Basics of pointer, pointer expressions, pointer to pointer and pointer in functions

Basics of pointer, pointer expressions, pointer to pointer and pointer in functions

• 1.
  Introduction to Pointers • A Pointer is a derived data type in ‘C’ . • It is built from one of the fundamental data types available in ‘C’ . • Pointers contain the memory addresses as their values . • Memory addresses, being the location of computer memory, can be accessed & used to store data via pointers .
• 2.
  Understanding Pointers •During the whole program execution the variable num is associated with the address 6843. This value of address, being a simple integer, can bee stored in another variable which is called pointer. • Pointer, again, is stored in some another memory location 6894 which too is accessible. • The link between address & value of variable can be visualized with the help of pointer in figure.
• 3.
   The terminstructs the system to find a location for integer variable ‘a’ and assign 100 value in that location.  Pointers, on the other side, spot the address or location area of the variable and not directly on the intermediate value of that variable.  The coding… …reflects how to declare a pointer variable. 1. Using asterisk ‘*’ with the data type before the variable name declares it as a ‘pointer’ . 2. The address operator ‘&’ assigns the address of the specified variable to the pointer variable. 3. Variable that hasn’t been assigned any value may contain garbage and make the pointer point to unknown locations.
• 4.
  • Pointer variablescan be initialized either in their declaration part OR in between a couple of (The variable must be function statements declared before the initialization. Also the data type of pointer variable & the variable to which it is assigned should be the same.) • Pointers, being flexible, can be used in different ways A single pointer A single variable to to many variables in different many pointers statements
• 5.
  • Pointers maybe used to assign a value to a variable based on the other one like… …assigns 223 to ‘n’ in two ways 1. By using the pointer to extract the value stored in ‘a’ . 2. By directly using the address of ‘a’ .  NOTE : A value stored in address 4243 or any other can’t be accessed by ‘ *4243 ‘ .
• 6.
  6 Understanding Pointersby Examples x : 4892 ip : 4904 int x = 70, y = 80, z[4] = {10, 20, 30, 40 }; int *ip; // int pointer ip ip = &x; // ip is assigned to address of x *ip = 200; // content of ip is assigned to 200 y = *ip; // y is assigned to content of ip ip = &z[2]; *ip = *ip + 20; // same as *ip += 20; y = *ip+1; y : 4894 Z, Z[0] : 4896 Z[1] : 4898 Z[2] : 4900 Z[3] : 4902 200 70 200 80 51 10 20 30 50 40 ???? 4892 4900
• 7.
  Pointer to Pointer • Pointer itself are variables whose locations are specifies on memory and their storage address too can be known by assigning a pointer. • We can access a target value indirectly pointed to by a pointer by applying the indirection operator or the asterisk mark twice. … ‘a’ is assigned a value ‘100’ and it’s location stored in ‘p1’ whose location in turn is stored in ‘p2’ . ‘*p1’ refers to ‘100’ so does ‘**p2’ . • REMEMBER to assign similar data types to chain pointing variables.
• 8.
  Pointer Expressions •Arithmetic operations between two or more pointer is not possible. • But pointers can be used to perform arithmetic operations on the value they point to. e.g.: …same as ((*p1) * (*p2)) / (*p3) …same as (10 * (-(*p3))) / (*p2)  Note to keep a space between / and * to not to make compiler interpret it to be a comment. • Pointer incrementation is valid in ‘C’ . e.g.: p++; OR p=p1+2; are valid statements . • A pointer, when incremented, it increases it’s value by the length of the data type it points to. 1. characters – 1 byte 3. Float – 4 bytes 2. integer – 2 bytes 4. double – 8 bytes
• 9.
  Illustration Of ‘Pointerto Pointer’ + ‘Expressions using Pointer’ 1. int a,b,c,*p,**q; 2. a=10; 3. b=20; 4. c=30; 5. printf(“%d %d %d”,a,b,c); 6. p=&a; 7. q=&p; 8. b=b/ (( *p * **q ) / 10); 9. c=c+ ( 2 * *p) - **q; 10. printf(“n%d %d %d”,a,b,c); Output: 10 20 30 10 2 40 b= 20/ ( ( ( value indicated by pointer p) * ( value indicated by chain pointer q ) ) / 10 ) c=30 + (2 * (value indicated by pointer p) ) - ( value indicated by chain pointer q )
• 10.
  Pointer & Arrays • The compiler, by default, allocates sufficient amount of storage to contain all elements when an array is declared. • These memory locations are contiguous as shown below. Elements Value Address a[0] a[1] a[2] a[3] a[4] 31 24 43 6 13 1030 1032 1034 1036 1038 • The memory address increases by the bits of data the data type of the variable occupies.
