D - Arrays

D programming language provides a data structure, the array, which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.

Instead of declaring individual variables, such as number0, number1, ..., and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and ..., numbers[99] to represent individual variables. A specific element in an array is accessed by an index.
All arrays consist of contiguous memory locations. The lowest address corresponds to the first element and the highest address to the last element.

Declaring Arrays:

To declare an array in D programming language, the programmer specifies the type of the elements and the number of elements required by an array as follows:

type arrayName [ arraySize ];

This is called a single-dimension array. The arraySize must be an integer constant greater than zero and type can be any valid D programming language data type. For example, to declare a 10-element array called balance of type double, use this statement:

double balance[10];

Initializing Arrays:

You can initialize D programming language array elements either one by one or using a single statement as follows:

double balance[5] = [1000.0, 2.0, 3.4, 17.0, 50.0];

The number of values between square brackets[ ] on right side cannot be larger than the number of elements that we declare for the array between square brackets [ ]. Following is an example to assign a single element of the array:
If you omit the size of the array, an array just big enough to hold the initialization is created. Therefore, if you write:

double balance[] = [1000.0, 2.0, 3.4, 17.0, 50.0];

You will create exactly the same array as you did in the previous example.

balance[4] = 50.0;

The above statement assigns element number 5th in the array a value of 50.0. Array with 4th index will be 5th, i.e., last element because all arrays have 0 as the index of their first element which is also called base index. Following is the pictorial representaion of the same array we discussed above:

Accessing Array Elements:

An element is accessed by indexing the array name. This is done by placing the index of the element within square brackets after the name of the array. For example:

double salary = balance[9];

The above statement will take 10th element from the array and assign the value to salary variable. Following is an example, which will use all the above-mentioned three concepts viz. declaration, assignment and accessing arrays:

import std.stdio;

void main()
{
   int n[ 10 ]; // n is an array of 10 integers

   // initialize elements of array n to 0
   for ( int i = 0; i < 10; i++ )
   {
      n[ i ] = i + 100; // set element at location i to i + 100
   }

   writeln("Element \t Value");

   // output each array element's value
   for ( int j = 0; j < 10; j++ )
   {
      writeln(j," \t ",n[j]);
   }
}

When the above code is compiled and executed, it produces the following result:

Element   Value
0     100
1     101
2     102
3     103
4     104
5     105
6     106
7     107
8     108
9     109

Static arrays vs. dynamic arrays

When the length of an array is specified when the program is written, that array is a static array. When the length can change during the execution of the program, that array is a dynamic array.
Defining dynamic arrays is simpler than defining fixed-length arrays because omitting the length makes a dynamic array:

int[] dynamicArray;

Array properties

Property	Description
.init	Static array returns an array literal with each element of the literal being the .init property of the array element type.
.sizeof	Static array returns the array length multiplied by the number of bytes per array element while dynamic arrays returns the size of the dynamic array reference, which is 8 in 32-bit builds and 16 on 64-bit builds.
.length	Static array returns the number of elements in the array while dynamic arrays is used to get/set number of elements in the array. Length is of type size_t.
.ptr	Returns a pointer to the first element of the array.
.dup	Create a dynamic array of the same size and copy the contents of the array into it.
.idup	Create a dynamic array of the same size and copy the contents of the array into it. The copy is typed as being immutable.
.reverse	Reverses in place the order of the elements in the array. Returns the array.
.sort	Sorts in place the order of the elements in the array. Returns the array.

The following example explains the various properties available for an array.

import std.stdio;

void main()
{
   int n[ 5 ]; // n is an array of 5 integers

   // initialize elements of array n to 0
   for ( int i = 0; i < 5; i++ )
   {
      n[ i ] = i + 100; // set element at location i to i + 100
   }
   writeln("Initialized value:",n.init);

   writeln("Length: ",n.length);
   writeln("Size of: ",n.sizeof);
   writeln("Pointer:",n.ptr);

   writeln("Duplicate Array: ",n.dup);
   writeln("iDuplicate Array: ",n.idup);

   n = n.reverse.dup;
   writeln("Reversed Array: ",n);

   writeln("Sorted Array: ",n.sort);
}

When the above code is compiled and executed, it produces the following result:

Initialized value:[0, 0, 0, 0, 0]
Length: 5
Size of: 20
Pointer:7FFF5A373920
Duplicate Array: [100, 101, 102, 103, 104]
iDuplicate Array: [100, 101, 102, 103, 104]
Reversed Array: [104, 103, 102, 101, 100]
Sorted Array: [100, 101, 102, 103, 104]

Multi - Dimensional Array

D programming allows multidimensional arrays. Here is the general form of a multidimensional array declaration:

type name[size1][size2]...[sizeN];

