Difference between revisions of "Dynamic array"
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array of char | array of char | ||
</syntaxhighlight> | </syntaxhighlight> | ||
− | Note | + | 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 | + | In order to define a multidimensional array, an array itself is specified as the base type, thus creating an 'array of arrays': |
<syntaxhighlight lang="pascal"> | <syntaxhighlight lang="pascal"> | ||
array of array of longInt | array of array of longInt |
Revision as of 18:06, 10 March 2020
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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 2018 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, or to change the first element from being specified by an index of 0
.
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.
1program setLengthDemo(input, output, stdErr);
2var
3 sieve: array of longWord;
4begin
5 setLength(sieve, 1337);
6end.
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.
Multidimensional arrays can be resized with setLength
, too.
1program multidimensionalSetLengthDemo(input, output, stdErr);
2var
3 samples: array of array of smallInt;
4begin
5 setLength(samples, 12, 64);
6end.
Valid indices for samples
' first dimension are in 0..11
, while valid indices for its second dimension are within the range 0..63
.
One quite useful fact is, the limitation all dimensions have to be of the same size does not apply to dynamic arrays.
1program binomialPotence(input, output, stdErr);
2var
3 pascalsTriangle: array of array of longWord;
4 exponent: longInt;
5 factor: longInt;
6begin
7 setLength(pascalsTriangle, 20);
8
9 setLength(pascalsTriangle[0], 1);
10 pascalsTriangle[0][0] := 1;
11
12 setLength(pascalsTriangle[1], 2);
13 pascalsTriangle[1][0] := 1;
14 pascalsTriangle[1][1] := 1;
15
16 // construct values by simple addition
17 for exponent := 2 to high(pascalsTriangle) do
18 begin
19 setLength(pascalsTriangle[exponent], exponent + 1);
20 pascalsTriangle[exponent][0] := 1;
21 pascalsTriangle[exponent][exponent] := 1;
22 for factor := 1 to exponent - 1 do
23 begin
24 pascalsTriangle[exponent][factor] :=
25 pascalsTriangle[exponent - 1][factor - 1] +
26 pascalsTriangle[exponent - 1][factor];
27 end;
28 end;
29
30 // ...
initializing
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:
1program dynamicArrayCreateDemo(input, output, stdErr);
2type
3 truths = array of boolean;
4var
5 l: truths;
6begin
7 l := truths.create(false, true, true, false, true, false, false);
8end.
Of course you can nest arrays as well:
1program nestedDynamicArrayCreateDemo(input, output, stdErr);
2type
3 truths = array of boolean;
4 pattern = array of truths;
5var
6 p: pattern;
7begin
8 p := pattern.create(
9 truths.create(false, false),
10 truths.create(true, false),
11 truths.create(true, false, false),
12 truths.create(true, true, false)
13 );
14end.
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
.
1program dynamicArrayCopyDemo(input, output, stdErr);
2
3var
4 foo, bar: array of char;
5
6procedure printArrays;
7begin
8 writeLn('foo[0] = ', foo[0], '; bar[0] = ', bar[0]);
9end;
10
11begin
12 setLength(foo, 1);
13 foo[0] := 'X';
14 // copies _reference_
15 bar := foo;
16 write(' initial values: ');
17 printArrays;
18
19 // change content _via_ _second_ reference
20 bar[0] := 'O';
21 write('changed via 2nd ref: ');
22 printArrays;
23
24 // copy content
25 bar := copy(foo, 0, length(foo));
26 bar[0] := 'X';
27 write(' copied and changed: ');
28 printArrays;
29end.
Only by using copy
both arrays can be modified independently.
As stated above, setLength
copies data.
The highlighted line in the example above is (semantically) equivalent to setLength(bar, length(bar))
.
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.
1program dynamicArrayNilDemo(input, output, stdErr);
2var
3 foo, bar: array of char;
4begin
5 setLength(foo, 1);
6 foo[0] := 'X';
7 // copy _reference_, increase reference count
8 bar := foo;
9 // foo becomes nil, reference count is decreased
10 setLength(foo, 0);
11 writeLn('length(foo) = ', length(foo),
12 '; length(bar) = ', length(bar));
13
14 // decrease reference count another time
15 bar := nil;
16 writeLn('length(foo) = ', length(foo),
17 '; length(bar) = ', length(bar));
18end.
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
andobjPas.tBoundArray