Department of Computer and Information Science, School of Science, IUPUI

1. Department of Computer and Information Science,School of Science, IUPUI CSCI N305 Pointers Introduction

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

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

4. 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. Pointer Variable Declarations and Initialization • Pointers • Definition: 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. count 7 • 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 countPtr count 7

7. Pointer Variable Declarations • Pointer Declarations type *variable_name • * 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;

8. ptr x FFFF 5000 x ptr FFFF 5000 10 ptr x FFFF 5000 FFFF 10 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;

9. yPtr y 5 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 y 5 500000 600000 600000 address of y is the value of yPtr

10. Pointer Operators • * : Indirection / De-referencing Operator • Returns a synonym/alias of what its operand points to • *yptr returns y (because yptr points to y) • * 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. pi i 902 874 874 5 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;

12. ppi pi i 108 104 100 5 104 100 Example: int i,*pi,**ppi; i = 5; pi = &i; ppi = π