arrays.dd

Ddoc

$(SPEC_S Arrays,

        $(P There are four kinds of arrays:)

    $(TABLE2 Kinds of Arrays,
        $(TR $(TH Syntax)       $(TH Description))
        $(TR $(TD $(I type)*)   $(TD $(LINK2 #pointers, Pointers to data)))

        $(TR $(TD $(I type)[$(I integer)])      $(TD $(LINK2 #static-arrays, Static arrays)))

        $(TR $(TD $(I type)[])  $(TD $(LINK2 #dynamic-arrays, Dynamic arrays)))

        $(TR $(TD $(I type)[$(I type)]) $(TD $(LINK2 $(WEB hash-map.html)$(EBOOK #Associative Arrays), Associative arrays)))
    )

<h3>$(LNAME2 pointers, Pointers)</h3>

---------
int* p;
---------

        $(P These are simple pointers to data, analogous to C pointers.
        Pointers are provided for interfacing with C and for
        specialized systems work.
        There
        is no length associated with it, and so there is no way for the
        compiler or runtime to do bounds checking, etc., on it.
        Most conventional uses for pointers can be replaced with
        dynamic arrays, $(TT out) and $(TT ref) parameters,
        and reference types.
        )

<h3>$(LNAME2 static-arrays, Static Arrays)</h3>

---------
int[3] s;
---------

        $(P These are analogous to C arrays. Static arrays are distinguished
        by having a length fixed at compile time.
        )

        $(P The total size of a static array cannot exceed 16Mb.
        A dynamic array should be used instead for such large arrays.
        )

        $(P A static array with a dimension of 0 is allowed, but no
        space is allocated for it. It's useful as the last member
        of a variable length struct, or as the degenerate case of
        a template expansion.
        )

$(V1
        $(P Static arrays are value types, but as in C
        static arrays are passed to functions by reference
        and cannot be returned from functions.
        )
)
$(V2
        $(P Static arrays are value types. Unlike in C and D version 1,
        static arrays are passed to functions by value.
        Static arrays can also be returned by functions.
        )
)

<h3>$(LNAME2 dynamic-arrays, Dynamic Arrays)</h3>

---------
int[] a;
---------

        $(P Dynamic arrays consist of a length and a pointer to the array data.
        Multiple dynamic arrays can share all or parts of the array data.
        )

<h2>Array Declarations</h2>

        $(P There are two ways to declare arrays, prefix and postfix.
        The prefix form is the preferred method, especially for
        non-trivial types.
        )

<h4>Prefix Array Declarations</h4>

        $(P Prefix declarations appear before the identifier being
        declared and read right to left, so:
        )

---------
int[] a;      // dynamic array of ints
int[4][3] b;  // array of 3 arrays of 4 ints each
int[][5] c;   // array of 5 dynamic arrays of ints.
int*[]*[3] d; // array of 3 pointers to dynamic arrays of pointers to ints
int[]* e;     // pointer to dynamic array of ints
---------


<h4>Postfix Array Declarations</h4>

        $(P Postfix declarations appear after the identifier being
        declared and read left to right.
        Each group lists equivalent declarations:
        )

---------
// dynamic array of ints
int[] a;
int a[];

// array of 3 arrays of 4 ints each
int[4][3] b;
int[4] b[3];
int b[3][4];

// array of 5 dynamic arrays of ints.
int[][5] c;
int[] c[5];
int c[5][];

// array of 3 pointers to dynamic arrays of pointers to ints
int*[]*[3] d;
int*[]* d[3];
int* (*d[3])[];

// pointer to dynamic array of ints
int[]* e;
int (*e)[];
---------

        $(P $(B Rationale:) The postfix form matches the way arrays are
        declared in C and C++, and supporting this form provides an
        easy migration path for programmers used to it.
        )

<h2>$(LNAME2 usage, Usage)</h2>

        $(P There are two broad kinds of operations to do on an array -
        affecting
        the handle to the array,
        and affecting the contents of the array.
        C only has
        operators to affect the handle. In D, both are accessible.
        )

        $(P The handle to an array is specified by naming the array, as
        in p, s or a:
        )

---------
int* p;
int[3] s;
int[] a;

int* q;
int[3] t;
int[] b;

p = q;     // p points to the same thing q does.
p = s.ptr; // p points to the first element of the array s.
p = a.ptr; // p points to the first element of the array a.

s = ...;   // error, since s is a compiled in static
           // reference to an array.

a = p;     // error, since the length of the array pointed
           // to by p is unknown
a = s;     // a is initialized to point to the s array
a = b;     // a points to the same array as b does
---------

