3.5.2. Accessing the Elements of an Array

As with the library vector and string types, we can use a range for or the subscript operator to access elements of an array. As usual, the indices start at 0. For an array of ten elements, the indices are 0 through 9, not 1 through 10.

When we use a variable to subscript an array, we normally should define that variable to have type size_t . size_t is a machine-specific unsigned type that is guaranteed to be large enough to hold the size of any object in memory. The size_t type is defined in the cstddef header, which is the C++ version of the stddef.h header from the C library.

With the exception that arrays are fixed size, we use arrays in ways that are similar to how we use vectors. For example, we can reimplement our grading program from § 3.3.3 (p. 104) to use an array to hold the cluster counters:

// count the number of grades by clusters of ten: 0--9, 10--19, ... 90--99, 100
unsigned scores[11] = {}; // 11 buckets, all value initialized to 0
unsigned grade;
while (cin >> grade) {
    if (grade <= 100)
        ++scores[grade/10]; // increment the counter for the current cluster
}

The only obvious difference between this program and the one on page 104 is the declaration of scores. In this program scores is an array of 11 unsigned elements. The not so obvious difference is that the subscript operator in this program is the one that is defined as part of the language. This operator can be used on operands of array type. The subscript operator used in the program on page 104 was defined by the library vector template and applies to operands of type vector.

As in the case of string or vector, it is best to use a range for when we want to traverse the entire array. For example, we can print the resulting scores as follows:

for (auto i : scores)      // for each counter in scores
    cout << i << " ";      // print the value of that counter
cout << endl;

Because the dimension is part of each array type, the system knows how many elements are in scores. Using a range for means that we don’t have to manage the traversal ourselves.

Checking Subscript Values

As with string and vector, it is up to the programmer to ensure that the subscript value is in range—that is, that the index value is equal to or greater than zero and less than the size of the array. Nothing stops a program from stepping across an array boundary except careful attention to detail and thorough testing of the code. It is possible for programs to compile and execute yet still be fatally wrong.