For example, the following declaration creates a three dimensional 5 . 10 . 4 integer array:

int threedim[5][10][4];

Two-Dimensional Arrays:

The simplest form of the multidimensional array is the two-dimensional array. A two-dimensional array is, in essence, a list of one-dimensional arrays. To declare a two-dimensional integer array of size x,y, you would write something as follows:

type arrayName [ x ][ y ];

Where type can be any valid D programming data type and arrayName will be a valid D programming identifier.
A two-dimensional array can be think as a table, which will have x number of rows and y number of columns. A 2-dimensional array a, which contains three rows and four columns can be shown as below:
Thus, every element in array a is identified by an element name of the form a[ i ][ j ], where a is the name of the array, and i and j are the subscripts that uniquely identify each element in a.

Initializing Two-Dimensional Arrays:

Multidimensioned arrays may be initialized by specifying bracketed values for each row. Following is an array with 3 rows and each row have 4 columns.

int a[3][4] = [  
 [0, 1, 2, 3] ,   /*  initializers for row indexed by 0 */
 [4, 5, 6, 7] ,   /*  initializers for row indexed by 1 */
 [8, 9, 10, 11]   /*  initializers for row indexed by 2 */
];

The nested braces, which indicate the intended row, are optional. The following initialization is equivalent to previous example:

int a[3][4] = [0,1,2,3,4,5,6,7,8,9,10,11];

Accessing Two-Dimensional Array Elements:

An element in 2-dimensional array is accessed by using the subscripts, i.e., row index and column index of the array. For example:

int val = a[2][3];

The above statement will take 4th element from the 3rd row of the array. You can verify it in the above digram.

import std.stdio;
 
void main ()
{
   // an array with 5 rows and 2 columns.
   int a[5][2] = [ [0,0], [1,2], [2,4], [3,6],[4,8]];

   // output each array element's value                      
   for ( int i = 0; i < 5; i++ )
      for ( int j = 0; j < 2; j++ )
      {
         writeln( "a[" , i , "][" , j , "]: ",a[i][j]);
      } 
}

When the above code is compiled and executed, it produces the following result:

a[0][0]: 0
a[0][1]: 0
a[1][0]: 1
a[1][1]: 2
a[2][0]: 2
a[2][1]: 4
a[3][0]: 3
a[3][1]: 6
a[4][0]: 4
a[4][1]: 8

Common Array operations

Array Slicing

We often use part of an array and slicing array is often quite helpul. A simple example for array slicing is shown below.

import std.stdio;

void main ()
{
   // an array with 5 elements.
   double a[5] = [1000.0, 2.0, 3.4, 17.0, 50.0];
   double[] b;

   b = a[1..3];
   writeln(b);
}

When the above code is compiled and executed, it produces the following result:

[2, 3.4]

Array Copying

We also use copying array . A simple example for array copying is shown below.

import std.stdio;

void main ()
{
   // an array with 5 elements.
   double a[5] = [1000.0, 2.0, 3.4, 17.0, 50.0];
   double b[5];
   writeln("Array a:",a);
   writeln("Array b:",b);

   b[] = a;      // the 5 elements of a[5] are copied into b[5]
   writeln("Array b:",b);

   b[] = a[];   // the 5 elements of a[3] are copied into b[5]
   writeln("Array b:",b);

   b[1..2] = a[0..1]; // same as b[1] = a[0]
   writeln("Array b:",b);

   b[0..2] = a[1..3]; // same as b[0] = a[1], b[1] = a[2]
   writeln("Array b:",b);
}

When the above code is compiled and executed, it produces the following result:

Array a:[1000, 2, 3.4, 17, 50]
Array b:[nan, nan, nan, nan, nan]
Array b:[1000, 2, 3.4, 17, 50]
Array b:[1000, 2, 3.4, 17, 50]
Array b:[1000, 1000, 3.4, 17, 50]
Array b:[2, 3.4, 3.4, 17, 50]

Array Setting

A simple example for setting value in an array is shown below.

import std.stdio;

void main ()
{
   // an array with 5 elements.
   double a[5];
   a[] = 5;
   writeln("Array a:",a);
}

When the above code is compiled and executed, it produces the following result:

Array a:[5, 5, 5, 5, 5]

Array Concatenation

A simple example for concatenation of two arrays is shown below.

import std.stdio;

void main ()
{
   // an array with 5 elements.
   double a[5] = 5;
   double b[5] = 10;
   double [] c;
   c = a~b;
   writeln("Array c: ",c);
}

When the above code is compiled and executed, it produces the following result:

Array c: [5, 5, 5, 5, 5, 10, 10, 10, 10, 10]