By Mike Prestwood

Delphi versus Access VBA: A side by side comparison between Delphi and Access VBA.

 
Data Structures
 

Data structures allow you to store and work with data. Common data structures include arrays, associative arrays, etc.

Array

[Other Languages] 

Languages Focus

A data structure in which individual values (called elements or items) may be located by reference to one or more integer index variables, the number of such indices being the number of dimensions in the array.

Arrays can start with an index value of 0 or 1, sometimes referred to as 0 based or 1 based.

Delphi:   x=Array[0..3] of string;

Delphi supports both static and dynamic arrays as well as single and multi dimensional arrays.

Syntax Example:
var
  MyArray: array[0..3] of string;
  i: Integer;
begin
  MyArray[0] := 'Mike';
  MyArray[1] := 'Lisa';
  MyArray[2] := 'Felicia';
  MyArray[3] := 'Nathan';
  
  for i := 0 to High(MyArray) do
    ShowMessage(MyArray[i]);
end;
Access VBA:   x = Array()

Arrays in Access VBA use a 0-based indice. Use UBound to get the number of elements. UBound returns -1 if the array has no elements, 0 if it has 1, 1 if it has 2, etc.

Syntax Example:  
Dim MyArray As Variant
Dim i As Integer
 
MyArray = Array("Mike", "Lisa", "Felicia", "Nathan")
 
If UBound(MyArray) > -1 Then
  For i = 0 To UBound(MyArray)
    MsgBox (MyArray(i))
  Next
End If




Associative Array

[Other Languages] 
A set of unique keys linked to a set of values. Each unique key is associated with a value. Think of it as a two column table. MyArray['CA'] = 'California' MyArray['AR'] = 'Arizona'

Languages Focus

Associative arrays are also known as a dictionary or a hash table in other languages.

Delphi:   TStringList Assoc Array

Object Pascal doesn't have a native associative array, but you can use a TStringList the same way. (Alternatively, search the Internet for TStringHash and THashedStringList classes for implementations of a true associative array).

Syntax Example:
var
  StateList : TStringList;
begin
StateList := TStringList.Create; 
  StateList.CommaText := 'CA=California, FL=Florida';
  ShowMessage('FL is ' + StateList.Values['FL']);
end;
Access VBA:   Collection

In addition to Add and Item, collections also offer Count and Remove. Notice that Add uses the format of Value, Key (which is backwards from many other languages).

Syntax Example:
Dim States As New Collection
   
States.Add "California", "CA"
States.Add "Nevada", "NV"
    
MsgBox (States.Item("CA"))




Pointers

[Other Languages] 

General Info: Pointers / References

A pointer is a variable type that allows you to refer indirectly to another object. Instead of holding data, a pointer holds the address to data -- the address of another variable or object. You can change the address value a pointer points to thus changing the variable or object the pointer is pointing to.

A reference is a type of pointer that cannot change and it must always point to a valid storage (no nulls).

Delphi: 

Although pointer data types in Delphi coding are less important and not required for most general coding, Delphi fully supports developer defined pointers. Use a carrot (^) to declare a pointer data type. Use the @ operator or Addr function to return the current address of a variable.

Delphi provides typed pointer types such as PChar and PExtended as well as a generic point to anything Pointer type.

Nil is a special pointer value that you can assign to any type of pointer. Nil never points to any valid memory and indicates an unassigned or empty pointer.

Syntax Example:
//Declare a pointer of type integer.
PCounter : ^Integer;
  
//Assign a value to the location of a pointer.
//Also known as dereferencing.
PCounter^ := 8;
  
//Assign address of A to B.
PointerB := @PointerA;  //or...PointerB := Addr(PointerA);
Access VBA:   Not Supported

Same as VB Classic. Access VBA does not offer developer defined pointers.






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