<h2>$(LNAME2 slicing, Slicing)</h2>

        $(P $(I Slicing) an array means to specify a subarray of it.
        An array slice does not copy the data, it is only another
        reference to it.
        For example:
        )

---------
int[10] a;   // declare array of 10 ints
int[] b;

b = a[1..3]; // a[1..3] is a 2 element array consisting of
             // a[1] and a[2]
foo(b[1]);   // equivalent to foo(0)
a[2] = 3;
foo(b[1]);   // equivalent to foo(3)
---------

        $(P The [] is shorthand for a slice of the entire array.
        For example, the assignments to b:
        )

---------
int[10] a;
int[] b;

b = a;
b = a[];
b = a[0 .. a.length];
---------

        $(P are all semantically equivalent.
        )

        $(P Slicing
        is not only handy for referring to parts of other arrays,
        but for converting pointers into bounds-checked arrays:
        )

---------
int* p;
int[] b = p[0..8];
---------

<h2>$(LNAME2 array-copying, Array Copying)</a></h2>

        $(P When the slice operator appears as the lvalue of an assignment
        expression, it means that the contents of the array are the
        target of the assignment rather than a reference to the array.
        Array copying happens when the lvalue is a slice, and the rvalue
        is an array of or pointer to the same type.
        )

---------
int[3] s;
int[3] t;

s[] = t;           // the 3 elements of t[3] are copied into s[3]
s[] = t[];         // the 3 elements of t[3] are copied into s[3]
s[1..2] = t[0..1]; // same as s[1] = t[0]
s[0..2] = t[1..3]; // same as s[0] = t[1], s[1] = t[2]
s[0..4] = t[0..4]; // error, only 3 elements in s
s[0..2] = t;       // error, different lengths for lvalue and rvalue
---------

        $(P Overlapping copies are an error:)

---------
s[0..2] = s[1..3]; // error, overlapping copy
s[1..3] = s[0..2]; // error, overlapping copy
---------

        $(P Disallowing overlapping makes it possible for more aggressive
        parallel code optimizations than possible with the serial
        semantics of C.
        )

<h2>$(LNAME2 array-setting, Array Setting)</h2>

        $(P If a slice operator appears as the lvalue of an assignment
        expression, and the type of the rvalue is the same as the element
        type of the lvalue, then the lvalue's array contents
        are set to the rvalue.
        )

---------
int[3] s;
int* p;

s[] = 3;     // same as s[0] = 3, s[1] = 3, s[2] = 3
p[0..2] = 3; // same as p[0] = 3, p[1] = 3
---------

<h2>$(LNAME2 array-concatenation, Array Concatenation)</a></h2>

        $(P The binary operator ~ is the $(I cat) operator. It is used
        to concatenate arrays:
        )

---------
int[] a;
int[] b;
int[] c;

a = b ~ c; // Create an array from the concatenation
           // of the b and c arrays
---------

        $(P Many languages overload the + operator to mean concatenation.
        This confusingly leads to, does:
        )

---------
"10" + 3 + 4
---------

        $(P produce the number 17, the string "1034" or the string "107" as the
        result? It isn't obvious, and the language designers wind up carefully
        writing rules to disambiguate it - rules that get incorrectly
        implemented, overlooked, forgotten, and ignored. It's much better to
        have + mean addition, and a separate operator to be array
        concatenation.
        )

        $(P Similarly, the ~= operator means append, as in:
        )

---------
a ~= b; // a becomes the concatenation of a and b
---------

        $(P Concatenation always creates a copy of its operands, even
        if one of the operands is a 0 length array, so:
        )

---------
a = b;           // a refers to b
a = b ~ c[0..0]; // a refers to a copy of b
---------

        $(P Appending does not always create a copy, see $(LINK2 #resize,
        setting dynamic array length) for details.
        )

<h2>$(LNAME2 array-operations, Array Operations)</h2>

        $(P Many array operations, also known as vector operations,
        can be expressed at a high level rather than as a loop.
        For example, the loop:
        )

---
T[] a, b;
...
for (size_t i = 0; i < a.length; i++)
  a[i] = b[i] + 4;
---

        $(P assigns to the elements of $(CODE a) the elements of $(CODE b)
        with $(CODE 4) added to each. This can also be expressed in
        vector notation as:
        )

---
T[] a, b;
...
a[] = b[] + 4;
---

        $(P A vector operation is indicated by the slice operator appearing
        as the lvalue of an =, +=, -=, *=, /=, %=, ^=, &amp;= or |= operator.
        The rvalue can be an expression consisting either of an array
        slice of the same length and type as the lvalue or an expression
        of the element type of the lvalue, in any combination.
        The operators supported for vector operations are the binary
        operators +, -, *, /, %, ^, &amp; and |, and the unary operators
        - and ~.
        )

