C programming - Pointers

Workshop India
Wilson Wingston Sharon
wingston.sharon@gmail.com

#include <stdio.h>

int main()
 A variable is something with a {
name. printf("size of a short is
%dn", sizeof(short));
 The value of that variable is not printf("size of a int is
fixed. %dn", sizeof(int));
printf("size of a long is
%dn", sizeof(long));
 Size of the variable depends on the }
datatype.

 The program on the right will tell
you the size of the variables in your
system.

 int k;

 What does the above statement mean?
 Int means the compiler needs to set aside 4 bytes of
memory to hold this particular value k.
 This variable k is going to be stored in the computers
RAM.
 What is the value of k at this time?
 k = 2;
 Now the value 2 is going to be placed in the memory
location for k.
 So if we check the place in RAM where the variable is
stored, we will find 2 there.

 If you take an = operator
 Lvalue = Rvalue

 These Values have their own rules.
 Lvalue must be a named region of storage.
 Rvalue must be an expression that returns some
value.
 k = 2;
 j = 3;
 k = j;

 2 = k; makes no sense.

k = 2;
 Lvalue is variable K
 Rvalue is 2
 2 is stored in variable k

k = j;
 Lvalue is variable k
 Rvalue is value stored in variable j
 Value of variable j is now copied to k.

2 = k;
 Really? Can this be possible?

k

k = 2; 2

 Name is k 0x001

 Value is 2
 Starting address is
0x001
c
?
 char c = „b‟; 0x00A
 Value = ?
 Address = ?

 Pointers are variables who‟s values is the address
of another memory location on the RAM.
 Pointers are variables like any other data type.

 The value they contain are special – they store an
address.

 int *ptr;
 This statement informs the compiler that we want a
variable balled ptr that will hold the address of another
integer variable.
 What does ptr at this point?

k

& is an operator that 2
returns the address of 0x001
the variable.

 printf(“%d”, k );
c
 printf(“%d”, &k );
„c‟
0x00A
 printf(“%c”, c );
 printf(“%d”, &c );

 intk;
var k
 k = 2;
? 2
0x00A 0x001
 int *ptr;
 ptr = &k;
ptr
?
 int *var = k;
0x00A

 * called a deferefencer.
 It is applicable only for a pointer data type and it
goes to memory location of that pointer.

 &k
 Return the address of the variable k in RAM
 Can only be Rvalue.
 k
 Return that value that k has in RAM
 Can be both Rvalue and Lvalue
 *k
 Go to the memory address in k and lets work with that
value.
 Can be both Rvalue and Lvalue

var k
0x001 2
 printf(“%d”, ptr );
0x00A 0x001

 printf(“%d”, *var );
ptr
0x001
 printf(“%d”, &var);
0x00A

 printf(“%d”, *ptr);

 2 var k

 printf(“%d”, ptr ); 0x001 2

 0x001 0x00A 0x001

 printf(“%d”, *var );
 2 ptr

 printf(“%d”, &var); 0x001

 0x00A 0x00A

 printf(“%d”, *ptr);
 2

A variable is declared by giving it a type and a
name (e.g. int k;)
 A pointer variable is declared by giving it a
type and a name (e.g. int *ptr) where the
asterisk tells the compiler that the variable
named ptr is a pointer variable and the type
tells the compiler what type the pointer is to
point to (integer in this case).
 Once a variable is declared, we can get its
address by preceding its name with the
unary &operator, as in &k.

 Time: 15 minutes

 Write
3 different programs that uses the
concepts you just learnt

 Familiarize yourselves with pointers.

 If*ptr is a pointer pointing to a variable,
 ptr + 1 doesn‟t simply increment the pointer
by 1

 The compiler changes the expression to
 ptr + 1 * sizeof(<datatype>)

 Ifptr was an integer of size()=4, then the
next integer will be 4 memory blocks away.

 int k = 2; k
ptr 2
 int *ptr = &k;
? 0x001
 int *v1 = ptr + 2;
0x00A 7
 int *v2 = v + 1;
v1 0x005

29
 printf(“%d”, *ptr ); ?
0x00B 0x001
 printf(“%d”, *v1 );
-2
 printf(“%d”, *v2 ); v2
0x001
 printf(“%d”, v2 ); ?

 printf(“%d”, &v2 ); 0x00C

 int k = 2; k
 int *ptr = &k;
ptr 2
 int *v1 = ptr + 2;
 int *v2 = v + 1; 0x001 0x001
0x01A 7
 printf(“%d”, *ptr );
 2 0x005
v1
 printf(“%d”, *v1 );
 29 0x009 29
 printf(“%d”, *v2 ); 0x01E 0x009
 -2
 printf(“%d”, v2 ); -2
v2
 0x00D 0x00D
 printf(“%d”, &v2 ); 0x00D
 0x03F 0x03F

 int array[] = {1,23,4,5,-100};

