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Chapter 7 Pointers

Chapter 7 Pointers. Associate Prof. Yuh-Shyan Chen Dept. of Computer Science and Information Engineering National Chung-Cheng University. Outline. Outline 7.1 Introduction 7.2 Pointer Variable Declarations and Initialization 7.3 Pointer Operators 7.4 Calling Functions by Reference

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Chapter 7 Pointers

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  1. Chapter 7Pointers Associate Prof. Yuh-Shyan Chen Dept. of Computer Science and Information Engineering National Chung-Cheng University

  2. Outline Outline 7.1 Introduction 7.2 Pointer Variable Declarations and Initialization 7.3 Pointer Operators 7.4 Calling Functions by Reference 7.5 Using the Const Qualifier with Pointers 7.6 Bubble Sort Using Call by Reference 7.7 Pointer Expressions and Pointer Arithmetic 7.8 The Relationship between Pointers and Arrays 7.9 Arrays of Pointers 7.10 Case Study: A Card Shuffling and Dealing Simulation 7.11 Pointers to Functions

  3. 7.1 Introduction • Pointers • Powerful, but difficult to master • Simulate call-by-reference • Create and manipulate dynamic data structures • Close relationship with arrays and strings

  4. count countPtr count 7 7 7.2 Pointer Variable Declarations and Initialization • Pointer variables • Contain memory addresses as their values • Normal variables contain a specific value (direct reference) • Pointers contain address of a variable that has a specific value (indirect reference) • Indirection - referencing a pointer value

  5. myPtr1 myPtr2 7 7.2 Pointer Variable Declarations and Initialization (II) • Pointer declarations • * used with pointer variables int *myPtr; • Declares a pointer to an int(pointer of type int *) • Multiple pointers, multiple * int *myPtr1, *myPtr2; • Can declare pointers to any data type int *myPtr1; float *myPtr2; • Initialize pointers to 0, NULL, or an address • 0 or NULL - points to nothing (NULL is preferred) int float 1234.5

  6. count count directly references a variable whose value is 7 countPtr indirectly references a variable whose value is 7 countPtr count 7 7 Fig. 7.1 Directly and indirectly referencing a variable

  7. Operator precedence

  8. yptr y y 5 yPtr Address of y is value of yptr 500000 600000 600000 5 7.3 Pointer Operators • & (address operator) • Returns address of operand int y = 5; int *yPtr; yPtr = &y; //yPtr gets address of y • yPtr“points to” y

  9. yptr y y 5 yPtr Address of y is value of yptr 500000 600000 600000 5 Common Programming Error 7.2 • Dereferencing a pointer that has not been initialized or that has not been assigned to point to a specific location in memory. • This could cause a fatal execution time error, or it could accidentally modify important data and allow the program to run to completion providing incorrect results. • int y = 5; • int *yPtr; • yPtr = &y;

  10. yptr y y 5 yPtr Address of y is value of yptr 500000 600000 600000 5 7.3 Pointer Operators (II) • *(indirection/dereferencing operator) • Returns a synonym/alias of what its operand points to *yptr returns y(because yptr points to y) • * can be used for assignment • Returns alias to an object *yptr = 7; // changes y to 7 • Dereferenced pointer (operand of *) must be an lvalue (no constants) • int y = 5; • int *yPtr; • yPtr = &y;

  11. yptr y y 5 yPtr Address of y is value of yptr 500000 600000 600000 5 7.3 Pointer Operators (III) • *and& are inverses • They cancel each other out *&yptr -> * (&yptr) -> * (address of yptr)-> returns alias of what operand points to -> yptr &*yptr -> &(*yptr) -> &(y) -> returns address of y, which is yptr -> yptr • int y = 5; • int *yPtr; • yPtr = &y;

  12. The address of a is the value of aPtr. The * operator returns an alias to what its operand points to. aPtr points to a, so *aPtr returns a. Notice how * and & are inverses

  13. The address of a is 0012FF7C The value of aPtr is 0012FF7C The value of a is 7 The value of *aPtr is 7 Showing that * and & are complements of each other. &*aPtr = 0012FF7C *&aPtr = 0012FF7C

  14. 7.4 Calling Functions by Reference • Call by reference with pointer arguments • Pass address of argument using & operator • Allows you to change actual location in memory • Arrays are not passed with & because the array name is already a pointer • * operator • Used as alias/nickname for variable inside of function void double(int *number) { *number = 2 * (*number); } *number used as nickname for the variable passed

  15. The original value of number is 5 The new value of number is 125

  16. Before calls : main cubeByValue int main() int cubeByValue( int n ) number { { 5 int number = 5 ; return n * n * n; } n number=cubeByValue(number); } undefined After cubeByValue receives the call: int main() int cubeByValue( int n ) number { { int number = 5 ; 5 return n * n * n; } n number = cubeByValue( number ); } 5 After cubes parameter and before returns to : cubeByValue n cubeByValue main int main() int cubeByValue( int n ) number { { 125 int number = 5 ; 5 return n * n * n; } n number = cubeByValue( number ); } 5 Fig. 7.8 Analysis of a typical call-by-value. (Part 1 of 2.)

  17. After returns to cubeByValue main and before assigning the result to number : int main() int cubeByValue( int n ) number { { 5 int number = 5 ; return n * n * n; 125 } n number = cubeByValue( number ); } undefined After main completes the assignment to number : int main() int cubeByValue( int n ) number { { 125 int number = 5 ; return n * n * n; 125 125 } n number = cubeByValue( number ); } undefined Fig. 7.8 Analysis of a typical call-by-value. (Part 2 of 2.)

  18. Notice that the function prototype takes a pointer to an integer. Notice how the address of number is given - cubeByReference expects a pointer (an address of a variable). fig07_07.c Inside cubeByReference, *nPtr is used (*nPtr is number). The original value of number is 5 The new value of number is 125

  19. Before main calls cubeByReference : number int main() void cubeByReference( int *nPtr ) { { 5 int number = 5 ; *nPtr = *nPtr * *nPtr * *nPtr; } nPtr cubeByReference( &number ); } undefined After receives the call and before cubeByReference *nPtr is cubed: number int main() void cubeByReference( int *nPtr ) { { 5 int number = 5 ; *nPtr = *nPtr * *nPtr * *nPtr; } nPtr cubeByReference( &number ); call establishes this pointer } After *nPtr is cubed and before program control returns to main : number int main() void cubeByReference( int *nPtr ) { { 125 125 int number = 5 ; *nPtr = *nPtr * *nPtr * *nPtr; } nPtr cubeByReference( &number ); called function modifies caller’s variable } Fig. 7.9 Analysis of a typical call-by-reference with a pointer argument.

  20. 7.5 Using the Const Qualifier with Pointers • constqualifier - variable cannot be changed • Good idea to have const if function does not need to change a variable • Attempting to change a const is a compiler error • const pointers - point to same memory location • Must be initialized when declared int *const myPtr = &x; • Type int *const - constant pointer to an int const int *myPtr = &x; • Regular pointer to a const int const int *const Ptr = &x; • const pointer to a const int • x can be changed, but not *Ptr

  21. Changing *ptr is allowed – x is not a constant. Changing ptr is an error – ptr is a constant pointer. Compiling... FIG07_13.c D:\books\2003\chtp4\Examples\ch07\FIG07_13.c(15) : error C2166: l-value specifies­ const object Error executing cl.exe. FIG07_13.exe - 1 error(s), 0 warning(s)

  22. Compiling... FIG07_14.c D:\books\2003\chtp4\Examples\ch07\FIG07_14.c(17) : error C2166: l-value specifies­ const object D:\books\2003\chtp4\Examples\ch07\FIG07_14.c(18) : error C2166: l-value specifies­ const object Error executing cl.exe. FIG07_12.exe - 2 error(s), 0 warning(s)

  23. 7.6 Bubble Sort Using Call-by-reference • Implement bubblesort using pointers • Swap two elements • swap function must receive address (using &) of array elements • Array elements have call-by-value default • Using pointers and the * operator, swap can switch array elements • Psuedocode Initialize array print data in original order Call function bubblesort print sorted array Define bubblesort

  24. 7.6 Bubble Sort Using Call-by-reference (II) • sizeof • To determine the size in bytes of an array • For arrays: size of 1 element * number of elements • if sizeof(int) = 4 bytes, then int myArray[10]; printf( "%d", sizeof( myArray ) ); will print 40 • sizeofcan be used with • Variable names • Type name • Constant values

  25. Example of sizeof • To determine the number of elements in the array double real[22]; sizeof (real) / sizeof (double) /* size_t is defined in stdio.h */ size_t getSize ( float * ) size_t getsize ( float *ptr) { reutrn sizeof (ptr); }

  26. Bubblesort gets passed the address of array elements (pointers). The name of an array is a pointer.

  27. Data items in original order 2 6 4 8 10 12 89 68 45 37 Data items in ascending order 2 4 6 8 10 12 37 45 68 89

  28. 7.7 Pointer Expressions and Pointer Arithmetic • Arithmetic operations can be performed on pointers • Increment/decrement pointer (++ or --) • An integer may be added (subtracted) to a pointer( + or +=, - or -=) • Pointers may be subtracted from each other • Operations meaningless unless performed on an array

  29. location 3000 3004 3008 3012 3016 pointer variable vPtr v[0] v[1] v[2] v[4] v[3] 7.7 Pointer Expressions and Pointer Arithmetic (II) • int v[10], *vPtr; /* declare a v array and a pointer variable */ • 5 element int array on machine with 4 byte ints • vPtr points to first element v[0] vPtr = v; at location 3000. (vPtr = 3000) or vPtr = & v[0] • vPtr +=2; /* sets vPtr to 3008 = 3000 + 2 *4 */ • vPtr points to v[2] (incremented by 2), but machine has 4 byte ints.

