1. Module 9 Pointers

2. @Copyright UMBC Training Centers 2012
Pointer Basics
In this lesson we will learn the basic syntax for declaring and using pointers
Upon completion of this lesson, you will be able to
Declare pointer variables
Use pointer syntax to assign and dereference pointers
Evaluate expressions that use pointers
@Copyright UMBC Training Centers 2012

3. @Copyright UMBC Training Centers 2012
What is a Pointer
A pointer variable (or simply “pointer”) is a variable that contains the memory address (location) of another variable
Powerful, but dangerous if not used with discipline
The variable to which the pointer points is known as the “pointee”
@Copyright UMBC Training Centers 2012

4. @Copyright UMBC Training Centers 2012
Why Pointers?
Pointers…
lead to compact and efficient code.
facilitate sharing data between different parts of an application.
make it possible to get dynamically allocated memory when your application is executing.
make it easy to represent relationships among data items.
@Copyright UMBC Training Centers 2012

5. @Copyright UMBC Training Centers 2012
Pointer Caution
They are a powerful low-level device.
Undisciplined use can be confusing and thus the source of subtle, hard-to-find bugs.
Program crashes
Memory leaks
Unpredictable results
@Copyright UMBC Training Centers 2012

6. @Copyright UMBC Training Centers 2012
Declaring Pointers
The general declaration of a pointer is
type *nameOfPointer;
For example
int *pInt;
@Copyright UMBC Training Centers 2012

7. @Copyright UMBC Training Centers 2012
Pointer Operators
The & operator gives the address of a variable
pInt = &age;
assigns the address of age to the pointer pInt, and we say that pInt “points to” age;
The * operator uses the pointer to access the pointee
x = *pInt;
assigns the value in the variable to which pInt points (the pointee) to x, and we say that we are “dereferencing” the pointer.
@Copyright UMBC Training Centers 2012

8. @Copyright UMBC Training Centers 2012
Pointers in Memory
int myAge, yourAge, *pAge; myAge = 42; pAge = &myAge; yourAge = *pAge + 3;
@Copyright UMBC Training Centers 2012

9. @Copyright UMBC Training Centers 2012
More Pointer Code
int *ip = NULL, x = 1, y = 2; int z[3] = {0}; ip = &x; y = *ip; *ip = 0; *ip = *ip + 10; ip = &y; printf( “%d”, *ip); scanf(“%d”, ip); ip = &z[2]; *ip = x + y;
@Copyright UMBC Training Centers 2012

10. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B – pointer1
Show 1st half of the code, have students predict the output
Trace the code on board, then run the code
Do the same with 2nd half of code
@Copyright UMBC Training Centers 2012

11. @Copyright UMBC Training Centers 2012
Broken swap( )
void swap (int a, int b) {
int temp = a;
a = b;
b = temp; }
int main( ) {
int x = 42, y = 77;
swap(x, y);
printf(“x = %d, y = %d\n”, x, y);
@Copyright UMBC Training Centers 2012

12. @Copyright UMBC Training Centers 2012
Exercises
pointers2 – students trace code and predict output
Pg 277
#10 – rewriting char array code to use char*
@Copyright UMBC Training Centers 2012

13. Initialized Strings Revisited
As an array of chars
char name1[ ] = “Jimmy”;
Using a CHAR pointer
char *name2 = “Jimmy”;
@Copyright UMBC Training Centers 2012

14. @Copyright UMBC Training Centers 2012
String Parameters
void printString1( char[ ] string) { printf(“%s\n”, string); } void printString2( char* string ) … printManyStrings( char* strings[], int count) for(int s = 0; s < count; s++) printf(“%s\n”, strings[s]);
@Copyright UMBC Training Centers 2012

15. Initialized Array of Strings
char* names[3] = { “Bobby”, “Jim”, “Mary” };
printf (“My name is %s\n”, names[1]);
@Copyright UMBC Training Centers 2012

16. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B – copyString (From the text; we would write this using while loop)
Explain the array of char version
General array copying algorithm
Must append NUL char to end of “to” to make it a C-String
Reveal and explain the pointer version
@Copyright UMBC Training Centers 2012

17. @Copyright UMBC Training Centers 2012
End of Day 1 Exercises
Pg 276
#1 (11.1, )
#10 – Rewrite compareStrings from Ch10 using pointers
#12 – printf with strings; predict first
Ex5-DateStrings – create and use arrays of strings
@Copyright UMBC Training Centers 2012

18. @Copyright UMBC Training Centers 2012
Pointer Parameters
Pointer parameters allow us to simulate passing arguments “by reference”
Argument is a pointer to a variable
Parameter is a copy of the pointer
Allows a function to alter the argument from within the function
@Copyright UMBC Training Centers 2012