        $(P The lvalue slice and any rvalue slices must not overlap.
        The vector assignment operators are evaluated right to left,
        and the other binary operators are evaluated left to right.
        All operands are evaluated exactly once, even if the array slice
        has zero elements in it.
        )

        $(P The order in which the array elements are computed
        is implementation defined, and may even occur in parallel.
        An application must not depend on this order.
        )

        $(P Implementation note: many of the more common vector
        operations are expected to take advantage of any vector
        math instructions available on the target computer.
        )

<h2>$(LNAME2 pointer-arithmetic, Pointer Arithmetic)</h2>

---------
int[3] abc;              // static array of 3 ints
int[] def = [ 1, 2, 3 ]; // dynamic array of 3 ints

void dibb(int* array) {
  array[2];     // means same thing as *(array + 2)
  *(array + 2); // get 3rd element
}

void diss(int[] array) {
  array[2];     // ok
  *(array + 2); // error, array is not a pointer
}

void ditt(int[3] array) {
  array[2];     // ok
  *(array + 2); // error, array is not a pointer
}
---------

<h2>$(LNAME2 rectangular-arrays, Rectangular Arrays)</h2>

        $(P Experienced FORTRAN numerics programmers know that multidimensional
        "rectangular" arrays for things like matrix operations are much faster than trying to
        access them via pointers to pointers resulting from "array of pointers to array" semantics.
        For example, the D syntax:
        )

---------
double[][] matrix;
---------

        $(P declares matrix as an array of pointers to arrays. (Dynamic arrays are implemented as
        pointers to the array data.) Since the arrays can have varying sizes (being dynamically
        sized), this is sometimes called "jagged" arrays. Even worse for optimizing the code, the
        array rows can sometimes point to each other! Fortunately, D static arrays, while using
        the same syntax, are implemented as a fixed rectangular layout:
        )

---------
double[3][3] matrix;
---------

        $(P declares a rectangular matrix with 3 rows and 3 columns, all contiguously in memory. In
        other languages, this would be called a multidimensional array and be declared as:
        )
---------
double matrix[3,3];
---------

<h2>$(LNAME2 array-length, Array Length)</h2>

$(V1
        $(P Within the [ ] of a static or a dynamic array,
        the variable $(B length)
        is implicitly declared and set to the length of the array.
        The symbol $(B $) can also be so used.
        )
)
$(V2
        $(P Within the [ ] of a static or a dynamic array,
        the symbol $(B $)
        represents the length of the array.
        )
)

---------
int[4] foo;
int[]  bar = foo;
int*   p = &foo[0];

// These expressions are equivalent:
bar[]
bar[0 .. 4]
$(V1 bar[0 .. $(B length)]
)bar[0 .. $(B $)]
bar[0 .. bar.length]

$(V1 p[0 .. length]         // 'length' is not defined, since p is not an array
bar[0]+length       // 'length' is not defined, out of scope of [ ]

bar[$(B length)-1]  // retrieves last element of the array
)
$(V2 p[0 .. $(DOLLAR)]      // '$' is not defined, since p is not an array
bar[0]+$(DOLLAR)            // '$' is not defined, out of scope of [ ]

bar[$(B $(DOLLAR))-1] // retrieves last element of the array
)
---------

<h2>$(LNAME2 array-properties, Array Properties)</h2>

        $(P Static array properties are:)

    $(TABLE2 Static Array Properties,
        $(TR $(TH Property) $(TH Description))

        $(TR
        $(TD $(B .init))
$(V1    $(TD returns the default initializer for the element type.)
)
$(V2    $(TD Returns an array literal with each element of the literal being the $(B .init)
        property of the array element type.
)
        )
        )

        $(TR
        $(TD $(B .sizeof))
        $(TD Returns the array length multiplied by the number of
        bytes per array element.
        )
        )

        $(TR
        $(TD $(B .length))
        $(TD Returns the number of elements in the array.
        This is a fixed quantity for static arrays.
        It is of type $(B size_t).
        )
        )

        $(TR
        $(TD $(B .ptr))
        $(TD Returns a pointer to the first element of the array.
        )
        )

        $(TR
        $(TD $(B .dup))
        $(TD Create a dynamic array of the same size
        and copy the contents of the array into it.
        )
        )

