16.1 Accessing Array Elements

If the variable a is an array, the nth element of a is a[n]. You can use that expression to access an element’s value or to assign to it:

x = a[5];
a[6] = 1;

Since the variable a is an lvalue, a[n] is also an lvalue.

The lowest valid index in an array is 0, not 1, and the highest valid index is one less than the number of elements.

The C language does not check whether array indices are in bounds, so if the code uses an out-of-range index, it will access memory outside the array.

Warning: Using only valid index values in C is the programmer’s responsibility.

Array indexing in C is not a primitive operation: it is defined in terms of pointer arithmetic and dereferencing. Now that we know what a[i] does, we can ask how a[i] does its job.

In C, x[y] is an abbreviation for *(x+y). Thus, a[i] really means *(a+i). See Pointers and Arrays.

When an expression with array type (such as a) appears as part of a larger C expression, it is converted automatically to a pointer to element zero of that array. For instance, a in an expression is equivalent to &a[0]. Thus, *(a+i) is computed as *(&a[0]+i).

Now we can analyze how that expression gives us the desired element of the array. It makes a pointer to element 0 of a, advances it by the value of i, and dereferences that pointer.

Another equivalent way to write the expression is (&a[0])[i].