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].