1. Dale Roberts Department of Computer and Information Science, School of Science, IUPUI CSCI 230 PointersIntroduction Dale Roberts, Lecturer Computer Science, IUPUI E-mail: droberts@cs.iupui.edu

2. Dale Roberts What is Pointer Bring your money to IUPUI SL-280 to exchange hostage “Find Instruction Under The crabapple tree next to old law school ” “Find next message on the top of traffic light on Michigan St. and West St. Phone rings and says “Deliver ransom to I-65 and West St. intersection Hostage Location A: Highway Intersection Location B: Traffic Light Location C: Crabapple Tree Location D: IUPUI SL-280 CS Dept. CSABCD Address of next location A pointer is an address.

3. Dale Roberts What is Pointer Pig Pen points to Lucy; Lucy points to Sally; Sally points to Linus; Linus points to Charlie; Charlie points to Snoopy; Snoopy has Woodstock Example: Find out who is with Woodstock among peanut gang ? Snoopy CharlieLinus … Pig Pen “ Lucy knows ” Lucy “Sally knows” Sally “ Linus knows ” Linus “ Charlie knows ” Charlie “ Snoopy knows ” Woodstock Woodstock is with Snoopy Snoopy An instance uses a pointer to link to a next instance making a chain.

4. Dale Roberts Pointer declarations: datatype snoopy, *charlie, **linus; snoopy = ; /* snoopy’s content is Woodstock */ charlie = &snoopy; /* charlie’s content is the info (pointer) to locate snoopy, which is snoopy’s address */ linus = &charlie; /* linus’s content is the info (pointer) to locate charlie, which is charlie’s address*/ In general, we can rewrite charlie using variable of snoopyPtr (Snoopy’s pointer) and rewrite linus using variable of snoopyPtrPtr (pointer to Snoopy’s pointer); Note that this is only a naming convention. The only requirement is that the variable be a valid identifier: begin with a letter followed by letters, digits or _.

5. Dale Roberts Pointer Variable Declarations and Initialization PointersDefinition: A pointer is a variable that contains address of another variable. A powerful feature of C, but difficult to master Pointers enable programs to simulate call-by- reference and create/manipulate dynamic data structures Close relationship with arrays and strings

6. Dale Roberts 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 count 7 7 countPtr

7. Dale Roberts Pointer Declarations type *variable_name * used with pointer variables * used with pointer variables Example: int *myPtr; Declares a pointer to an int (pointer of type int * ) Multiple pointers require using a * before each variable declaration Can declare pointers to any data type Example: int *myPtr1, *myPtr2; float *pq; char *pc; Pointer Variable Declarations

8. Dale Roberts Pointer Variable Initialization Initialize pointers to 0, NULL, or an address 0 or NULL – points to nothing ( NULL preferred) Example: int *ptr, x; x=10; ptr = &x; 5000 ptr FFFF x 5000 ptr x 10 FFFF 5000 ptr FFFF x 10

9. Dale Roberts Pointer Operators & : Address Operator Returns address of operand int y = 5; int *yPtr; yPtr = &y; /* yPtr “points to” y */ /* yPtr gets address of y */ yptr 500000600000 y 5 yPtr y 5 address of y is the value of yPtr

10. Dale Roberts Pointer Operators * : Indirection / De-referencing Operator Returns a synonym/alias of what its operand points to *yptr returns y (because yptr points to y ) *yptr returns y (because yptr points to y ) * can be used for assignment that returns alias to an object * can be used for assignment that returns alias to an object *yptr = 7; // changes y to 7 Dereferenced pointer (operand of * ) must be a variable (no constants) * and & are inverses They cancel each other out

11. Dale Roberts Pointer Operators Example: int i = 5; int *pi; pi = &i; /* place the address of i into pi */ Assume Symbol Table i.e. &i = 874, i = 5; &pi = 902, pi = 874; *pi = ? *pi = 5; // same as i = 5; *pi = *pi * 2; // same as i = i * 2; *pi *= 2; // same as i *= 2; 874902 5 pii 874 variableaddressvalue at the address i 8745 pi 902874

12. Dale Roberts Example: int i,*pi,**ppi; i = 5; pi = &i; ppi = π 100104108 5 ppipii 100104 VariableAddressValue at address i1005 pi104100 ppi108104 UsageMeaningValue pi address of int 100 *piint value 5 &pi address of pointer 104 ppi address of pointer 104 *ppi address of int 100 **ppiint value 5