19. @Copyright UMBC Training Centers 2012
swap( )
/* swap the values of pointees */ void swap (int *ip1, int *ip2) { int temp = *ip1; *ip1 = *ip2; *ip2 = temp; } int main() int x = 42, y = 77; int *ptrToX, *ptrToY; ptrToX = &x; ptrToY = &y; swap( ptrToX, ptrToY ); return 0;
@Copyright UMBC Training Centers 2012

20. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B PointersAndFunction1
@Copyright UMBC Training Centers 2012

21. @Copyright UMBC Training Centers 2012
“Output” Parameters
void convertTime (int time, int *pHours, int *pMins) {
*pHours = time / 60;
*pMins = time % 60; }
int main( )
int time, hours, minutes;
printf("Enter a time duration in minutes: ");
scanf ("%d", &time);
convertTime (time, &hours, &minutes);
printf("HH:MM format: %02d:%02d\n", hours, minutes);
return 0;
@Copyright UMBC Training Centers 2012

22. @Copyright UMBC Training Centers 2012
An Exercise
void myFunction (int a, int *b) {
a = 7 ; // line 1
*b = a ; // line 2
b = &a ; // line 3
*b = 4 ; // line 4
printf("%d, %d\n", a, *b) ; // line 5 }
int main()
int m = 3, n = 5; // line 6
myFunction(m, &n) ; // line 7
printf("%d, %d\n", m, n) ; // line 8
return 0;
@Copyright UMBC Training Centers 2012

23. @Copyright UMBC Training Centers 2012
Pointer to Structure
typedef struct point { int x, y; } POINT; POINT center = {5, 10}; // structure initialization POINT corner = {3, 9}; POINT *pPoint; pPoint = &center; printf(“Center at (%d, %d)\n”, pPoint->x, pPoint->y); pPoint = &corner; printf(“Corner at (%d, %d)\n”, pPoint->x, pPoint->y); // less common but equivalent printf(“Center at (%d, %d)\n”, (*pPoint).x, (*pPoint.y));
@Copyright UMBC Training Centers 2012

24. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B – nap (name, address, person)
Lot of functions with pointer to struct as parameters
Start discussion with NAME, then ADDRESS, then finally PERSON
@Copyright UMBC Training Centers 2012

25. Pointer to struct parameter
void inputPoint( struct point *pPoint) { printf(“Please input point coordinates: ”); scan(“%d %d”, &(pPoint->x), &(pPoint->y)); } void printPoint( struct point *pPoint) printf(“(%d, %d)”, pPoint->x, pPoint->y); int main( ) struct point center; inputPoint( &center ); printPoint( &center ); return 0;
@Copyright UMBC Training Centers 2012

26. Structures Containing Pointers
struct intPtrs {
int *p1, *p2; };
struct intPtrs bob;
int int1 = 100, int2;
bob.p1 = &int1;
bob.p2 = &int2;
*bob.p2 = -97;
@Copyright UMBC Training Centers 2012

27. @Copyright UMBC Training Centers 2012
NULL
NULL is a predefined special value for pointers
Points to “nothing”
Some programmers like to initialize all pointers to NULL when they are defined
double *pRatio = NULL;
Program termination if we dereference a pointer that is NULL
@Copyright UMBC Training Centers 2012

28. @Copyright UMBC Training Centers 2012
Exercises
Text, #1 pg 275
Ex1-Coins2 – nr coins from N cents (pointer parameters)
Ex2-Division
Ex3-Money – pointer to struct as function parameter
Ex4 – People – struct parameters/return values
@Copyright UMBC Training Centers 2012

29. @Copyright UMBC Training Centers 2012
End of Day 1??
@Copyright UMBC Training Centers 2012

30. @Copyright UMBC Training Centers 2012
Pointers and Arrays
Arrays are commonly used to store data
In C, there is an intimate relationship between pointers and arrays
@Copyright UMBC Training Centers 2012

31. @Copyright UMBC Training Centers 2012
Basics
int a[4] = {11, 13, 15, 17}; int *ip, x; ip = &a[0]; x = *ip;
x = *(ip + 1);
x = *(ip + 2);
@Copyright UMBC Training Centers 2012

32. @Copyright UMBC Training Centers 2012
The Big Ideas
If ip points to a[0], then ip + i points to a[i] and *(ip + i) is synonymous with a[i]
More generally, ip + i points i elements beyond where ip is pointing
The statement ip = ip + i; moves ip to point i elements beyond where it currently points
@Copyright UMBC Training Centers 2012

33. @Copyright UMBC Training Centers 2012
More Big Ideas
Since the name of an array is synonymous with the address of (pointer to) its first element…
The name of an array can be thought of as a pointer to the first element of the array and vice-versa
ip = a; is equivalent to ip = &a[0];
*(ip + i), *(a + i) and ip[i] are all equivalent to a[i]
@Copyright UMBC Training Centers 2012

34. @Copyright UMBC Training Centers 2012
Let’s take a look
CB – PointersAndArrays1
typical code and you should use [ ] with arrays – clearer
going to see array code that uses pointers instead
it’s important that we examine the different ways to write this code
First create a new pointer variable and change the code in the for-loop to use pointer arithmetic
ADD int *pWeight = &weights[ 0 ] ; change weights[i] to *(pWeight+i)
As the for-loop changes the value of I, pWeight+I iterates through the elements of the array, just like before
To show that “weights” acts like a pointer, change pWeight to “weights”
@Copyright UMBC Training Centers 2012

35. Arrays and Functions Revisited
Array name is a pointer to its first element
Array parameter could be replaced with a pointer parameter
Compiler treats array parameters as pointers
Reflected in error messages
Use an array parameter if the function uses an index to access the members of an array
@Copyright UMBC Training Centers 2012

36. @Copyright UMBC Training Centers 2012
Let’s take a look
CB -- PointersAndArrays2
change array parameter to a[300] to illustrate that parameter is just the address of where the array is located in memory… a “pointer”
change array parameter to pointer parameter
change code to *(a + i)
Error messages related to the array parameter will use pointer terminology
Change int a[] to float a[] and look at the log
@Copyright UMBC Training Centers 2012

37. @Copyright UMBC Training Centers 2012
What’s the difference?
Between an array and a pointer variable
A pointer is a variable that can point to different variables during execution
Array name is NOT a variable and can only “point to” its first element
Now we know why scanf does not require & for %s arguments
@Copyright UMBC Training Centers 2012

38. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B - ConstPointers
CB: PtrExpressions – hand out table to be completed by students C::B CompareStrings
Modify to use pointer+offset
@Copyright UMBC Training Centers 2012

39. Initialized Strings Revisited
As an array of chars
char name1[ ] = “Jimmy”;
Using a pointer
char *name2 = “Jimmy”;
@Copyright UMBC Training Centers 2012

40. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B – copyString
Explain the array of char version
General array copying algorithm
Must append NUL char to end of “to” to make it a C-String
Reveal and explain the pointer version
@Copyright UMBC Training Centers 2012

41. @Copyright UMBC Training Centers 2012
Array of Strings
char *names[3] = { “Bobby”, “Jim”}; names[2] = “Mary”;
printf (“My name is %s\n”, names[1]);
@Copyright UMBC Training Centers 2012

42. @Copyright UMBC Training Centers 2012
String Parameters
printString1( char string[] ) { printf(“%s\n”, string); } printString2( char *string ) printManyStrings( char *strings[ ], int count) for(int s = 0; s < count; s++) printf(“%s\n”, strings[s]);
@Copyright UMBC Training Centers 2012

43. @Copyright UMBC Training Centers 2012
More String Functions
char *strchr(char * string, char c) returns a pointer to the first occurrence of c in string or NULL if not found
char *strpbrk(char *s1, char *s2) returns a pointer to the first occurrence of any character in s2 found in s1 or NULL if not found
char *strrchr(char * string, char c) returns a pointer to the last occurrence of c in string or NULL if not found
char* strstr(char *s1, char *s2) returns a pointer to the first occurrence of s2 in s1 or NULL if not found
@Copyright UMBC Training Centers 2012

44. @Copyright UMBC Training Centers 2012
Let’s take a look
C::B – String Functions
Add code after strchr() to find the next ‘w’ add code after strchr( ) call to print index where ‘w’ is found by showing pointer subtraction
@Copyright UMBC Training Centers 2012

45. @Copyright UMBC Training Centers 2012
Exercises
PtrExpressions.docx
experimenting with ++*p, etc
Ex4-DateStrings
@Copyright UMBC Training Centers 2012

46. More Pointer Operations
To access array elements and traverse through the array use *arrayPtr++
Dereferences arrayPtr,then increments it
void copyString( char *to, char *from ) {
while (*from)
*to++ = *from++;
*to = ‘\0’; }
@Copyright UMBC Training Centers 2012

47. @Copyright UMBC Training Centers 2012
Let’s take a look
Ex3-PointerExpression
@Copyright UMBC Training Centers 2012

48. @Copyright UMBC Training Centers 2012
Exercises
Text
#8 – function to sort 3 integers with pointer params (not array)
#12
Ex2-ExchangeSort (same as Ex7)
Ex5-DateStrings
Ex6 – Gymnastic Scores – array as function parameters
Ex7-Gettysburg – string functions that return pointer
@Copyright UMBC Training Centers 2012

49. @Copyright UMBC Training Centers 2012
If you have any comments about this video, please use the contact information in your registration materials
to let us know
@Copyright UMBC Training Centers 2012