  30. location 3000 3004 3008 3012 3016 pointer variable vPtr v[0] v[1] v[2] v[4] v[3] 7.7 Pointer Expressions and Pointer Arithmetic (III) • Subtracting pointers • Returns number of array elements from one to the other. • int *vPtr2, *vptr; vPtr2 = &v[2]; vPtr = &v[0]; x = vPtr2 – vPtr; /* x is equal to 2 */ • Pointer comparison ( <, == , > ) • See which pointer points to the higher numbered array element • Also, see if a pointer points to 0

  31. 7.7 Pointer Expressions and Pointer Arithmetic (IV) • Pointers of the same type can be assigned to each other • If not the same type, a cast operator must be used • Exception: pointer to void (type void *) • Generic pointer, represents any type • No casting needed to convert a pointer to void pointer • void pointers cannot be dereferenced

  32. 7.8 The Relationship Between Pointers and Arrays • Arrays and pointers closely related • Array name like a constant pointer • Pointers can do array subscripting operations • Declare an array b[5] and a pointer variable bPtr(int *bPtr;) bPtr = b; Array name actually a address of first element OR bPtr = &b[0]; Explicitly assign bPtr to address of first element

  33. 7.8 The Relationship Between Pointers and Arrays (II) • Element b[n] • can be accessed by *( bPtr + n ) • n - offset (pointer/offset notation) • The address &b[3] can be written with the pointer expression bPtr + 3 • The array itself can be treated as a pointer b[3] same as *(b + 3) • Pointers can be subscripted (pointer/subscript notation) bPtr[3] same as b[3]

  34. Array b printed with: Array subscript notation b[ 0 ] = 10 b[ 1 ] = 20 b[ 2 ] = 30 b[ 3 ] = 40 Pointer/offset notation where the pointer is the array name *( b + 0 ) = 10 *( b + 1 ) = 20 *( b + 2 ) = 30 *( b + 3 ) = 40 Pointer subscript notation bPtr[ 0 ] = 10 bPtr[ 1 ] = 20 bPtr[ 2 ] = 30 bPtr[ 3 ] = 40 Pointer/offset notation *( bPtr + 0 ) = 10 *( bPtr + 1 ) = 20 *( bPtr + 2 ) = 30 *( bPtr + 3 ) = 40

  35. An array name is essentially a constant pointer; it always points to the beginning of the array. Thus, the expression b += 3 is invalid.

  36. location 3000 3001 3002 3003 3004 pointer variable string2 ‘l’ ‘o’ ‘e’ ‘H’ ‘l’ A string is a one-dimensional char array. Literal Pool 3005 ‘\0’ String literal

  37. string1 = Hello string3 = Good Bye

  38. ’\0’ ’\0’ ’\0’ ’\0’ ’n’ ’d’ ’o’ ’u’ ’a’ ’s’ ’d’ ’b’ ’m’ ’H’ ’s’ ’a’ ’D’ ’i’ ’a’ ’s’ ’l’ ’C’ ’r’ ’s’ ’S’ ’p’ ’e’ ’t’ ’e’ suit[0] suit[1] suit[2] suit[3] 7.9 Arrays of Pointers • Arrays can contain pointers - array of strings char *suit[4] = {"Hearts", "Diamonds", "Clubs", "Spades" }; • String: pointer to first character • char * - each element of suit is a pointer to a char • Strings not actually in array - only pointers to string in array • suit array has a fixed size, but strings can be of any size.

  39. 7.11 Pointers to Functions • Pointer to function • Contains address of function • Similar to how array name is address of first element • Function name is starting address of code that defines function • Function pointers can be • Passed to functions • Stored in arrays • Assigned to other function pointers

  40. 7.11 Pointers to Functions (II) • Example: bubblesort • Function bubble takes a function pointer • bubble calls this helper function • this determines ascending or descending sorting • The argument in bubblesort for the function pointer: bool ( *compare )( int, int ) tells bubblesort to expect a pointer to a function that takes two ints and returns a bool. • If the parentheses were left out: bool *compare( int, int ) • Declares a function that receives two integers and returns a pointer to a bool

  41. Enter 1 to sort in ascending order, Enter 2 to sort in descending order: 1 Data items in original order 2 6 4 8 10 12 89 68 45 37 Data items in ascending order 2 4 6 8 10 12 37 45 68 89 Enter 1 to sort in ascending order, Enter 2 to sort in descending order: 2 Data items in original order 2 6 4 8 10 12 89 68 45 37 Data items in descending order 89 68 45 37 12 10 8 6 4 2

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