        $(TR
        $(TD $(B .idup))
        $(TD Create a dynamic array of the same size
        and copy the contents of the array into it.
        The copy is typed as being immutable.
        $(I D 2.0 only)
        )
        )

        $(TR
        $(TD $(B .reverse))
        $(TD Reverses in place the order of the elements in the array.
        Returns the array.
        )
        )

        $(TR
        $(TD $(B .sort))
        $(TD Sorts in place the order of the elements in the array.
        Returns the array.
        )
        )

    )

        $(P Dynamic array properties are:)

    $(TABLE2 Dynamic Array Properties,
        $(TR $(TH Property) $(TH Description))

        $(TR
        $(TD $(B .init))
        $(TD Returns null.)
        )

        $(TR
        $(TD $(B .sizeof))
        $(TD Returns the size of the dynamic array reference,
        which is 8 on 32 bit machines.
        )
        )

        $(TR
        $(TD $(B .length))
        $(TD Get/set number of elements in the array.
        It is of type $(B size_t).
        )
        )

        $(TR
        $(TD $(B .ptr))
        $(TD Returns a pointer to the first element of the array.
        )
        )

        $(TR
        $(TD $(B .dup))
        $(TD Create a dynamic array of the same size
        and copy the contents of the array into it.
        )
        )

        $(TR
        $(TD $(B .idup))
        $(TD Create a dynamic array of the same size
        and copy the contents of the array into it.
        The copy is typed as being immutable.
        $(I D 2.0 only)
        )
        )

        $(TR
        $(TD $(B .reverse))
        $(TD Reverses in place the order of the elements in the array.
        Returns the array.
        )
        )

        $(TR
        $(TD $(B .sort))
        $(TD Sorts in place the order of the elements in the array.
        Returns the array.
        )
        )

    )

        $(P For the $(B .sort) property to work on arrays of class
        objects, the class definition must define the function:
        $(TT int opCmp(Object)). This is used to determine the
        ordering of the class objects. Note that the parameter
        is of type $(TT Object), not the type of the class.)

        $(P For the $(B .sort) property to work on arrays of
        structs or unions, the struct or union definition must
        define the function:
        $(TT int opCmp(S)) or
        $(TT int opCmp(S*)).
        The type $(TT S) is the type of the struct or union.
        This function will determine the sort ordering.
        )

    $(P Examples:)

---------
int* p;
int[3] s;
int[] a;

p.length; // error, length not known for pointer
s.length; // compile time constant 3
a.length; // runtime value

p.dup;    // error, length not known
s.dup;    // creates an array of 3 elements, copies
          // elements s into it
a.dup;    // creates an array of a.length elements, copies
          // elements of a into it
---------

<h3>$(LNAME2 resize, Setting Dynamic Array Length)</h3>

        $(P The $(B $(TT .length)) property of a dynamic array can be set
        as the lvalue of an = operator:
        )

---------
array.length = 7;
---------

        $(P This causes the array to be reallocated in place, and the existing
        contents copied over to the new array. If the new array length is
        shorter, the array is not reallocated, and no data is copied.  It is
        equivalent to slicing the array:

---------
array = array[0..7];
---------

        If the new array length is longer, the remainder is filled out with the
        default initializer.
        )

        $(P To maximize efficiency, the runtime always tries to resize the
        array in place to avoid extra copying.
        $(V1 It will always do a copy if the new size is larger and the array
        was not allocated via the new operator or a previous resize operation.)
        $(V2 It will always do a copy if the new size is larger and the array
        was not allocated via the new operator or resizing in place would
        overwrite valid data in the array.)
        )

$(V1    $(P This means that if there is an array slice immediately following the
        array being resized, the resized array could overlap the slice; i.e.:
        )

---------
char[] a = new char[20];
char[] b = a[0..10];
char[] c = a[10..20];

b.length = 15; // always resized in place because it is sliced
               // from a[] which has enough memory for 15 chars
b[11] = 'x';   // a[11] and c[1] are also affected

a.length = 1;
a.length = 20; // no net change to memory layout

c.length = 12; // always does a copy because c[] is not at the
               // start of a gc allocation block
c[5] = 'y';    // does not affect contents of a[] or b[]

a.length = 25; // may or may not do a copy
a[3] = 'z';    // may or may not affect b[3] which still overlaps
               // the old a[3]
---------
)

$(V2
        For example:

---------
char[] a = new char[20];
char[] b = a[0..10];
char[] c = a[10..20];
char[] d = a;

b.length = 15; // always reallocates because extending in place would
               // overwrite other data in a.
b[11] = 'x';   // a[11] and c[1] are not affected

d.length = 1;
d.length = 20; // also reallocates, because doing this will overwrite a and
               // c

c.length = 12; // may reallocate in place if space allows, because nothing
               // was allocated after c.
c[5] = 'y';    // may affect contents of a, but not b or d because those
               // were reallocated.

