Dynamic array

A dynamic array is an array whose dimensions are not known at compile-time. The dynamic array type is not the only type providing variable-length arrays, but as of 2022 it is the only one FPC supports.

usage

concept

A dynamic array’s definition will only allocate space for a pointer. During runtime, various routines will ensure convenient usage but, most importantly, the syntax of accessing an array’s elements by placing indices in square brackets is supported by the compiler, implemented as automatic de-referencing of the pointer.

Dynamic arrays’ indices are always non-negative integers starting at zero for the first element. It is not possible to use an enumerative type or any other ordinal type as an index. The first element is always specified by an index of 0 – this cannot be changed.

definition

A one-dimensional dynamic array is defined like this:

array of char

Note how no dimensions’ size is specified. This is what distinguishes the definition from a normal array.

In order to define a multidimensional array, an array itself is specified as the base type, thus creating an “array of arrays”:

array of array of longInt

sizing

The compiler procedure setLength will change a dynamic array’s length, provided there is enough memory.

program setLengthDemo(input, output, stdErr);
var
    sieve: array of longWord;
begin
    setLength(sieve, 1337);
end.

The procedure allocates memory for as many records of the base type as specified, plus some management data. It then copies all elements of the old incarnation to the new one. New fields that did not exist before are initialized with the default intrinsic.

It is important to remember that, since all elements are copied during setLength, it can become extremely inefficient to use it with large arrays, or inside inner loops.

Multidimensional arrays can be resized with setLength, too, by specifying multiple sizes in the call, like this:

program multidimensionalSetLengthDemo(input, output, stdErr);
var
    samples: array of array of smallInt;
begin
    setLength(samples, 12, 64);
end.

In this example, valid indices for samples’ first dimension are in the range 0..11, while valid indices for its second dimension are within 0..63.

One quite useful fact is that, because multidimensional dynamic arrays are always “arrays of arrays”, the limitation that all dimensions must be of the same size does not apply to them. Different dimensions are implemented as arrays, and can each have their own size. For instance,

program binomialPotence(input, output, stdErr);
var
    pascalsTriangle: array of array of longWord;
    exponent: longInt;
    factor: longInt;
begin
    setLength(pascalsTriangle, 20);
    
    setLength(pascalsTriangle[0], 1);
    pascalsTriangle[0][0] := 1;
    
    setLength(pascalsTriangle[1], 2);
    pascalsTriangle[1][0] := 1;
    pascalsTriangle[1][1] := 1;
    
    // construct values by simple addition
    for exponent := 2 to high(pascalsTriangle) do
    begin
        setLength(pascalsTriangle[exponent], exponent + 1);
        pascalsTriangle[exponent][0] := 1;
        pascalsTriangle[exponent][exponent] := 1;
        for factor := 1 to exponent - 1 do
        begin
            pascalsTriangle[exponent][factor] :=
                pascalsTriangle[exponent - 1][factor - 1] +
                pascalsTriangle[exponent - 1][factor];
        end;
    end;
    
    // ...

initializing

constructor-like syntax

Since FPC 3.0.0 dynamic array types that are not anonymous are automatically equipped with a “constructor” as it might be familiar from object-oriented programming. This lets you unite setLength calls and a series of assignments in one statement:

program dynamicArrayCreateDemo(input, output, stdErr);
type
    truths = array of boolean;
var
    l: truths;
begin
    l := truths.create(false, true, true, false, true, false, false);
end.

Of course you can nest arrays as well:

program nestedDynamicArrayCreateDemo(input, output, stdErr);
type
    truths = array of boolean;
    pattern = array of truths;
var
    p: pattern;
begin
    p := pattern.create(
            truths.create(false, false),
            truths.create(true, false),
            truths.create(true, false, false),
            truths.create(true, true, false)
        );
end.

