[ Pobierz całość w formacie PDF ]
Every set type can hold the empty set, denoted by [].
Arrays
An array represents an indexed collection of elements of the same type (called the base type). Because each element
has a unique index, arrays, unlike sets, can meaningfully contain the same value more than once. Arrays can be
allocated statically or dynamically.
Static Arrays
Static array types are denoted by constructions of the form
array[ indexType1, ..., indexTypen ] of baseType;
where each indexType is an ordinal type whose range does not exceed 2GB. Since the indexTypes index the array,
the number of elements an array can hold is limited by the product of the sizes of the indexTypes. In practice,
indexTypes are usually integer subranges.
In the simplest case of a one-dimensional array, there is only a single indexType. For example,
var MyArray: array [1..100] of Char;
declares a variable called MyArray that holds an array of 100 character values. Given this declaration, MyArray
[3] denotes the third character in MyArray. If you create a static array but don't assign values to all its elements,
the unused elements are still allocated and contain random data; they are like uninitialized variables.
A multidimensional array is an array of arrays. For example,
type TMatrix = array[1..10] of array[1..50] of Real;
is equivalent to
type TMatrix = array[1..10, 1..50] of Real;
Whichever way TMatrix is declared, it represents an array of 500 real values. A variable MyMatrix of type
TMatrix can be indexed like this: MyMatrix[2,45]; or like this: MyMatrix[2][45]. Similarly,
77
packed array[Boolean, 1..10, TShoeSize] of Integer;
is equivalent to
packed array[Boolean] of packed array[1..10] of packed array[TShoeSize] of Integer;
The standard functions Low and High operate on array type identifiers and variables. They return the low and high
bounds of the array's first index type. The standard function Length returns the number of elements in the array's
first dimension.
A one-dimensional, packed, static array of Char values is called a packed string. Packed-string types are compatible
with string types and with other packed-string types that have the same number of elements. See Type compatibility
and identity.
An array type of the form array[0..x] of Char is called a zero-based character array. Zero-based character
arrays are used to store null-terminated strings and are compatible with PChar values. See Working with null-
terminated strings.
Dynamic Arrays
Dynamic arrays do not have a fixed size or length. Instead, memory for a dynamic array is reallocated when you
assign a value to the array or pass it to the SetLength procedure. Dynamic-array types are denoted by constructions
of the form
array of baseType
For example,
var MyFlexibleArray: array of Real;
declares a one-dimensional dynamic array of reals. The declaration does not allocate memory for
MyFlexibleArray. To create the array in memory, call SetLength. For example, given the previous declaration,
SetLength(MyFlexibleArray, 20);
allocates an array of 20 reals, indexed 0 to 19. Dynamic arrays are always integer-indexed, always starting from 0.
Dynamic-array variables are implicitly pointers and are managed by the same reference-counting technique used
for long strings. To deallocate a dynamic array, assign nil to a variable that references the array or pass the variable
to Finalize; either of these methods disposes of the array, provided there are no other references to it. Dynamic
arrays are automatically released when their reference-count drops to zero. Dynamic arrays of length 0 have the
value nil. Do not apply the dereference operator (^) to a dynamic-array variable or pass it to the New or Dispose
procedure.
If X and Y are variables of the same dynamic-array type, X := Y points X to the same array as Y. (There is no need
to allocate memory for X before performing this operation.) Unlike strings and static arrays, copy-on-write is not
employed for dynamic arrays, so they are not automatically copied before they are written to. For example, after this
code executes,
var
A, B: array of Integer;
begin
SetLength(A, 1);
A[0] := 1;
B := A;
78
B[0] := 2;
end;
the value of A[0] is 2. (If A and B were static arrays, A[0] would still be 1.)
Assigning to a dynamic-array index (for example, MyFlexibleArray[2] := 7) does not reallocate the array.
Out-of-range indexes are not reported at compile time.
In contrast, to make an independent copy of a dynamic array, you must use the global Copy function:
var
A, B: array of Integer;
begin
SetLength(A, 1);
A[0] := 1;
B := Copy(A);
B[0] := 2; { B[0] A[0] }
end;
When dynamic-array variables are compared, their references are compared, not their array values. Thus, after
execution of the code
var
A, B: array of Integer;
begin
SetLength(A, 1);
SetLength(B, 1);
A[0] := 2;
B[0] := 2;
end;
A = B returns False but A[0] = B[0] returns True.
To truncate a dynamic array, pass it to SetLength, or pass it to Copy and assign the result back to the array variable.
(The SetLength procedure is usually faster.) For example, if A is a dynamic array, A := SetLength(A, 0,
20) truncates all but the first 20 elements of A.
Once a dynamic array has been allocated, you can pass it to the standard functions Length, High, and Low.
Length returns the number of elements in the array, High returns the array's highest index (that is, Length -
1), and Low returns 0. In the case of a zero-length array, High returns 1 (with the anomalous consequence that
High
Note: In some function and procedure declarations, array parameters are represented as array of baseType,
without any index types specified. For example,function CheckStrings(A: array of string):
Boolean;
This indicates that the function operates on all arrays of the specified base type, regardless of their size, how they
are indexed, or whether they are allocated statically or dynamically.
Multidimensional Dynamic Arrays
To declare multidimensional dynamic arrays, use iterated array of ... constructions. For example,
79
type TMessageGrid = array of array of string;
var Msgs: TMessageGrid;
declares a two-dimensional array of strings. To instantiate this array, call SetLength with two integer arguments.
For example, if I and J are integer-valued variables,
SetLength(Msgs,I,J);
allocates an I-by-J array, and Msgs[0,0] denotes an element of that array.
You can create multidimensional dynamic arrays that are not rectangular. The first step is to call SetLength, passing
it parameters for the first n dimensions of the array. For example,
var Ints: array of array of Integer;
SetLength(Ints,10);
allocates ten rows for Ints but no columns. Later, you can allocate the columns one at a time (giving them different
lengths); for example
SetLength(Ints[2], 5);
makes the third column of Ints five integers long. At this point (even if the other columns haven't been allocated)
you can assign values to the third column - for example, Ints[2,4] := 6.
The following example uses dynamic arrays (and the IntToStr function declared in the SysUtils unit) to create a
triangular matrix of strings.
var
A : array of array of string;
I, J : Integer;
begin
SetLength(A, 10);
for I := Low(A) to High(A) do
begin
SetLength(A[I], I);
for J := Low(A[I]) to High(A[I]) do
A[I,J] := IntToStr(I) + ',' + IntToStr(J) + ' ';
end;
end;
Array Types and Assignments
Arrays are assignment-compatible only if they are of the same type. Because the Delphi language uses name-
equivalence for types, the following code will not compile.
var
Int1: array[1..10] of Integer;
array[1..10] of Integer;
...
Int1 := Int2;
80
To make the assignment work, declare the variables as
var Int1, Int2: array[1..10] of Integer;
or
type IntArray = array[1..10] of Integer;
var
Int1: IntArray;
Int2: IntArray;
Dynamically allocated multidimensional arrays (.NET)
[ Pobierz całość w formacie PDF ]