Definition. Pointers. Outline. Why pointers? Definition. Memory Organization Overview. by Ziad Kobti. Definition. Pointers enable programmers to:

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1 Pointers by Ziad Kobti Deinition When you declare a variable o any tye, say: int = ; The system will automatically allocated the required memory sace in a seciic location (tained by the system) to store an integer size value (4 bytes). When you request this value, you simly reer to its name without any other concerns about its actual (logical or hysical) memory address until now ointer is a secial tye o variable that only holds a memory address. In this case, an integer ointer would hold the memory address o an integer variable (int *;). Note that the ointer is also a variable, and itsel has its own memory address! To store the address o a ointer variable you need another secial kind o variable a ointer to ointer variable (int **;) Outline Deinition Pointer Variable Declaration Pointer Initialization Reerencing and Dereerencing Oerators rray Maniulation Passing Pointers to Functions (Pass-By- Reerence) Why ointers? Pointers enable rogrammers to: simulate call by reerence create and maniulate dynamic data structures (linked lists, stacks ) Deinition ointer variable is one that holds a valid memory address. Pointers allow direct access to memory locations, rovided you suly or query the correct address. The C language allows arithmetic maniulations on ointers. Pointer values can be incremented, decremented, etc.. By altering the ointer value we are altering the memory address value it contains, thereby making it oint to a dierent memory location. CUTION: Extreme care has to be exercised in order to avoid dangling ointers. Oten, accessing invalid or rotected memory saces lead to sudden rogram and even system crashes. Errors reorted by the comiler or at run time such as Segmentation ault, core dumed, and bus error usually hint to a aulty ointer value. Memory Organization Overview Memory cells or blocks are sequentially organized Each memory cell has a UNIQUE address memory address is reresented by a hexadecimal number (0x indicates hex) ddresses increase in one direction (ositive direction) We don t need to know the hex system details, other than be able to recognize a hex number when you see one. 0xFC00 0xFFFF 1

2 Declaring Pointer Variables Just like any other variable, a ointer variable needs to be declared. NOTE: You will see a some symbols like * and & and it is imortant to know that they will have dierent meaning in dierent contexts o the rogram! To declare a ointer variable: <data_tye> *<ointer_variable_name>; Examles: int *; loat *; Naming convention: you may call your ointer variable anything you like, however it would be more useul to tell that it is a ointer, and what it would be used to oint at. (good names:,, Ptr the ointer oints to ) Basic Oerators * Indirection or dereerencing oerator. Returns the value o the object to which its oerand (ie. a ointer) oints. rint( %d, *); means: since oints to, we want the value o (an integer). Note that this * symbol was also used in declaring the ointer variable! Initializing Pointer Variables Memory Organization Overview There are three legal values you may store in a ointer: 1. the integer 0 (the only legal integer you may assign to a ointer) 2. NULL (ointer to nothing) 3. or a valid address (best to query it using &) int *; 0xFC Basic Oerators Memory Organization Overview & ddress oerator, unary oerator that returns the address o its oerand. (also called reerencing oerator) = &; means: the ointer will have the address o variable. So now oints to. int *; = ; 0xFC 2

3 Memory Organization Overview Samle Pointer ccess: int *; = ; = &; 0xFC int *; = ; = &; rint( %d, *&); rint( %d, &*); ERROR! 0xFC Why? because * next to an integer reers to the multilication binary oerator! Reer to igure.5 age 253 Samle Pointer ccess: Relationshi between ointers and arrays int *; = ; = &; rint( %, &); rint( %, ); rint( %d, *); 0xFC rray basics: 1. arrays are a list o sequential memory cells 2. Each array element has a unique memory address 3. Every memory cell is equal to the next since all array elements are o the same tye (int=4 bytes) 4. The name o the array is a ointer to the irst element 0xFC00 0xFC04 0xFC08 0xFC10 0xFC Reer to igure.5 age 253 Samle Pointer ccess: Tracing Ti: int *; = ; = &; Scoe/Function Name: rint( %, &); Var xFC rint( %, &*); rint( %d, *&); Reer to igure.5 age 253 0xFC trvar lways draw your memory structures i you are tracing code. It hels i you draw the variables as rectangular boxes and distinguish ointers by oval shaes. The arrow ointing to Var rom trvar indicates that: trvar = &Var; 3

4 rray ccess int [5] = {0, 0, 0, 0, 0; int * = ; or int * = &[0]; rray ccess In this scenario, * and [1] reer to the same element in the array rray ccess * = ; rray ccess * = 4; rray ccess ++; This increments the ointer value by one unit address (unit is int=4bytes) rray ccess *++; Increments the contents

5 rray ccess = + 3; Passing Parameters by Reerence Pass by value is deault arameter assing or variables. That is a coy o the arameter is sent to the unction to view/edit it locally. The outcome o editing the coy does not aect the original. Pass by reerence asses a coy o the memory address or ointer that tells the unction where to ind the original value. Eicient because you don t have to dulicate memory sace usage and ass a coy Dangerous because you are telling the unction where to ind the original coy (use const i you wish to make it read only) rray ccess ++; Pass-by-Reerence void cube (int *n); Runtime ERROR! void cube (int *n) { *n = *n * *n * *n; rray ccess: Examle 1 /* oulate the array with 0 s */ int [5]; /* declare the array size 5 */ int *; /* declare a ointer */ = ; /* oints to array */ int i; /* declare a counter */ or (i = 0; i < 5; i++) { * = 0; /* same as [i] = 0; */ ++; Pass-by-Reerence void cube (int *n); <- no return, since the outut is by modiying the arameter void cube (int *n) <- declares a ointer { *n = *n * *n * *n; <- * = multily (rename n to trn would make more sense) 5

6 Examle: Examle: void (int *) { * = * * 2; int () { int a = ; int *atr = &a; (atr); (&a); a atr -> void (int *) { * = * * 2; int () { int a = ; int *atr = &a; (atr); (&a); a 28 atr Examle: Examle: void (int *) { * = * * 2; int () { int a = ; int *atr = &a; (atr); (&a); a 14 atr void (int *) { * = * * 2; int () { int a = ; int *atr = &a; (atr); (&a); a 28 atr -> 28 Examle: void (int *) { * = * * 2; int () { int a = ; int *atr = &a; (atr); (&a); a 14 atr -> 14 Passing an rray by Reerence int BubbleSort( int [], int size, int order); int BubbleSort( int *, int size, int order); Both rototyes accomlish a ass by reerence. It is more eicient to ass a ointer to the beginning o the array than to coy all the elements! Function call: #deine SCENDING 1 #deine DESCENDING 1... myrray[4] = {1, 5, 2, 65; BubbleSort( myrray, 4, SCENDING); 6

7 Swa 2 array elements Swa 2 array elements int [] = { 5, 8, 2, 6, 3; int tem; int * = ; tem = *; * = *( + 1); *( + 1) = tem; int [] = { 5, 8, 2, 6, 3; int tem; int * = ; tem = *; * = *( + 1); *( + 1) = tem; tem 5 Swa 2 array elements Swa 2 array elements int [] = { 5, 8, 2, 6, 3; int [] = { 5, 8, 2, 6, 3; int tem; int * = ; tem = *; int tem; int * = ; tem = *; * = *( + 1); *( + 1) = tem; tem * = *( + 1); *( + 1) = tem; tem 5 Swa 2 array elements Next Time: int [] = { 5, 8, 2, 6, 3; int tem; int * = ; tem = *; * = *( + 1); *( + 1) = tem; tem 5 Character rrays rrays o Pointers (section.9) Chater 8: C Characters and Strings