C++ Primer Plus

You accomplish this in C++ by modifying the declaration for a simple variable and adding brackets that contain the number of elements. For example, the following declaration creates an array named months that has 12 elements, each of which can hold a type short value:

short months[12];     // creates array of 12 short

Each element, in essence, is a variable that you can treat as a simple variable.

This is the general form for declaring an array:

typeName arrayName[arraySize];

The expression arraySize, which is the number of elements, must be an integer constant, such as 10 or a const value, or a constant expression, such as 8 * sizeof (int), for which all values are known at the time compilation takes place. In particular, arraySize cannot be a variable whose value is set while the program is running. However, later in this chapter you’ll learn how to use the new operator to get around that restriction.

The Array as Compound Type

An array is called a compound type because it is built from some other type. (C uses the term derived type, but because C++ uses the term derived for class relationships, it had to come up with a new term.) You can’t simply declare that something is an array; it always has to be an array of some particular type. There is no generalized array type. Instead, there are many specific array types, such as array of char or array of long. For example, consider this declaration:

float loans[20];

The type for loans is not “array”; rather, it is “array of float.” This emphasizes that the loans array is built from the float type.

Much of the usefulness of the array comes from the fact that you can access array elements individually. The way to do this is to use a subscript, or an index, to number the elements. C++ array numbering starts with zero. (This is nonnegotiable; you have to start at zero. Pascal and BASIC users will have to adjust.) C++ uses a bracket notation with the index to specify an array element. For example, months[0] is the first element of the months array, and months[11] is the last element. Note that the index of the last element is one less than the size of the array (see Figure 4.1). Thus, an array declaration enables you to create a lot of variables with a single declaration, and you can then use an index to identify and access individual elements.

Figure 4.1. Creating an array.

The Importance of Valid Subscript Values

The compiler does not check to see if you use a valid subscript. For instance, the compiler won’t complain if you assign a value to the nonexistent element months[101]. But that assignment could cause problems when the program runs, possibly corrupting data or code, possibly causing the program to abort. So it is your responsibility to make sure that your program uses only valid subscript values.

The yam analysis program in Listing 4.1 demonstrates a few properties of arrays, including declaring an array, assigning values to array elements, and initializing an array.

Listing 4.1. arrayone.cpp

// arrayone.cpp -- small arrays of integers

#include

int main()

{

    using namespace std;

    int yams[3];    // creates array with three elements

    yams[0] = 7;    // assign value to first element

    yams[1] = 8;

    yams[2] = 6;

    int yamcosts[3] = {20, 30, 5}; // create, initialize array

// NOTE: If your C++ compiler or translator can't initialize

// this array, use static int yamcosts[3] instead of

// int yamcosts[3]

    cout << "Total yams = ";

    cout << yams[0] + yams[1] + yams[2] << endl;

    cout << "The package with " << yams[1] << " yams costs ";

    cout << yamcosts[1] << " cents per yam.\n";

    int total = yams[0] * yamcosts[0] + yams[1] * yamcosts[1];

    total = total + yams[2] * yamcosts[2];

    cout << "The total yam expense is " << total << " cents.\n";

    cout << "\nSize of yams array = " << sizeof yams;

    cout << " bytes.\n";

    cout << "Size of one element = " << sizeof yams[0];

    cout << " bytes.\n";

    return 0;

}

Here is the output from the program in Listing 4.1:

Total yams = 21

The package with 8 yams costs 30 cents per yam.

The total yam expense is 410 cents.

Size of yams array = 12 bytes.

Size of one element = 4 bytes.

Program Notes

First, the program in Listing 4.1 creates a three-element array called yams. Because yams has three elements, the elements are numbered from 0 through 2, and arrayone.cpp uses index values of 0 through 2 to assign values to the three individual elements. Each individual yam element is an int with all the rights and privileges of an int type, so arrayone.cpp can, and does, assign values to elements, add elements, multiply elements, and display elements.

The program uses the long way to assign values to the yam elements. C++ also lets you initialize array elements within the declaration statement. Listing 4.1 uses this shortcut to assign values to the yamcosts array:

int yamcosts[3] = {20, 30, 5};