7.2. Arrays

An array is a consecutive group of memory locations that all have the same type. To refer to a particular location or element in the array, we specify the name of the array and the position number of the particular element in the array.

Figure 7.1 shows an integer array called c. This array contains 12 elements. A program refers to any one of these elements by giving the name of the array followed by the position number of the particular element in square brackets ([]). The position number is more formally called a subscript or index (this number specifies the number of elements from the beginning of the array). The first element in every array has subscript 0 (zero) and is sometimes called the zeroth element. Thus, the elements of array c are c[0] (pronounced "c sub zero"), c[1], c[2] and so on. The highest subscript in array c is 11, which is 1 less than the number of elements in the array (12). Array names follow the same conventions as other variable names, i.e., they must be identifiers.

A subscript must be an integer or integer expression (using any integral type). If a program uses an expression as a subscript, then the program evaluates the expression to determine the subscript. For example, if we assume that variable a is equal to 5 and that variable b is equal to 6, then the statement

c[ a + b ] += 2;

adds 2 to array element c[ 11 ]. Note that a subscripted array name is an lvalue—it can be used on the left side of an assignment, just as nonarray variable names can.

Let us examine array c in Fig. 7.1 more closely. The name of the entire array is c. Its 12 elements are referred to as c[0] to c[ 11 ]. The value of c[0] is -45, the value of c[1] is 6, the value of c[2] is 0, the value of c[7] is 62, and the value of c[ 11 ] is 78. To print the sum of the values contained in the first three elements of array c, we'd write

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

To divide the value of c[ 6 ] by 2 and assign the result to the variable x, we would write

x = c[ 6 ] / 2;

Common Programming Error 7.1

Note the difference between the "seventh element of the array" and "array element 7." Array subscripts begin at 0, so the "seventh element of the array" has a subscript of 6, while "array element 7" has a subscript of 7 and is actually the eighth element of the array. Unfortunately, this distinction frequently is a source of off-by-one errors. To avoid such errors, we refer to specific array elements explicitly by their array name and subscript number (e.g., c[6] or c[7]).

The brackets used to enclose the subscript of an array are actually an operator. Brackets have the same level of precedence as parentheses. Figure 7.2 shows the precedence and associativity of the operators introduced so far. Note that brackets ([]) have been added to the first row of Fig. 7.2. The operators are shown top to bottom in decreasing order of precedence with their associativity and type.