Dynamic array: Difference between revisions
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A dynamic array is an [[Array|array]] | A dynamic array is an [[Array|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. | The dynamic array type is not the only type providing variable-length arrays, but as of 2018 it is the only one [[FPC]] supports. | ||
Revision as of 08:57, 27 November 2018
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 run-time various routines will ensure convenient usage, but most importantly the syntax how to access array's elements by placing indices in square brackets, is supported by the compiler (implemented as automatic de-referencing 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, any other ordinal type as index, or to change the first element being specified by an index of 1
.
definition
A one-dimensional dynamic array is defined like this:
array of char
Note, how no dimensions' size is specified.
In order to define a multidimensional array, an array itself is specified as the base type.
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.
Multidimensional arrays can be resized with setLength
, too.
program multidimensionalSetLengthDemo(input, output, stdErr);
var
samples: array of array of smallInt;
begin
setLength(samples, 12, 64);
end.
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.
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
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.
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.
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.
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
andobjPas.tBoundArray