Lec 13. Introduction to Pointers

Lecture 13Lecture 13
Version 1.0Version 1.0
Introduction to PointersIntroduction to Pointers

2Rushdi Shams, Dept of CSE, KUET, Bangladesh
PreludePrelude
 The most crucial of all of C’s grammars isThe most crucial of all of C’s grammars is
pointerspointers
 Many other programming languages have theMany other programming languages have the
similar concept as pointer in Csimilar concept as pointer in C
 they merely do not use the best of pointersthey merely do not use the best of pointers
 C uses pointers as its inevitable partC uses pointers as its inevitable part
 once you have the clean idea on pointers, youonce you have the clean idea on pointers, you
will see that by using pointers, you can solve anywill see that by using pointers, you can solve any
problem in Cproblem in C

Concept of Variable, Value &Concept of Variable, Value &
AddressAddress
int i = 3;int i = 3;
 This declaration tells the C compiler to:This declaration tells the C compiler to:
(a) Reserve space in memory to hold the integer(a) Reserve space in memory to hold the integer
value.value.
(b) Associate the name(b) Associate the name ii with this memorywith this memory
location.location.
(c) Store the value 3 at this location.(c) Store the value 3 at this location.

AddressAddress
 The important point is,The important point is, ii’s address in memory is a’s address in memory is a
number.number.

AddressAddress

AddressAddress
 The other pointer operator available in C is ‘The other pointer operator available in C is ‘*’*’
 called ‘value at address’ operatorcalled ‘value at address’ operator
 It gives the value stored at a particular addressIt gives the value stored at a particular address
 The ‘value at address’ operator is also calledThe ‘value at address’ operator is also called
‘indirection’ operator‘indirection’ operator

AddressAddress

Concept of PointerConcept of Pointer
 The expressionThe expression &i&i gives the address of thegives the address of the
variablevariable ii
 This address can be collected in a variableThis address can be collected in a variable
j = &i ;j = &i ;
 But remember thatBut remember that jj is not an ordinary variableis not an ordinary variable
like any other integer variablelike any other integer variable
 It is a variable that contains the address of otherIt is a variable that contains the address of other
variablevariable

 As you can see,As you can see, ii’s value is 3 and’s value is 3 and jj’s value is’s value is ii’s address’s address

 But wait, we can’t useBut wait, we can’t use jj in a program withoutin a program without
declaring itdeclaring it
 sincesince jj is a variable that contains the address ofis a variable that contains the address of ii, it is, it is
declared as,declared as,
int *j ;int *j ;
 This declaration tells the compiler thatThis declaration tells the compiler that jj will be usedwill be used
to store the address of an integer valueto store the address of an integer value
 In other wordsIn other words jj points to an integerpoints to an integer

 int *jint *j would mean, the value at the address containedwould mean, the value at the address contained
inin jj is anis an intint

 Look at the following declarations,Look at the following declarations,
1.1. int *alpha ;int *alpha ;
2.2. char *ch ;char *ch ;
3.3. float *s ;float *s ;
 The declarationThe declaration float *sfloat *s does not mean thatdoes not mean that ss
is going to contain a floating-point valueis going to contain a floating-point value
 ss is going to contain the address of a floating-is going to contain the address of a floating-
point valuepoint value

 Pointer, we know is a variable that containsPointer, we know is a variable that contains
address of another variableaddress of another variable
 Now this variable itself might be another pointerNow this variable itself might be another pointer
 Thus, we now have a pointer that containsThus, we now have a pointer that contains
another pointer’s addressanother pointer’s address

 i is an ordinaryi is an ordinary intint
 jj is a pointer to anis a pointer to an intint
 kk is a pointer to an integer pointeris a pointer to an integer pointer

 what will be the output?what will be the output?

Lec 13. Introduction to Pointers

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Lec 13. Introduction to Pointers