a.length = 25; // This always reallocates because if c extended in place,
               // then extending a would overwrite c.  If c didn't
               // reallocate in place, it means there was not enough space,
               // which will still be true for a.
a[15] = 'z';   // does not affect c, because either a or c has reallocated.
---------
)

        $(P To guarantee copying behavior, use the .dup property to ensure
        a unique array that can be resized. $(V2 Also, one may use the phobos
        $(TT .capacity) property to determine how many elements can be appended
        to the array without reallocating.)
        )

        $(P These issues also apply to appending arrays with the ~= operator.
        Concatenation using the ~ operator is not affected since it always
        reallocates.
        )

        $(P Resizing a dynamic array is a relatively expensive operation.
        So, while the following method of filling an array:
        )

---------
int[] array;
while (1) {
  c = getinput();
  if (!c)
    break;
  array.length = array.length + 1;
  array[array.length - 1] = c;
}
---------

        $(P will work, it will be inefficient. A more practical
        approach would be to minimize the number of resizes:
        )

---------
int[] array;
array.length = 100;        // guess
for (i = 0; 1; i++) {
  c = getinput();
  if (!c)
    break;
  if (i == array.length)
    array.length = array.length * 2;
  array[i] = c;
}
array.length = i;
---------

        $(P Picking a good initial guess is an art, but you usually can
        pick a value covering 99% of the cases.
        For example, when gathering user
        input from the console - it's unlikely to be longer than 80.
        )

        $(V2 $(P Also, you may wish to utilize the phobos $(TT reserve)
        function to pre-allocate array data to use with the append operator.))

<h3>$(LNAME2 func-as-property, Functions as Array Properties)</h3>

        $(P If the first parameter to a function is an array, the
        function can be called as if it were a property of the array:
        )

---
int[] array;
void foo(int[] a, int x);

foo(array, 3);
array.foo(3);   // means the same thing
---

<h2>$(LNAME2 bounds, Array Bounds Checking)</h2>

        $(P It is an error to index an array with an index that is less than
        0 or greater than or equal to the array length. If an index is
        out of bounds, $(V1 an ArrayBoundsError)$(V2 a RangeError) exception is
        raised if detected at runtime, and an error if detected at compile
        time.  A program may not rely on array bounds checking happening, for
        example, the following program is incorrect:
        )

---------
try {
  for (i = 0; ; i++) {
    array[i] = 5;
  }
}
catch (RangeError) {
  // terminate loop
}
---------

        The loop is correctly written:

---------
for (i = 0; i < array.length; i++) {
  array[i] = 5;
}
---------

        $(P $(B Implementation Note:) Compilers should attempt to detect
        array bounds errors at compile time, for example:
        )

---------
int[3] foo;
int x = foo[3]; // error, out of bounds
---------

        $(P Insertion of array bounds checking code at runtime should be
        turned on and off
        with a compile time switch.
        )

<h2>$(LNAME2 array-initialization, Array Initialization)</h2>

<h3>$(LNAME2 default-initialization, Default Initialization)</h3>

        $(UL
        $(LI Pointers are initialized to $(B null).)
        $(LI Static array contents are initialized to the default
        initializer for the array element type.)
        $(LI Dynamic arrays are initialized to having 0 elements.)
        $(LI Associative arrays are initialized to having 0 elements.)
        )

<h3>$(LNAME2 void-initialization, Void Initialization)</h3>

        $(P Void initialization happens when the $(I Initializer) for
        an array is $(B void). What it means is that no initialization
        is done, i.e. the contents of the array will be undefined.
        This is most useful as an efficiency optimization.
        Void initializations are an advanced technique and should only be used
        when profiling indicates that it matters.
        )

<h3>$(LNAME2 static-init-static, Static Initialization of Static Arrays)</h3>

        $(P Static initalizations are supplied by a list of array
        element values enclosed in [ ]. The values can be optionally
        preceded by an index and a :.
        If an index is not supplied, it is set to the previous index
        plus 1, or 0 if it is the first value.
        )

---------
int[3] a = [ 1:2, 3 ]; // a[0] = 0, a[1] = 2, a[2] = 3
---------

        $(P This is most handy when the array indices are given by enums:)

---------
enum Color { red, blue, green };

int value[Color.max + 1] =
  [ Color.blue :6,
    Color.green:2,
    Color.red  :5 ];
---------

        $(P These arrays are static when they appear in global scope.
        Otherwise, they need to be marked with $(B const) or $(B static)
        storage classes to make them static arrays.)