Non-Constructor like syntax

As announced https://lists.freepascal.org/pipermail/fpc-pascal/2018-May/053892.html its now possible to initialize dynamic array constants and variables directly -

const
   Test1: array of LongInt = (1, 2, 3);

var
    DArray : array of string  = ('one', 'two', 'three');  // A dynamic Array
    IArray : array of integer = (1,2,3);                  // A dynamic array

This is available in at least FPC 3.2.2. A very minor note, the above will not work with the predefined FPC colors such as colRed and colBlue because the TFCColors are Typed Constants. I bet you needed to know that !

dynamic array literals

Since FPC 3.2.0 it is possible to specify dynamic array literals. This works for anonymous data types as well.

program dynamicArrayLiteralDemo(input, output, stdErr);

procedure printArray(const list: array of integer);
var
    i: ALUSInt;
begin
    for i := low(list) to high(list) do
    begin
        writeLn(list[i]);
    end;
end;

begin
    printArray([123, 456, 789]);
end.

Note that the data type of the formal parameter list does not have a name, yet we are able to specify a dynamic array of integer literal as an actual parameter.

Insert, Delete

Dynamic Arrays have their content managed with the Insert and Delete intrinsics.

var
    IArray  : array of integer;                    // A dynamic array
    i : integer;
begin
    iArray := [7];                                 // A very small array
    iArray := [7, 8, 9];                           // A bit bigger
    for i in IArray do
        writeln(i, ' ');                           // prints "7, 8, 9"
    writeln('');
    for i := 0 to 3 do
        insert(i, IArray, 0);                      // four more elements, at the start of array
    for i in IArray do
        write(i, ' ');                             // prints "3 2 1 0 7 8 9"
    delete(IArray, 0, 6);                          // back to a one element array, just "9" 
end;

Used this way, the compiler generates code to increase and decrease the memory allocated for the array, while this is convenient, it may not be best for large data sets or the situations that need the fastest response.

handling

Keep in mind dynamic arrays are pointers. Assigning dynamic array variables to each other does not copy any payload, but just the address. This differs from static arrays’ behavior.

If you want to duplicate data you have to use system.copy.

program dynamicArrayCopyDemo(input, output, stdErr);

var
    foo, bar: array of char;

procedure printArrays;
begin
    writeLn('foo[0] = ', foo[0], '; bar[0] = ', bar[0]);
end;

begin
    setLength(foo, 1);
    foo[0] := 'X';
    // copies _reference_
    bar := foo;
    write('     initial values: ');
    printArrays;
    
    // change content _via_ _second_ reference
    bar[0] := 'O';
    write('changed via 2nd ref: ');
    printArrays;
    
    // copy content
    bar := copy(foo, 0, length(foo));
    bar[0] := 'X';
    write(' copied and changed: ');
    printArrays;
end.

Only by using copy both arrays can be modified independently.

As stated above, setLength copies data. This procedure can be used to make data unique again, despite the fact only the references, the pointers (so just addresses) have been copied (bar := foo). The highlighted line in the example above could be replaced by setLength(bar, length(bar)), though the copy function is preferred as it is more explanatory.

Dynamic arrays are reference counted. Calling setLength(myDynamicArrayVariable, 0) virtually does myDynamicArrayVariable := nil and decreases the reference count. Only when the reference count hits zero, the memory block is released.

program dynamicArrayNilDemo(input, output, stdErr);
var
    foo, bar: array of char;
begin
    setLength(foo, 1);
    foo[0] := 'X';
    // copy _reference_, increase reference count
    bar := foo;
    // foo becomes nil, reference count is decreased
    setLength(foo, 0);
    writeLn('length(foo) = ', length(foo),
        '; length(bar) = ', length(bar));
    
    // decrease reference count another time
    bar := nil;
    writeLn('length(foo) = ', length(foo),
        '; length(bar) = ', length(bar));
end.

Nonetheless, dynamic arrays are finalized automatically. It is not necessary to manually setLength(…, 0) on all your references when the program comes to end, or when leaving a scope in general.

Without {$rangeChecks on} it is possible to reach beyond an array’s limits. That means when iterating over dynamic arrays, it is impossible to work without low and high to determine valid indices (the former being optional, since dynamic arrays always start at zero). Alternatively, for … in loops can be used, if no index is required.

Remember, sizeOf of a dynamic array evaluates to the size of a pointer.

application

• there is system.tBoundArray and objPas.tBoundArray

see also

• string
• example: multidimensional dynamic array
• Free Pascal Reference guide: “Dynamic arrays”

From the Free Pascal wiki, page "Dynamic array", as it stood on 2022-12-14 after a change by Dbannon, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.