• 11.
  • These memorylocations, being contiguous, can be used by pointers to access the exact locations of any specific variable of an array. E.g. :- int a[5],*p; p=a; /* by default p is the address of a[0] */ p+1=4; /* assigning ‘4’ to a[1], shown by ‘p+1’ */ p+2=12; /* assigning ‘12’ to a[3], shown by ‘p+2’ */ p+3=10; /* assigning ‘10’ to a[2], shown by ‘p+3’ */ • Also a[1], a[2],etc. can be directly referred by using *(p+1), *(p+2), etc. Pointer & Arrays
• 12.
  Examples of ‘ArithmeticOperation On Pointer’ as well as ‘Pointers & Arrays’ float a[4]; float *ptr; ptr = &(a[2]); *ptr = 3.14; ptr++; *ptr = 9.0; ptr = ptr - 3; *ptr = 6.0; ptr += 2; *ptr = 7.0; Data Table Name Type Description Value a[0] float float array element (variable) ? a[1] float float array element (variable) ? a[2] float float array element (variable) ? a[3] float float array element (variable) ? ptr float * float pointer variable *ptr float de-reference of float pointer variable 3.14 7.0 address of a[2] 3.14 ? 3] 9.0 9.0 0] 6.0 6.0 7.0
• 13.
  Pointer & Functions: Pointer as function arguments • By using pointer as parameter, addresses of variables is passed to the called function. This process of calling a function to pass address of variables is called ‘Call By Reference’ OR ‘Pass By Pointers’ . • The function called by ‘reference’ can change the value of the variable used in the call. • E.g. :- The function value() receives the address of variable a & not the value. Inside value(), a is pointer & therefore it increments the value of variable a by 50. OUTPUT : 70
• 14.
  Pointer & Functions: Function Returning • As pointers are a data type in ‘C’ , a function can return a pointer to the calling function. • E.g. :- The coding aside shows the function addvalue() receiving address of a as a parameter. It increments the value stored in the address of a & then returns that specific address to the calling function, which is then assigned to pointer variable p. OUTPUT :- 40 Pointers
• 15.
  Pointer & Functions: Pointers to Functions • Function too has an address location as well as a type in the memory. So, it is thereby possible to use pointer to point to a specific function, which can then be used as argument in another function. • The declaration of pointer to a function takes place as follows: data_type (*pointer_name) (data);  Here, the data type specifies must be the same the function, which the pointer points to, is going to return.  Moreover a pointer can be assigned to a function by simply equating the pointer name to the name of the function. e.g. :- float add(int, int); float (*p) (int, int); p=add;  A function can also be called using pointer like :- (*p)(a,b); /* equivalent to [ add(x,y); ] */
• 16.
  Pointer & Functions: Pointers to Functions • An Illustration to add two integral numbers :- #include<stdio.h> #include<conio.h> #include<stdlib.h> int (*p)(int, int); /*declaration of function pointer ‘p’ which points function ‘add ‘*/ void print(int (*p)(int, int)); /* declaration of function ‘print’ */ int add(int, int); /* declaration of function ‘add’ */ void main() { p=add; /* initializing pointer */ print(p); /* calling function ‘print’ which receives the address of function ‘add’ through pointer ‘p’ */ } /* Continued */
• 17.
  Pointer & Functions: Pointers to Functions • An Illustration to add two integral numbers (continued) :- OUTPUT void printf(int (*p)(int, int)) { int a,b; scanf(“%d %d”,&a,&b); printf(“n%d”,(*p)(a,b)); /* passes values of ‘a’ & ‘b’ to ‘add’ through ‘p’ */ } int add(int a, int b) { return(a+b); /* adds ‘a’ & ‘b’ */ } /* program over */ 30 50 80
• 18.
  Uses of Pointers i. Pointers can be used to return multiple values from a function via function arguments . ii. They prove to be an efficient tool for manipulating dynamic data structures such as Linked Lists, Queens, Stacks & Trees. iii. They reduce the program execution speed as well as their altitude of complexity . iv. Pointers save a lot of data storage space in memory when used with character strings
• 19.
  Pitfalls Of Pointer • Since Pointer holds addresses of memory location, it must never be used without proper initialization. • An uninitialized pointer may hold addresses of some memory location that is protected by the Operating System. In such cases, de-referencing a pointer may crash the program. • Pointer can’t track the boundaries of an array.