Dynamic array

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Revision as of 11:36, 17 March 2014 by HowardPC (talk | contribs)

The dynamic array type is a very nice feature of FreePascal (and Delphi) syntax. It is very similar to the array type, but allows more flexibility for the programmer since the number of elements does not need to be known until program execution.

The declaration part is just as simple as for the array type:

  MyVariable : array of type;

The number of elements can be set or modified whenever needed during the execution of the program by inserting the statement:

  SetLength(MyVariable, ItsNewLength);

You can put as many SetLength statements as you want in your program in order to expand or truncate an array but you must put at least one statement before you can use the variable for the first time.

The individual elements can be accessed as follows:

MyVariable[18] := 123;
MyOtherVariable := MyVariable[0];
WriteLn('MyVariable has ', Length(MyVariable), ' elements');  {should be 19}
WriteLn('Its range is ', Low(MyVariable), ' to ', High(MyVariable)); {should be 0 to 18}

The index of a dynamic array is ZERO based, i.e. it must be within the range from 0 to (Length-1). It is NOT possible to change this to a ONE based system.

Actually, dynamic arrays are pointers with automatic dereferencing. They are initialized to nil automatically. This means, that MyVariable is interpreted as a pointer variable when handed over to low level routines like fillchar, sizeof, etc. but it is automatically expanded to MyVariable^ when indexing its elements (as in MyVariable[2]) or when handing it over to routines that expect array types.

From a memory management view, dynamic array variables are simple pointers. SetLength allocates and frees memory on the heap as needed. When used in functions or procedures only the pointer is added to the stack. When the procedure exits, the dynamic array variable is removed and the memory is made available again. In fact, the memory management procedures are inserted in the executable program and the result is totally transparent to the programmer.

Assigning nil to a dynamic array variable automatically frees the memory where the pointer pointed to. It's identical to SetLength(MyVariable, 0). This can have a side effect, if the pointer value is not valid for some reason (i.e., if it was read from disk where it was stored from previous program runs). To init such an invalid pointer you have to use FillChar(MyVariable,sizeof(MyVariable), #0).

Although writing to elements of dynamic arrays does not create a new instance of the array (no copy-on-write as it exists for Ansistrings) using SetLength on such arrays does create a copy! So if 2 dynamic array variables point to the same array (one has been assigned to the other) they do not do so after using SetLength on one (or both) of them. After the SetLength() call the two variables are distinct arrays each whose elements are independent from the other array.

See also: