One-Dimensional Arrays
A postfix expression followed by an expression in square brackets ([ ]) is a subscripted representation of an element of an array object. A subscript expression represents the value at the address that is expression positions beyond postfix-expression when expressed as
postfix-expression[expression]
Usually, the value represented by postfix-expression is a pointer value, such as an array identifier, and expression is an integral value. However, all that is required syntactically is that one of the expressions be of pointer type and the other be of integral type. Thus the integral value could be in the postfix-expression position and the pointer value could be in the brackets in the expression, or "subscript," position. For example, this code is legal:
// one_dimensional_arrays.c
int sum, *ptr, a[10];
int main() {
ptr = a;
sum = 4[ptr];
}
Subscript expressions are generally used to refer to array elements, but you can apply a subscript to any pointer. Whatever the order of values, expression must be enclosed in brackets ([ ]).
The subscript expression is evaluated by adding the integral value to the pointer value, then applying the indirection operator (*) to the result. (See Indirection and Address-of Operators for a discussion of the indirection operator.) In effect, for a one-dimensional array, the following four expressions are equivalent, assuming that a is a pointer and b is an integer:
a[b]
*(a + b)
*(b + a)
b[a]
According to the conversion rules for the addition operator (given in Additive Operators), the integral value is converted to an address offset by multiplying it by the length of the type addressed by the pointer.
For example, suppose the identifier line refers to an array of int values. The following procedure is used to evaluate the subscript expression line[ i ]:
The integer value i is multiplied by the number of bytes defined as the length of an int item. The converted value of i represents i int positions.
This converted value is added to the original pointer value (line) to yield an address that is offset i int positions from line.
The indirection operator is applied to the new address. The result is the value of the array element at that position (intuitively, line [ i ]).
The subscript expression line[0] represents the value of the first element of line, since the offset from the address represented by line is 0. Similarly, an expression such as line[5] refers to the element offset five positions from line, or the sixth element of the array.