Subscript Operator:


The latest version of this topic can be found at Subscript Operator:.


postfix-expression [ expression ]  

A postfix expression (which can also be a primary expression) followed by the subscript operator, [ ], specifies array indexing.

For information about managed arrays, see Arrays.

Usually, the value represented by postfix-expression is a pointer value, such as an array identifier, and expression is an integral value (including enumerated types). 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. Consider the following code fragment:

int nArray[5] = { 0, 1, 2, 3, 4 };  
cout << nArray[2] << endl;            // prints "2"  
cout << 2[nArray] << endl;            // prints "2"  

In the preceding example, the expression nArray[2] is identical to 2[nArray]. The reason is that the result of a subscript expression e1[ e2 ] is given by:

*( ( e2 ) + (e1) )

The address yielded by the expression is not e2 bytes from the address e1. Rather, the address is scaled to yield the next object in the array e2. For example:

double aDbl[2];  

The addresses of aDb[0] and aDb[1] are 8 bytes apart — the size of an object of type double. This scaling according to object type is done automatically by the C++ language and is defined in Additive Operators where addition and subtraction of operands of pointer type is discussed.

A subscript expression can also have multiple subscripts, as follows:

expression1 [expression2] [expression3]...

Subscript expressions associate from left to right. The leftmost subscript expression, expression1[expression2], is evaluated first. The address that results from adding expression1 and expression2 forms a pointer expression; then expression3 is added to this pointer expression to form a new pointer expression, and so on until the last subscript expression has been added. The indirection operator (*) is applied after the last subscripted expression is evaluated, unless the final pointer value addresses an array type.

Expressions with multiple subscripts refer to elements of multidimensional arrays. A multidimensional array is an array whose elements are arrays. For example, the first element of a three-dimensional array is an array with two dimensions. The following example declares and initializes a simple two-dimensional array of characters:

// expre_Subscript_Operator.cpp  
// compile with: /EHsc  
#include <iostream>  
using namespace std;  
#define MAX_ROWS 2  
#define MAX_COLS 2  
int main() {  
   char c[ MAX_ROWS ][ MAX_COLS ] = { { 'a', 'b' }, { 'c', 'd' } };  
   for ( int i = 0; i < MAX_ROWS; i++ )  
      for ( int j = 0; j < MAX_COLS; j++ )  
         cout << c[ i ][ j ] << endl;  

The first element of an array is element 0. The range of a C++ array is from array[0] to array[size – 1]. However, C++ supports positive and negative subscripts. Negative subscripts must fall within array boundaries; if they do not, the results are unpredictable. The following code shows positive and negative array subscripts:

#include <iostream>  
using namespace std;  
int main() {  
    int intArray[1024];  
    for (int i = 0, j = 0; i < 1024; i++)  
        intArray[i] = j++;  
    cout << intArray[512] << endl;// 512  
    int *midArray = &intArray[512];  // pointer to the middle of the array  
    cout << midArray[-256] << endl;   // 256  
    cout << intArray[-256] << endl; // unpredictable  

The negative subscript in the lasta line can produce a run-time error because it points an address 256 bytes lower in memory than the origin of the array. The pointer midArray is initialized to the middle of intArray; it is therefore possible to use both positive and negative array indices on it. Array subscript errors do not generate compile-time errors, but they yield unpredictable results.

The subscript operator is commutative. Therefore, the expressions array[index] and array[array] are guaranteed to be equivalent as long as the subscript operator is not overloaded (see Overloaded Operators). The first form is the most common coding practice, but either works.

Postfix Expressions
C++ Operators
C++ Built-in Operators, Precedence and Associativity
One-Dimensional Arrays
Multidimensional Arrays