An array is a variable capable of storing multiple values. When we declare an array we tell the compiler how many values we want the array to hold. We also tell the compiler what type of values the array can store. All of the values in an array must be of the same type.

Here is a declaration of an array called numlist that will be used to store 8 integers:

int numlist[8];       // declaring an integer array that can store 8 values

The individual values stored in an array are called the elements of the array. You will also hear them called indexed variables or subscripted variables. Each of the elements of an array is assigned an index. An index is a natural number in the range {0,1,2,...}.  Note that the array index started from 0.

As it is shown in the above table, to access one of the elements of an array, you put the index of that element in square brackets after the name of the array. The 0^th element in the array called numlist is numlist[0], the next one is numlist[1], and so forth. Since we start the numbering with 0, the last element in numlist is numlist[7].

To put the value of 12 into the 0^th element of numlist, we will use:

numlist[0] = 12;

cin >> numlist[1];

if(numlist[2] > numlist[1]) // Compares the third element of the array with
                            // the second element of the array
cout << numlist[5];         // Displays the sixth element of the array

sum = sum + numlist[7];     // Adds the 8th element to sum

for(int i = 0; i < 3; i++)
     numlist[2*i+1] = 0;  // set the odd elements of the array to 0

If you wish to fill the array numlist with the integers typed from the keyboard, you can use a for loop too. Here is a for loop that will allow you to enter 8 values from the keyboard and will store them in the array numlist. Notice that we have used the variable i as an index for array numlist.

for (int i=0; i<8; ++i)
{
        cout << "Enter the next value: ";
        cin >> numlist[i];
}

for (int i=0; i<8; ++i)
{
        cout << "Enter value #" << i+1 << ": ";
        cin >> numlist[i];
}

The following program allows you to enter 8 integers from the keyboard and will store those values in array numlist.

// P9_1.cpp - A program that uses an array of integers
#include <iostream>
using namespace std;
int main(void)
{
        int numlist[8];

        // Read 8 integers from the keyboard
        for (int i = 0; i<8; i++ )
        {
                cout << "Enter value #" << i+1 << ": ";
                cin >> numlist[i];
        }
        // Display the numbers in a reverse order
        for (int i = 8; i > 0; i-- )
        {
                cout << "Value #" << i << ": ";
                cout << numlist[i-1] << endl;  //Pay attention to i-1!
        }

        return 0;
}

Array Index Out of Range

// P9_1a.cpp - This program illustrates array index out of range.

#include <iostream>
using namespace std;
int main(void)
{
        int numlist[8];

        cout << " \t i \t numlist[i] \n";
        cout << " \t ===== \t ======== \n";

        for (int i = 0; i <= 8; i++)
        {
                numlist[i] = i*2;
                cout << " \t " << i << "\t " << numlist[i] << endl;
        }
        return 0;
}

Here is the output of this program:
          i               numlist[i]
        =====    ========
         0             0
         1             2
         2             4
         3             6
         4             8
         5            10
         6            12
         7            14
         8            ?????      // Do you see any thing wrong here?

Initializing Arrays

int num = 4;

int numlist[8] = {12,  8, 10, 123, 1000, 23, 4, 10};

If you list fewer values within the braces { and } than the declared size (8 in the above example) of the array, our C++ compiler will initialize all the rest of the elements to 0. However, not all C++ compilers will do this. If you initialize an array when it is declared, you can omit the size of the array. C++ will use the number of initializers in your list as the size of the array. Here is an example:

char vowels[] = {'a', 'e', 'i', 'o', 'u'};

Creating Arrays of Flexible Size
One way to create an array with a particular size is to use a global variable to define the size of that array. Thus, every time one can change that number to increase or decrease the size of an array.  This requires you to recompile the program for the new size to take effect.  Let's modify program  P9_1.cpp to work on flexible size arrays.

// P9_1b.cpp - A program that uses a flexible size array of integers
#include <iostream>
using namespace std;
const int SIZE = 8;  // Set the maximum size for the array

int main(void)
{
        int numlist[SIZE];

        // Read SIZE integers from the keyboard
        for (int i = 0; i<SIZE; i++ )
        {
                cout << "Enter value #" << i+1 << ": ";
                cin >> numlist[i];
        }
        // Display the numbers in a reverse order
        for (int i = SIZE; i > 0; i-- )
        {
                cout << "Value #" << i << ": ";
                cout << numlist[i-1] << endl;  //Pay attention to i-1!
        }

        return 0;
}

This produces the same result as P9_1.cpp.  Now, you are limited to array of 8 integers.  By changing the value for SIZE, you can read as many numbers as you wish.

Exercise 9.1
Modify program P9_1b.cpp such that it reads some number of integers as defined by SIZE, stores them in array numlist, displays the array numlist, then reverses the contents of the array, and at last displays the contents of the reversed array. Call your program ex91.cpp. Make your program as general as possible.  Thus, your program should be able to reverse the contents of an array of any size defined by SIZE.  Note that I asked to display the array twice, the first time in the original content, the second time with the reverse content.  Contrary to the program P9 _1b.cpp which only displays the array in reverse order, you must effectively reverse the contents of the array, then display the reversed array.  Example:

int A = {1, 2, 4, 5, 8, 2, 0, 9};

After you reverse the contents of array A, that array would become: {9, 0, 2, 8, 5, 4, 2, 1}.  So, you will display the array A before you reverse the content and after you have reversed the content. Plan ahead how to reverse the array. Remember that after you have reversed the array, the element which used to be first in the original array will be last in the reversed array, the element which used to be second in the original array, will be the one before the last in the reversed array, and so on. Use this reasoning to reverse the array.