1 Lecture 25:Pointers Introduction to Computer Science Spring 2006

2 Contents Review searching algorithm on an ordered list Pointers

3 Contents Review searching algorithm on an ordered list Pointers

4 Sequential Search on an Ordered List General form of sequential search algorithm on a sorted list:

5 int seqOrderedSearch(const int list[], int listLength, int searchItem) { int loc;//Line 1 bool found = false;//Line 2 for (loc = 0; loc < listLength; loc++)//Line 3 if (list[loc] >= searchItem)//Line 4 { found = true;//Line 5 break;//Line 6 } if (found)//Line 7 if (list[loc] == searchItem)//Line 8 return loc;//Line 9 else//Line 10 return -1;//Line 11 else//Line 12 return -1;//Line 13 } Sequential Search on an Ordered List

6 Binary Search Binary search can be applied to sorted lists Uses the “ divide and conquer ” technique Compare search item to middle element mid= (first+last)/2 first is the starting index of the search list last is the ending index of the search list mid is the index of the middle element of the search list If search item is less than middle element, restrict the search to the lower half of the list Otherwise search the upper half of the list

7 Search 75 in the following list: search list

8 int binarySearch(const int list[], int listLength, int searchItem) { int first = 0; int last = listLength - 1; int mid; bool found = false; while(first <= last && !found) { mid = (first + last) / 2; if(list[mid] == searchItem) found = true; else if(list[mid] > searchItem) last = mid - 1; else first = mid + 1; } if(found) return mid; else return -1; } Binary Search

9 Contents Review searching algorithm on an ordered list Pointers

10 Three categories of data type Simple data type Structured data type Pointer

11 Pointer Variables int count = 5; The value "5" is stored in memory and can be accessed by using the variable "count". Pointer variable: content is a memory address Declaring Pointer Variables dataType *identifier; int *p; char *ch;

12 Pointer Variables (continued) These statements are equivalent int *p; The character * can appear anywhere between type name and variable name int* p, q; Only p is the pointer variable, not q. Here q is an int variable To avoid confusion, attach the character * to the variable name int *p, q; The following statement declares both p and q to be pointer variables of the type int int *p, *q;

13 Address Of Operator (&) Example: int count = 5; int *p; The ampersand, &, is called the address of operator p = &count; The address of operator is a unary operator that returns the address of its operand

14 Dereferencing Operator (*) C++ uses * as the binary multiplication operator and as a unary operator When used as a unary operator, * Called dereferencing operator or indirection operator Refers to object to which its operand (that is, a pointer) points Example: int count = 5; int *p; p = &count; // Stores the address of count in p // The unary operator & returns the address of a variable int total; total = *p; // The value in the address stored in p is assigned to total

15 int *p; int num; num=78;p = &num;*p = 24;

16 An application of pointer: parameter passed by address void swap(int *x, int *y) { int tmp=*x; *x=*y; *y=tmp; } int a=1, b=2; swap(&a, &b); 1 ab 2 xy tmp=1 21

17 Difference between pointers and references Reference is always the alias of some variable, but pointer does not have to be. –int a; int &ra=a; –int *pa; //pa is not an alias of any variable, we call it dangling. Reference can not refer other variable after it is defined. –int a, b; int &ra=a; ra=b; //ra is not the alias of b. Here, ra=b is equivalent to a=b –int *pa=&x; pa=&y; //pa refers to y now pointer supports arithmetic operation, reference does not. Conclusion: reference is more restricted, more safe than pointer. But Pointer is more powerful than reference.

18 Pointer Arithmetic Operations of Pointers: Add a number p=p+1; or p++ //assume p is a pointer Subtract a number p=p-1 or p--; //assume p is a pointer Taken the difference of two pointers int d=p1-p2 //assume p1, p2 are pointers

19 The meaning of Pointer Arithmetic p p+3p-2 q q-p=5

20 Another application of pointer: access array by using pointer 100 int A[10]; int *p = &A[0]; *(p+4)=100; p

21 Other applications of Pointers Pointers of other data type Pointer of array, pointer of function, pointer of pointer. Dynamic memory allocation Heap based memory allocation. We skip these issues because time limit.

22 End of lecture 23 Thank you!