<h2>$(LNAME2 special-array, Special Array Types)</h2>

<h3>$(LNAME2 strings, Strings)</h3>

        $(P A string is
        an array of characters. String literals are just
        an easy way to write character arrays.
        String literals are immutable (read only).
        )

$(V1
---------
char[] str;
char[] str1 = "abc";
str[0] = 'b';  // error, "abc" is read only, may crash
---------

        $(P The name $(CODE string) is aliased to $(CODE char[]),
        so the above declarations could be equivalently written as:
        )

---------
string str;
string str1 = "abc";
---------
)
$(V2
---------
char[] str1 = "abc";                // error, "abc" is not mutable
char[] str2 = "abc".dup;            // ok, make mutable copy
immutable(char)[] str3 = "abc";     // ok
immutable(char)[] str4 = str1;      // error, str4 is not mutable
immutable(char)[] str5 = str1.idup; // ok, make immutable copy
---------

        $(P The name $(CODE string) is aliased to $(CODE immutable(char)[]),
        so the above declarations could be equivalently written as:
        )
---------
char[] str1 = "abc";     // error, "abc" is not mutable
char[] str2 = "abc".dup; // ok, make mutable copy
string str3 = "abc";     // ok
string str4 = str1;      // error, str4 is not mutable
string str5 = str1.idup; // ok, make immutable copy
---------
)
        $(P $(CODE char[]) strings are in UTF-8 format.
        $(CODE wchar[]) strings are in UTF-16 format.
        $(CODE dchar[]) strings are in UTF-32 format.
        )

        $(P Strings can be copied, compared, concatenated, and appended:)

---------
str1 = str2;
if (str1 < str3) ...
func(str3 ~ str4);
str4 ~= str1;
---------

        $(P with the obvious semantics. Any generated temporaries get cleaned up
        by the garbage collector (or by using $(CODE alloca())).
        Not only that, this works with any
        array not just a special String array.
        )

        $(P A pointer to a char can be generated:
        )

---------
char* p = &str[3]; // pointer to 4th element
char* p = str;     // pointer to 1st element
---------

        $(P Since strings, however, are not 0 terminated in D,
        when transferring a pointer
        to a string to C, add a terminating 0:
        )

---------
str ~= "\0";
---------

        $(P or use the function $(TT std.string.toStringz).)

        $(P The type of a string is determined by the semantic phase of
        compilation. The type is
        one of: char[], wchar[], dchar[], and is determined by
        implicit conversion rules.
        If there are two equally applicable implicit conversions,
        the result is an error. To
        disambiguate these cases, a cast or a postfix of $(B c),
        $(B w) or $(B d) can be used:
        )

---------
$(V1
cast(wchar [])"abc" // this is an array of wchar characters
"abc"w              // so is this
)
$(V2
cast(immutable(wchar) [])"abc" // this is an array of wchar characters
"abc"w                         // so is this
)
---------

        $(P String literals that do not have a postfix character and that
        have not been cast can be implicitly converted between char[],
        wchar[], and dchar[] as necessary.
        )

---------
char c;
wchar w;
dchar d;

c = 'b';     // c is assigned the character 'b'
w = 'b';     // w is assigned the wchar character 'b'
w = 'bc';    // error - only one wchar character at a time
w = "b"[0];  // w is assigned the wchar character 'b'
w = "\r"[0]; // w is assigned the carriage return wchar character
d = 'd';     // d is assigned the character 'd'
---------

<h4>$(LNAME2 printf, C's printf() and Strings)</h4>

        $(P $(B printf()) is a C function and is not part of D. $(B printf())
        will print C strings, which are 0 terminated. There are two ways
        to use $(B printf()) with D strings. The first is to add a
        terminating 0, and cast the result to a char*:
        )

---------
str ~= "\0";
printf("the string is '%s'\n", cast(char*)str);
---------

        $(P or:)

---------
import std.string;
printf("the string is '%s'\n", std.string.toStringz(str));
---------

        $(P String literals already have a 0 appended to them, so
        can be used directly:)

-----------
printf("the string is '%s'\n", cast(char*)"string literal");
-----------

        $(P So, why does the first string literal to printf not need
        the cast? The first parameter is prototyped as a char*, and
        a string literal can be implicitly cast to a char*.
        The rest of the arguments to printf, however, are variadic
        (specified by ...),
        and a string literal is passed as a (length,pointer) combination
        to variadic parameters.)