 The variable is a pointer pointing to the first
element of the array .
 All other arrays are stored sequentially after that.

 array[1] = ?
 *(arr+1) = ?
 *arr + 1 = ?

 arr[3] = ?
 *(arr+3) = ?
 *arr + 3 = ?

 int array[] = {1,23,4,5,-100};

 The variable is a pointer pointing to the first
element of the array .
 All other arrays are stored sequentially after that.

 array[1] = 23
 *(arr+1) = 23
 *arr + 1 = 24

 arr[3] = 5
 *(arr+3) = 5
 *arr + 3 = 8

 int array[] = {1,23,7,5,10,78,9,87};
 int *ptr = array + 2;

 array[1] =?
 *(ptr+1) = ?
 *array + 3 = ?
 array[4] = ?
 *ptr + 1 = ?
 &array[0] = ?

 int array[] = {1,23,7,5,10,78,9,87};
 int *ptr = array + 2;

 array[1] = 23
 *(ptr+1) = 5
 *array + 3 = 5
 array[4] = 10
 *ptr + 1 = 8
 &array[0] = array

 Most people will say the name of an array is a
pointer.

 No – the name of an array is a constant
pointer to the first element of the array.

 i.e. int array[5];
 Is array[1] an Lvalue or Rvalue?
 Is array an Lvalue or Rvalue?
 is *array an Lvalue or Rvalue?

 int arr[] = {1,2,3,4,5,6,7};

 arr[2] =?

 *(arr + 2) = ?

 *(2 + arr) = ?

 2[arr] =?

 int arr[] = {1,2,3,4,5,6,7};

 arr[2] =3

 *(arr + 2) = 3

 *(2 + arr) = 3

 2[arr] =3

 The best teacher is yourself.

 Make an array.
 Decide what output you want.
 Write code to check that output.
 If not, call us and we‟ll help you.

 Ifyou consistently get the output the same as
what you predicted. Congratulations – you are
learning!

 Used for packaging variables.

 Arrays
are collection of lots of variables of
same type under one name.

 Structsare collections of a number of
variables of different types under one name.

 Structure definition and variable creation are
different.

struct person
{
 The following code makes a char *name;
struct called person. int marks;
float phonenum;
 It defines a string pointer for person *sibling;
the name, an integer field for };
marks
 A float field for phone number

 And a pointer to another
person only to say that they‟re
a sibling.

struct person
 Once the definition is set, we {
char name[20];
make variables or objects based int marks;
on this template. float phonenum;
person *sibling;
 Now “person” is a valid data type. };
 struct person ram;
 Creates an object called ram of type
person.
 Strcpy(ram.name, “Hello”);
 ram.marks= “5”;
 ram.phonenum = 4374389;
 Is how you acces elements inside a
structure

struct person
{
 struct
person ali = {“Ali”, 78, char name[20];
893043, NULL}; int marks;
float phonenum;
 Creates new object of type person person *sibling;
with the above values. };

struct person ali = {“Ali”,
 Lookat mike definition. The 78, 893043, NULL};
person* field has been given &ali. struct person mike=
 So mikes sibling field points to ali! {“mike”, 78, 893043,
&ali};

struct person
 The . (dot) operator lets us {
char name[20];
access the structure elements. int marks;
float phonenum;
person *sibling;
 Ifyou are pointing to a structure };
you have to use. (->)
78, 893043, NULL};
 structperson *ptr = ali;
struct person mike=
 ptr->marks; {“mike”, 78, 893043,
 ? &ali};

 ptr->sibling;
 ?

struct person
 The . (dot) operator lets us {
char name[20];
access the structure elements. int marks;
float phonenum;
person *sibling;
 Ifyou are pointing to a structure };
you have to use. (->)
78, 893043, NULL};
 structperson *ptr = ali;
struct person mike=
 ptr->marks; {“mike”, 78, 893043,
 78 &ali};

 ptr->sibling;
 NULL

struct person
 we see the object mike of type {
person. char name[20];
int marks;
 Its sibling field has been set to point
float phonenum;
to ali. person *sibling;
};
 mike.name
 ? 78, 893043, NULL};
 mike.sibling
 ? struct person mike=
 mike.sibling->name {“mike”, 78, 893043,
&ali};
 ?
 Mike.sibling->sibling
 ?

struct person
 we see the object mike of type {
person. char name[20];
int marks;
 Its sibling field has been set to point
float phonenum;
to ali. person *sibling;
};
 mike.name
 mike 78, 893043, NULL};
 mike.sibling
 //memory address of object ali struct person mike=
 mike.sibling->name {“mike”, 78, 893043,
&ali};
 ali
 Mike.sibling->sibling
 NULL

C programming - Pointers

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C programming - Pointers