Dynamic Data Structures H&K Chapter 14 Instructor – Gokcen Cilingir Cpt S 121 (July 26, 2011) Washington State University

Dynamic Data Structures (1) Structures that expand and contract as a program executes Using dynamic data structures, you can allocate memory as needed (no need to constraint the maximum size of an array at compile time) Dynamic memory allocation allows us to dynamically allocate an array at run time. Image source:

Dynamic Data Structures (2) When allocating memory dynamically, a program can use the space to store any of the simple and structured data types (i.e. struct s, int s, double s, etc.) We can combine separately allocated structured blocks (called nodes) to form composite structures that expand and contract while the program executes. 527 head A node containing a data field and a pointer to the next node

C. Hundhausen, A. O’Fallon4 A pointer variable contains the address of another cell containing a data value Note that a pointer is “useless” unless we make sure that it points somewhere: ◦ int num = 3, int *nump = # The direct value of num is 3, while the direct value of nump is the address (1000) of the memory cell which holds the 3 Pointer Review I 3 numpnum

C. Hundhausen, A. O’Fallon5 Pointer Review II ReferenceExplanationValue numDirect value of num3 numpDirect value of nump1000 *numpIndirect value of nump3 &numpAddress of nump numpnum

6 Pointers as Function Parameters Recall that we define an output parameter to a function by passing the address (&) of the variable to the function The output parameter is defined as a pointer in the formal parameter list

7 Pointers Representing Arrays and Strings Consider these variable declarations: double gradeList[20]; char name[40]; When we want to pass either of these arrays to functions, we use the array name with no subscript average (gradeList); reverse (name); C interprets the array name as the address of the initial array element, so a whole array is always passed to a function as a pointer! Therefore, if we were passing an array to a function, the corresponding formal parameter might be declared in two ways. So, all of the following prototypes are valid for the above functions: double average (double list[]); double average (double *list); void reverse (char name[]); void reverse (char *name); array declarations pass an array to a function arrays as formal parameters

8 Pointers to Structures Recall that when we have a pointer to a structure, we can use the indirect component selection operator -> to access components within the structure typedef struct { double grade; int age; } Student; int main (void) { Student p1, *struct_ptr; p1.grade = 95; p1.age = 20; struct_ptr = &p1; struct_ptr->x; // Access grade field in Student. }

C. Hundhausen, A. O’Fallon9 malloc function (1) Dynamic memory allocation can be performed with the help of the function malloc() found in malloc() has the following prototype: void *malloc (size_t size); The argument to malloc is a number indicating the amount of memory space needed to be allocated We can apply the sizeof operator to a data type to determine the number (the number represents the number of bytes needed)

C. Hundhausen, A. O’Fallon10 malloc function (2) The statement: malloc (sizeof (int)) allocates exactly enough space to hold one integer A call to malloc returns a pointer to the block of memory allocated (if no memory can be allocated, NULL is returned) The return type is void * which is generic, thus we need to assign the pointer to a specific type by explicitly typecasting: int *nump = (int *) malloc (sizeof (int)); char *chp = (char *) malloc (sizeof (char));

C. Hundhausen, A. O’Fallon, G. Cilingir11 malloc function (3) The area in which the dynamic memory is allocated is called the heap The stack is the area that stores function data when functions are entered ◦ Remember that the memory allocated for a given function's data (on the stack) goes away when the function is exited Function data areaHeap nump chp

C. Hundhausen, A. O’Fallon12 Accessing a Component of a Dynamically Allocated Structure Accessing a component of a dynamically allocated structure is done in the same way as accessing a component of a statically allocated structure: Student *struct_ptr = (Student *) malloc (sizeof (Student)); struct_ptr->grade is equivalent to (*struct_ptr).grade

C. Hundhausen, A. O’Fallon13 Dynamic Array Allocation with calloc calloc() may be used to allocate contiguous blocks of memory for array elements The function is also found in The function prototype for calloc() is as follows: void *calloc (size_t nitems, size_t size); The two arguments to calloc() are the number of array elements needed and the size of one element, respectively.

C. Hundhausen, A. O’Fallon14 calloc As with malloc, calloc also returns a void * to the beginning of the allocated blocks of memory Thus, we can allocate 10 blocks of contiguous memory as follows: Student *struct_ptr = (Student *) calloc (10, sizeof (Student)); Note the above statement is similar to Student array[10]; except that it is allocated dynamically, instead of statically

C. Hundhausen, A. O’Fallon15 Applying Array Notation to Pointers If we executed the calloc statement on the previous slide, we could then use array notation to access each element in the block of dynamically allocated memory: ◦ struct_ptr[0] would access the first element in the array and struct_ptr[9] would access the last element in the array

C. Hundhausen, A. O’Fallon16 Freeing Allocated Memory We need to be able to give up memory after we are done using it We can use the function free() to return the allocated memory to the heap For example: free (struct_ptr); returns the dynamically-allocated memory block pointed to by struct_ptr Memory leaks occur if we do not free memory before the local variable that accesses the allocated memory goes out of scope Memory leaks are a common source of run-time program bugs; they can also slow down a program's execution

17 References J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (6 th Ed.), Addison- Wesley, 2010 P.J. Deitel & H.M. Deitel, C How to Program (5 th Ed.), Pearson Education, Inc., 2007.