1. Starting out with C++1 Chapter 9 – Pointers Getting the address of a Variable Why do we have pointers? Indirection – difference between –Will you go out with me? –Will you go out with person whose name is on cafeteria door? The address operator (&) returns the memory address of a variable.

2. Starting out with C++2 Addresses: for our machines: char 1 int 4, short int 2 float 4

3. Starting out with C++3 Use of & Operator – Program // This program uses the & operator to determine a variable’s // address and the sizeof operator to determine its size. sizeof(type) is also okay sizeof(array) gives length of types * number in array sizeof(pointer) gives length of pointer itself (4 for us) #include void main ( void ) { int arr[10]; int X = 25; int * a = new int[10]; cout << "The address of X is " << &X << endl; cout << "The size of X is " << sizeof(X) << " bytes\n"; cout << "The value in X is " << X << endl; } // main

4. Starting out with C++4 Pointer Variables Pointer variables, which are often just called pointers, are designed to hold memory addresses. With pointer variables you can indirectly manipulate data stored in other variables.

5. Starting out with C++5 Pointer Variables – Program // This program stores the address of a variable in a pointer. #include void main ( void ) { int X = 25; int *Ptr; //I like to read this backwards Ptr = &X; // Store the address of X in Ptr cout << "The value in X is " << X << endl; cout << "The address of X is " << Ptr << endl; } // main Note * and & are overloaded operators – you have seen them for other uses.

6. Starting out with C++6 Draw pictures to help you understand: int num1, num2=10; float f1=2.5; float * fptr; int * iptr; iptr = &num2; *iptr = 50; fptr = & f1; num1 = num2 + *iptr * *fptr; What is the value of num1?

7. Starting out with C++7 Pointer Variables What about taking the address of an address? int ** addP; addP = &Ptr Only works if address is actually stored somewhere

8. Starting out with C++8 Indirection Operator – Program // This program demonstrates the use of the indirection // operator. #include void main ( void ) { int X = 25; int *Ptr; Ptr = &X; // Store the address of X in Ptr cout << "Here is the value in X, printed twice:\n"; cout << X << " " << *Ptr << endl; *Ptr = 100; // Note indirection is a unary operator cout << "Once again, here is the value in X:\n"; cout << X << " " << *Ptr << endl; } // main

9. Starting out with C++9 Note int * x,y is the same as int *x; int y;

10. Starting out with C++10 At Seats Write the code to read in an integer, access it via a pointer, add five to it, and then print it out. Why are we using pointers? No real purpose here – just to show we can and to gain experience.

11. Starting out with C++11 Relationship Between Arrays and Pointers Array names can be used as pointers, and vice-versa. This is important in understanding how arrays are passed. This is also important in allocating dynamic arrays.

12. Starting out with C++12 Write a function void clear_array(int array[], int size) which sets each element of the array to zero. Try it three different ways (two with pointers)

13. Starting out with C++13 Here is the simplest way void clear(int a[], int size){ for (int i =0;i < size; i++) a[i] = 0; }

14. Starting out with C++14 void clear(int a[], int size){ int * p; for (int* p=a;p < a+size; p++) *p = 0; }

15. Starting out with C++15 void clear(int a[], int size){ for (int i =0;i < size; i++) *(a+i) = 0; }

16. Starting out with C++16 At seats Write a function int replace_char(char *str, char old_char, char new_char) which replaces each occurrence of the character old_char with the character new_char in the string str. The function should return the number of characters actually replaced.

17. Starting out with C++17 Repeat the previous exercise accessing the array via pointers

18. Starting out with C++18 Arrays and Pointers – Program #include void main ( void ) {short Numbers[] = {10, 20, 30, 40, 50}; // short *Numbers; would be similar, but no space allocated // an array of short and a pointer to short are the // SAME type to the compiler cout << "The first Oth element of the array is "; cout << *Numbers << endl; cout << "The third element of the array is "; cout << *(Numbers + 2) << endl; // Note, smart enough to add not just 2 but 2 times // the length of the type pointed to! } // main

19. Starting out with C++19 Using Pointer Arithmetic

20. Starting out with C++20 Pointers and Arrays – Program #include void main ( void ) { int Numbers[5]; cout << "Enter five numbers: "; for ( int Count = 0; Count < 5; Count++ ) cin >> *(Numbers + Count); cout << "Here are the numbers you entered:\n"; for ( int Count = 0; Count < 5; Count++ ) cout << *(Numbers + Count)<< " "; cout << endl; } // main

21. Starting out with C++21 Consider these two swaps. What happens when you pass two pointers to each? void swap(int *p, int*q){ int * t = p; p = q; q = t; } void swap2( int *p, int *q){ int *t = new int(); *t = *p; *p = *q; *q = *t; }

22. Starting out with C++22 Dynamic Arrays – Program float *Sales; int NumDays; Sales = new float[NumDays]; … delete [] Sales; float Sales2[10]; // Sales2 is a constant pointer

23. Starting out with C++23 At seats Define an int* pointer variable a. Then: (1) Use new to make a point to a dynamic array of 5 cells of type int. (2) Write a loop to fill a with values 3, 7, 11, 15, 19.

24. Starting out with C++24 At seats Write code to read in the size of an array from input. Allocate an array of strings of that size Print them out backwards (last string first) using pointer variables (rather than subscripts)

25. Starting out with C++25 Try this: Assume int x[5] = {0, 1, 2, 3, 4}. What does print3(&x[0]) print? print3(&x[2])? print3(&x[4])? void print3(int x[]) { int i; for (i = 0; i < 3; i++) cout << x[i] << " "; cout << endl; }

26. Starting out with C++26 Pointer Arithmetic Some mathematical operations may be performed on pointers. –The ++ and -- operators may be used to increment or decrement a pointer variable. –An integer may be added to or subtracted from a pointer variable. This may be performed with the +, -, +=, or -= operators. –A pointer may be subtracted from another pointer. (gives number of items between them not physical length)

27. Starting out with C++27 Pointer Arithmetic – Program #include void main ( void ) {int Set[8] = {5, 10, 15, 20, 25, 30, 35, 40}; int *Nums, Index; Nums = Set; cout << "The numbers in Set are:\n"; for ( Index = 0; Index < 8; Index++ ) { cout << *Nums << " "; Nums++; } // for cout << "\nThe numbers in Set backwards are:\n"; for ( Index = 0; Index < 8; Index++ ) { Nums--; cout << *Nums << " "; } // for } // main

28. Starting out with C++28 Comparing Pointers If one address comes before another address in memory, the first address is considered “less than” the second. C++’s relational operators maybe used to compare pointer values.

29. Starting out with C++29 Pointers as Function Parameters A pointer can be used as a function parameter. It gives the function access to the original argument, much like a reference parameter does.

30. Starting out with C++30 Dynamic Memory Allocation Variables may be created and destroyed while a program is running. This is help for linked lists. A pointer than contains the address 0 is called a null pointer. Use the new operator to dynamically allocate memory. Use delete to dynamically deallocate memory.