One-dimensional arrays

Previous Index Next

Declaration Referring an array element Initialising the elements of an array in the array declaration

Declaration Top

The following are some examples of array declarations:

C++ version Equivalent Pascal version
int a[5], b[10]; var
  a: array[0..4] of integer;
  b: array[0..9] of integer;

The integer constant in the square brackets ([]) specifies the number of elements in the array. In C++, the subscript of an array must start with 0.

Referring an array element Top

Referring an array element in C++ is the same as in Pascal: the array name, followed by a subscript (which is an integer expression) enclosed by square brackets. For example, "a[4]", "b[x + 1]" (assume that x is an integer variable).

Initialising the elements of an array in the array declaration Top

In addition to initialising the elements of an array one by one, they can also be initialised in the array declaration by following the declaration with "=", followed by a comma-separated list of initialisers (the initialisation values) which is enclosed in braces. For example:

C++ version Equivalent Pascal version
int n[5] = {14, 23, 7, 10, 5}; var n: array[0..4] of integer;
  ...
n[0] := 14;
n[1] := 23;
n[2] := 7;
n[3] := 10;
n[4] := 5;

The elements of an array must be initialised separately if the initialisation is not done in the array declaration.

If the number of elements in the array is more than the number of the initialisers, the remaining elements are automatically initialised to zero. For example:

C++ version Equivalent Pascal version
int n[5] = {14, 23}; var n: array[0..4] of integer;
  ...
n[0] := 14;
n[1] := 23;
n[2] := 0;
n[3] := 0;
n[4] := 0;

If an array is not initialised in array declaration, the values stored in the array are unknown until initialisation of the elements are done separately.

There will be a compile-time error if the number of initialisers is more than the number of the elements in the array.

If the array size is not specified in the array declaration, there should be an initialiser list. The number of elements in the array will be the numbers of elements in the initialiser list. For example, the statement
    int x[] = {4, 7, 10, 8, 5};
specifies that there are 5 elements in the array (range of subscripts is 0 to 4).

The following sample program uses a one-dimensional array to store odd numbers:

Program 24: Using a one-dimensional array
C++ version Pascal version
#include <iostream.h>
#define MAX 10

main()
{
  int a[MAX], i;

  for (i = 0; i < MAX; i++)
    a[i] = 2 * i + 1;
  cout << "Subscript? (-ve value to quit) ";
  cin >> i;
  while (i >= 0) {
    if (i >= MAX)
      cout << "Subscript out of range!"
           << endl;
    else
      cout << "a[" << i << "]: "
           << a[i] << endl;
    cout << "Subscript? (-ve value to quit) ";
    cin >> i;
  }
  return 0;
}		 program prog_24(input, output);
const
  MAX = 10;
var
  a: array[0..MAX - 1] of integer;
  i: integer;
begin
  for i := 0 to MAX - 1 do
    a[i] := 2 * i + 1;
  write('Subscript? (-ve value to quit) ');
  readln(i);
  while i >= 0 do begin
    if i >= MAX then
      writeln('Subscript out of range!')

    else
      writeln('a[', i, ']: ', a[i]);

    write('Subscript? (-ve value to quit) ');
    readln(i)
  end
end.
Sample output (text in red is entered by user):
Subscript? (-ve value to quit) 5
a[5]: 11
Subscript? (-ve value to quit) 9
a[9]: 19
Subscript? (-ve value to quit) 10
Subscript out of range!
Subscript? (-ve value to quit) 0
a[0]: 1
Subscript? (-ve value to quit) -2

Previous Index Next