        $(P The second way is to use the precision specifier. The way D arrays
        are laid out, the length comes first, so the following works:)

---------
printf("the string is '%.*s'\n", str);
---------

        $(P The best way is to use std.stdio.writefln, which can handle
        D strings:)

---------
import std.stdio;
writefln("the string is '%s'", str);
---------

<h3>$(LNAME2 implicit-conversions, Implicit Conversions)</h3>

        $(P A pointer $(TT $(I T)*) can be implicitly converted to
        one of the following:)

        $(UL
        $(LI $(TT void*))
        )

        $(P A static array $(TT $(I T)[$(I dim)]) can be implicitly
        converted to
        one of the following:
        )

        $(UL
        $(LI $(TT $(I T)[]))
        $(LI $(TT $(I U)[]))
        $(LI $(TT void[]))
        )

        $(P A dynamic array $(TT $(I T)[]) can be implicitly converted to
        one of the following:
        )

        $(UL
        $(LI $(TT $(I U)[]))
        $(LI $(TT void[]))
        )

        $(P Where $(I U) is a base class of $(I T).)

<hr>
<h1>$(LNAME2 associative, Associative Arrays)</h1>

        $(P Associative arrays have an index that is not necessarily an integer,
        and can be sparsely populated. The index for an associative array
        is called the $(I key), and its type is called the $(I KeyType).
        )

        $(P Associative arrays are declared by placing the $(I KeyType)
        within the [] of an array declaration:
        )

---------
int[string] b;    // associative array b of ints that are
                  // indexed by an array of characters.
                  // The $(I KeyType) is char[]
b["hello"] = 3;   // set value associated with key "hello" to 3
func(b["hello"]); // pass 3 as parameter to func()
---------

        $(P Particular keys in an associative array can be removed with the
        remove function:
        )

---------
b.$(B remove)("hello");
---------

        $(P The $(I InExpression) yields a pointer to the value
        if the key is in the associative array, or $(B null) if not:
        )

---------
int* p;
p = ("hello" $(B in) b);
if (p != $(B null))
        ...
---------

        $(P $(I KeyType)s cannot be functions or voids.
        )

<h3>$(LNAME2 classes-as-keys, Using Classes as the KeyType)</h3>

        $(P Classes can be used as the $(I KeyType). For this to work,
        the class definition must override the following member functions
        of class $(TT Object):)

        $(UL
        $(LI $(TT hash_t toHash()))
$(V1    $(LI $(TT int opEquals(Object))))
$(V2    $(LI $(TT bool opEquals(Object))))
        $(LI $(TT int opCmp(Object)))
        )

        $(P $(TT hash_t) is an alias to an integral type.)

        $(P Note that the parameter to $(TT opCmp) and $(TT opEquals) is
        of type
        $(TT Object), not the type of the class in which it is defined.)

        $(P For example:)

---
class Foo {
  int a, b;

  hash_t $(B toHash)() { return a + b; }

$(V1   int $(B opEquals)(Object o))$(V2   bool $(B opEquals)(Object o)) {
    Foo foo = cast(Foo) o;
    return foo && a == foo.a && b == foo.b;
  }

  int $(B opCmp)(Object o) {
    Foo foo = cast(Foo) o;
    if (!foo)
      return -1;
    if (a == foo.a)
      return b - foo.b;
    return a - foo.a;
  }
}
---

        $(P The implementation may use either $(TT opEquals) or $(TT opCmp) or
        both. Care should be taken so that the results of
        $(TT opEquals) and $(TT opCmp) are consistent with each other when
        the class objects are the same or not.)

<h3>$(LNAME2 structs-as-keys, Using Structs or Unions as the KeyType)</h3>

        $(P If the $(I KeyType) is a struct or union type,
        a default mechanism is used
        to compute the hash and comparisons of it based on the binary
        data within the struct value. A custom mechanism can be used
        by providing the following functions as struct members:
        )

---------
$(V2 const) hash_t $(B toHash)();
$(V1 int $(B opEquals)($(I KeyType)* s);)$(V2 const bool $(B opEquals)(ref const $(I KeyType) s);)
$(V1 int $(B opCmp)($(I KeyType)* s);)$(V2 const int $(B opCmp)(ref const $(I KeyType) s);)
---------

        $(P For example:)

