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

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Basics of Pointers in C language - Introduction, Pointer expressions, Pointer to Pointer, Pointer in Functions

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  • 1. Introduction to Pointers A Pointer is a derived data type in C . It is built from one of the fundamental data types available inC . 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 executionthe variable num is associated with theaddress 6843. This value of address,being a simple integer, can bee stored inanother variable which is called pointer. Pointer, again, is stored in someanother memory location 6894 whichtoo is accessible. The link between address & value ofvariable can be visualized with the helpof pointer in figure. 3. The term instructs the system to find a location forinteger variable a and assign 100 value in that location. Pointers, on the other side, spot the address or location area of thevariable and not directly on the intermediate value of that variable. The codingreflects how to declare a pointer variable.1. Using asterisk * with the data type before the variable namedeclares it as a pointer .2. The address operator & assigns the address of the specifiedvariable to the pointer variable.3. Variable that hasnt been assigned any value may contain garbageand make the pointer point to unknown locations. 4. Pointer variables can be initialized either in theirdeclaration part OR in between a couple of(The variable must be function statementsdeclared before theinitialization. Also thedata type of pointer variable& the variable to whichit is assigned should be the same.) Pointers, being flexible, can be used in different waysA single pointer A single variable toto many variables in different many pointersstatements 5. Pointers may be used to assign a value to a variable based on theother one likeassigns 223 to n in two ways1. 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 othercant be accessed by *4243 . 6. 6Understanding Pointers by Examplesx : 4892ip : 4904int x = 70, y = 80, z[4] = {10, 20, 30, 40 };int *ip; // int pointer ipip = &x; // ip is assigned to address of x*ip = 200; // content of ip is assigned to 200y = *ip; // y is assigned to content of ipip = &z[2];*ip = *ip + 20; // same as *ip += 20;y = *ip+1;y : 4894Z, Z[0] : 4896Z[1] : 4898Z[2] : 4900Z[3] : 49022007020080511020305040????48924900 7. Pointer to Pointer Pointer itself are variables whose locations are specifies on memoryand their storage address too can be known by assigning a pointer. We can access a target value indirectly pointed to by a pointer byapplying the indirection operator or the asterisk mark twice. a is assigned a value 100 and its location stored in p1whose 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 valuethey 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 interpretit 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 its value by the length of thedata type it points to.1. characters 1 byte 3. Float 4 bytes2. integer 2 bytes 4. double 8 bytes 9. Illustration Of Pointer to Pointer + Expressions using Pointer1. 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 3010 2 40b= 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 tocontain all elements when an array is declared. These memory locations are contiguous as shown below.ElementsValueAddressa[0] a[1] a[2] a[3] a[4]31 24 43 6 131030 1032 1034 1036 1038 The memory address increases by the bits of data the data type ofthe variable occupies. 11. These memory locations, being contiguous, can be used by pointersto 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 Arithmetic Operation On Pointeras well as Pointers & Arraysfloat 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 TableName Type Description Valuea[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 pointervariable3.147.0address of a[2]3.14?3]9.09.00]6.06.07.0 13. Pointer & Functions : Pointer as functionarguments By using pointer as parameter, addresses of variables is passed to the calledfunction. This process of calling a function to pass address of variables iscalled Call By Reference OR Pass By Pointers . The function called by reference can change the value of the variable usedin the call. E.g. :-The function value() receives the address ofvariable a & not the value. Inside value(), a ispointer & therefore it increments the value ofvariable 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 thecalling function. E.g. :-The coding aside shows the functionaddvalue() receiving address of a as aparameter. It increments the value stored inthe address of a & then returns that specificaddress to the calling function, which isthen assigned to pointer variable p.OUTPUT :-40Pointers 15. Pointer & Functions : Pointers to Functions Function too has an address location as well as a type in the memory. So, itis thereby possible to use pointer to point to a specific function, which canthen 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 thepointer points to, is going to return. Moreover a pointer can be assigned to a function by simply equating thepointer 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#include#includeint (*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 offunction add through pointer p */} /* Continued */ 17. Pointer & Functions : Pointers to Functions An Illustration to add two integral numbers (continued) :-OUTPUTvoid 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 5080 18. Uses of Pointersi. Pointers can be used to return multiple values from a functionvia function arguments .ii. They prove to be an efficient tool for manipulating dynamicdata structures such as Linked Lists, Queens, Stacks & Trees.iii. They reduce the program execution speed as well as theiraltitude of complexity .iv. Pointers save a lot of data storage space in memory whenused with character strings 19. Pitfalls Of Pointer Since Pointer holds addresses of memorylocation, it must never be used without properinitialization. An uninitialized pointer may hold addresses ofsome memory location that is protected by theOperating System. In such cases, de-referencinga pointer may crash the program. Pointer cant track the boundaries of an array.

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