CSC209: arrays & pointers

type in array

all elements in an array need to be the same type

allocation of arrays

need to allocate space for all the elements in the array

eg. an array with 4 int elements would be initialized like int arr[4];

what happens when you call arr?

you receive a pointer to the first element of the array

how to instantiate an array?

int arr[3] = {0, 1, 2};

it is cleaner to instantiate all the elements at the beginning and this is the only time you can do so, afterwards you must instantiate each element individually

size of array

an array cannot change size, if you need a larger array you would need to make a new one and copy all the elements into the new array from the original one

how to iterate through an array?

once we know the address of where the array starts, and the size of each element, we can calculate the address of each element

we rely on this fact when we use indexes to access elements of the array

big issue with arrays

C does not check that an array access is within the bounds of the array

what is a pointer?

stores the address of a variable

how to declare a pointer?

int *pt;

this will create a pointer that points at an integer

how to store an address in a pointer?

int i;
int *pt;
pt = &i;

using “&” allows you to access the address of a variable

how to access the value that is pointed to by a pointer?

int i = 209;
int *pt = &i;
printf("Value pointed to by pt: %d\n", *pt);

using “*” in this context is called dereferencing a pointer

i and *pt are aliases

how do changes persist?

when we pass a pointer to the first element of an array rather than passing a copy of the array

arithmetic on a pointer

when we use the + operator on a pointer, its behaviour depends on the type of the pointer

pointer to pointer

to initialize and dereference we need to use **