$(V1
---------
import std.string;

struct MyString {
  string str;

  hash_t $(B toHash)() {
    hash_t hash;
    foreach (char c; str)
      hash = (hash * 9) + c;
    return hash;
  }

  bool $(B opEquals)(MyString* s) {
    return std.string.cmp(this.str, s.str) == 0;
  }

  int $(B opCmp)(MyString* s) {
    return std.string.cmp(this.str, s.str);
  }
}
---------
)
$(V2
---------
import std.string;

struct MyString {
  string str;

  const hash_t $(B toHash)() {
    hash_t hash;
    foreach (char c; str)
      hash = (hash * 9) + c;
    return hash;
  }

  const bool $(B opEquals)(ref const MyString s) {
    return std.string.cmp(this.str, s.str) == 0;
  }

  const int $(B opCmp)(ref const MyString s) {
    return std.string.cmp(this.str, s.str);
  }
}
---------
)


        $(P The implementation may use either $(TT opEquals) or $(TT opCmp) or
        both. Care should be taken so that the results of
        $(TT opEquals) and $(TT opCmp) are consistent with each other when
        the struct/union objects are the same or not.)

<h3>$(LNAME2 aa-properties, Properties)</h3>

Properties for associative arrays are:

    $(TABLE2 Associative Array Properties,
        $(TR $(TH Property) $(TH Description))

        $(TR
        $(TD $(B .sizeof))
        $(TD Returns the size of the reference to the associative
        array; it is typically 8.
        )
        )

        $(TR
        $(TD $(B .length))
        $(TD Returns number of values in the associative array.
        Unlike for dynamic arrays, it is read-only.
        )
        )

        $(TR
        $(TD $(B .keys))
        $(TD Returns dynamic array, the elements of which are the keys in
        the associative array.
        )
        )

        $(TR
        $(TD $(B .values))
        $(TD Returns dynamic array, the elements of which are the values in
        the associative array.
        )
        )

        $(TR
        $(TD $(B .rehash))
        $(TD Reorganizes the associative array in place so that lookups
        are more efficient. rehash is effective when, for example,
        the program is done loading up a symbol table and now needs
        fast lookups in it.
        Returns a reference to the reorganized array.
        )
        )

$(V2
        $(TR
        $(TD $(B .byKey()))
        $(TD Returns a delegate suitable for use as an $(I Aggregate) to
        a $(GLINK2 statement, ForeachStatement)
        which will iterate over the keys
        of the associative array.
        )
        )

        $(TR
        $(TD $(B .byValue()))
        $(TD Returns a delegate suitable for use as an $(I Aggregate) to
        a $(GLINK2 statement, ForeachStatement)
        which will iterate over the values
        of the associative array.
        )
        )

        $(TR
        $(TD $(B .get(Key key, lazy Value defaultValue)))
        $(TD Looks up $(I key); if it exists returns corresponding $(I value)
        else evaluates and returns $(I defaultValue).
        )
        )
)
    )

<hr>
<h3>$(LNAME2 aa-example, Associative Array Example: word count)</h3>

---------
import std.file;         // D file I/O
import std.stdio;

int main (string[] args) {
  int word_total;
  int line_total;
  int char_total;
  int[string] dictionary;

  writefln("   lines   words   bytes file");
  for (int i = 1; i < args.length; ++i)      // program arguments
  {
    char[] input;            // input buffer
    int w_cnt, l_cnt, c_cnt; // word, line, char counts
    int inword;
    int wstart;

    // read file into input[]
    input = cast(char[])std.file.read(args[i]);

    foreach (j, char c; input)
    {
      if (c == '\n')
              ++l_cnt;
      if (c >= '0' && c <= '9')
      {
      }
      else if (c >= 'a' && c <= 'z' ||
               c >= 'A' && c <= 'Z')
      {
        if (!inword)
        {
          wstart = j;
          inword = 1;
          ++w_cnt;
        }
      }
      else if (inword)
      {
        char[] word = input[wstart .. j];
        dictionary[word]++;   // increment count for word
        inword = 0;
      }
      ++c_cnt;
    }
    if (inword)
    {
      char[] word = input[wstart .. input.length];
      dictionary[word]++;
    }
    writefln("%8d%8d%8d %s", l_cnt, w_cnt, c_cnt, args[i]);
    line_total += l_cnt;
    word_total += w_cnt;
    char_total += c_cnt;
  }

  if (args.length > 2)
  {
    writef("-------------------------------------\n%8d%8d%8d total",
            line_total, word_total, char_total);
  }

  writefln("-------------------------------------");
  foreach (word; dictionary.keys.sort)
  {
    writefln("%3d %s", dictionary[word], word);
  }
  return 0;
}
---------

)

Macros:
        TITLE=Arrays
        WIKI=Arrays
        CATEGORY_SPEC=$0
        FOO=