How to play with pointers in C


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An array is a contiguous memory location which holds data. The data can be integers, characters, floating point numbers, structures etc. Each array element takes a distinct memory location. They all have distinct address.

In the following example, int32_t data type is used. int32_t represents 32 bit signed integer and is defined in stdint.h header file in supported compilers. While int data type doesn't guarantee about its size, int32_t is guaranteed to be 4 bytes long in every compiler that supports it. If your compiler doesn't support int32_t data type, try an integer type (int, long, long long) which is 32 bit wide in your environment. Although according to C standard compilers aren't forced to implement int32_t data type, to make the size of integers predictable, int32_t type is used in following examples in this tutorial.

Notice that each array element is 4 bytes long and takes contiguous memory locations. If &arr[0] is 10, then &arr[1] is 14, and so on. Do you know that replacing &arr[i] with &i[arr] also works in the example above? Try this! Caution: Try this for fun. Don't write &i[arr] in your final code of your coding project as &i[arr] is less intuitive to get the address of the i-th element in array arr.

It works because for compiler arr[i] and i[arr] are same operation. Compiler converts arr[i] to arr + i and similarly i[arr] to i + arr. Since arr + i and i + arr produce same result, so do arr[i] and i[arr].

What does arr + i mean? It is the address of the i-th element of the memory location pointed to by arr. This sounds like arr is a pointer. In fact, an array name in C acts as a pointer to the first element of the array!

Try the following example and notice that &arr[i] and arr + i are producing the same address:

Explanation in words:

arr + 0 = address of 1st element of array arr
arr + 1 = address of 2nd element of array arr
arr + 2 = address of 3rd element of array arr
arr + n = address of (n + 1)-st element of array arr
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