Pointer and Dynamic Array Summary:

· General:

o A pointer variable holds an address.

o A pointer variable that points to a pointer variable holds an address that in turn points to an address.

o A Multi-dimensional array variable is a pointer to a pointer, because each row is referred to by a pointer.

o Print addresses with %p format character.

· Declare a pointer:

o Read backwards: double * p means: p is a pointer to a double

o Not dynamic: point to a variable that was created with a normal <type> <variable name> declaration

§ double x

§ double * p = &x

o Dynamic:

§ Create a pointer with a null value, and then assign a block of memory using malloc in the heap and then store the starting address in your pointer:

· double * p1;

· p1 = (double *) malloc(sizeof(int));

o (note cast of void pointer malloc returns to the type of pointer you want.)

o If malloc returns NULL, no memory was assigned. - You must check

§ When you are done using memory you grabbed with malloc, you must free it.

· free(p1);

§ Set pointer to NULL when it point to nothing: p2 = NULL;

· Use a pointer:

o Use the address: Just the variable name : ptr

o Set pointers to either the value of another pointer (such as an array) or the address of another variable (any type, even a pointer).

o Use the value at the address: deference the pointer : *ptr

o Get the address of another variable: &varname

o Remember that the array variable is a pointer, and the cells inside the array are not (unless you declare an array of pointers).

o You cannot dereference a value variable : cannot int x and then *x = 6

o Know whether you are dealing with a pointer or a value variable, and be able to set address or value as appropriate for the problem you are solving

§ Ex: scanf does not need the & for a pointer

§ Ex: printf does need a * for a pointer when not using %p so the pointed at value can print

· Pass to functions:

o Passing a pointer is called passing by reference

o Nothing in C passes by reference without you insisting that a pointer be passed (except you can say arrays naturally pass by pointer because arrays are pointers).

· Pointers and arrays

o An array variable is a pointer, but it also knows the full size of the array

o Making a pointer to an array (such as when you pass an argument to a function) only knows the size of the pointer and the address of the first element of the array.

o Pointers to arrays can use pointer arithmetic, moving through elements by adding 1.

o A dynamic array is created by using malloc to allocate the memory for the array, and then putting the address malloc returns into a pointer.

o For multidimensional dynamic arrays, malloc the memory for the array of pointer to each row, and then malloc each row. Remember to free all that you create.

Typedef:

· Like creating a variable type name, but really just substitute words

o Ex:

§ In heading of program: typdef int * IntPtr

§ Later use: IntPtr x

§ Means: int * x