Address
Review of Arrays
int m[4]
Array name m (int []) 100
refers to constant memory location
Cant change the memory location ??? 100
(int)
but the array elements can be
changed ??? 104
(int)
Space is allocated at declaration ??? 108
(int)
time ???
(int)
112
Review of call-by-value
Arguments passed by value
A temporary copy of each argument is created,
solely for use within the function call
f()
void f(int x, int y) { } 17 42
(int) (int )
x y
main() {
int a = 17, b = 42; 17 42
f(a,b); (int) (int)
.... } a b
main()
Passing Arrays to functions
Specify the name of the array without brackets [].
Name of array is address of first element.
Function knows where array is stored: modifies
original location f()
(Call by Reference) int 2
(int )
void f(int x[],int y) {} x[] y
main() {
int b=2, a[b]={1,2}; int 2
f(a,b); (int)
.... } a b
main()
#include<stdio.h>
float avg(int a[], int size)
{
int i;
float total = 0.;
for(i = 0; i < size; i++)
total+=a[i];
return total/size;
}
main(){
int array[4] = {2, 4, 6, 8};
printf("Average : %f",avg(array,4));
}
#include <stdio.h>
void printArray(int array[][4]); /* declare function */
int main() {
int array[3][4] = {0,1,2,3,4,5,6,7,8,9,10,11};
printArray(array);
return 0;
}
void printArray(int a[][4]) { /* define function */
int i, j;
for(i=0 ; i<3 ; i++) {
for(j=0 ; j<4 ; j++) {
printf("%2d ", a[i][j]);
Output
}
0 1 2 3
printf("\n");
4 5 6 7
}
8 9 10 11
}
Row of house
Row of memory cells House No.
Memory Address
Pointers
House No:
0x1234
0x1236
People inside house
Content of memory
cells 0x1238
0x1240
&<variable-name>: specifies the address of the variable
Print the address
#include <stdio.h>
int main() {
int x = 0;
printf("Address of x ");
printf("= 0x%p \n", &x);
return 0;
}
Address of x = 0x7fffbfbf34dc
What are pointers?
Variables that store the address of other
variables.
Declared just like a variable except that
it has the dereference operator
int *p;
Example
a b p
int a = 1, b = 2, *p; 1 2
p = &a; a b p
1 2
a b p
b = *p; 1
#include <stdio.h>
int main() {
int x = 12; int *ptr = &x;
printf("Address of x: 0x%p\n", ptr);
printf("Address of x: 0x%p\n", &x);
printf("Address of ptr: 0x%x\n", &ptr);
printf("Value of x: %d\n", *ptr);
return 0;
}
Declarations and Initializations
int i = 3, j = 5, *p = &i, *q = &j, *r;
double x;
Expression Value Type
p == &i p == (&i) 1
**&p *(*(&p)) 3
r = &x illegal
7 ** p / *q + 7 ((7 *(* p)) / (*q)) + 7 11
* (r = &j) *=* p (* (r = (&j))) *= (* p) 15
Call-by-Reference
void swap(int *p, int *q) {
int tmp;
tmp=*p; *p=*q; *q=tmp; }
int main(void) {
int i = 3, j = 5;
printf(Value of i=%d,j=%d\n,i,j);
}
Call-by-Reference
void swap(int *p, int *q) {
int tmp;
tmp=*p; *p=*q; *q=tmp;
}
p ?=100 q ?=200
100 int1 200 int2
Call-by-Reference
void swap(int *p, int *q) {
int tmp;
tmp=*p; *p=*q; *q=tmp;
}
p ?=100 q ?=200
100 int1 200 int2
tmp
int1
Call-by-Reference
void swap(int *p, int *q) {
int tmp;
tmp=*p; *p=*q; *q=tmp;
}
p ?=100 q ?=200
100 int2 200 int2
tmp
int1
Call-by-Reference
void swap(int *p, int *q) {
int tmp;
tmp=*p; *p=*q; *q=tmp;
}
p ?=100 q ?=200
100 int2 200 int1
tmp
int1
1. Declare function parameter
to be a pointer
Call-by-Reference
void swap(int *p, int *q) {
int tmp;
tmp=*p; *p=*q; *q=tmp; }
2. Use dereferenced pointer
int main(void) { in the function body
int i = 3, j = 5; 3. Pass address as argument
swap(&i, &j); to the function call
printf(Value of i=%d,j=%d\n,i,j);
}
Pointer Arithmetic
int *p = &i; p: 100
104
i: 104
int
Pointer Arithmetic (2)
p: 100
int *p = &i;
108
++p;
i: 104
Assuming an integer
int
is 4 bytes, it will store
the value 108. : 108
+= and -= are valid int
*= and /= are illegal
Arrays & Pointers
An array name stores the address of the
first element of the array. Pointers also
store address of memory locations.
An array name is an address or pointer
that is fixed but the values of pointer
variables are not fixed.
a[0] is equivalent to *a
a[i] is equivalent to *(a + i)
#include<stdio.h>
float avg(int a[], int size)
{
int i;
float total = 0.;
for(i = 0; i < size; i++)
total+=a[i];
return total/size;
}
main(){
int array[4] = {2, 4, 6, 8};
printf("Average : %f",avg(array,4));
}
#include<stdio.h>
float avg(int *a, int size)
{
int i;
float total = 0.;
for(i = 0; i < size; i++)
total+=*(a+i);
return total/size;
}
main(){
int array[4] = {2, 4, 6, 8};
printf("Average : %f",avg(array,4));
}
Write a program that takes as input N
integers and print the numbers in
reverse order?
Dynamic Memory Allocation
So far, memory allocation was handled
automatically at compile time.
Certain cases you dont know how
much memory to set aside
Dynamically allocate memory to variables
at run-time. Example is an unsized array.
The following four functions are used:
malloc(), calloc(), realloc() and free().
malloc()
Requires one argument, the number of
bytes you want to allocate dynamically
If successful, returns a void pointer
assign this to a pointer variable
int *p;
p = malloc(10 * sizeof(int));
If memory allocation fails, malloc will
return a NULL pointer.
free()
free(ptr) will release the memory that
was allocated to the pointer variable ptr.
It is good practice to free memory when
you have finished with it.
#include <stdio.h>
#include <stdlib.h> /* required for the malloc and free
functions */
int main() {
int number, i;
int *ptr;
printf("How many ints would you like store? ");
scanf("%d", &number);
/* allocate memory */
ptr = malloc(number*sizeof(int));
if(ptr!=NULL) {
for(i=0 ; i<number ; i++)
scanf("%d",(ptr+i));
/* print out in reverse order */
for(i=number - 1 ; i>=0 ; i--) {
printf("%d\n", *(ptr+i));
}
/* free allocated memory */
free(ptr);
return 0;
}
else {
printf("\nMemory allocation failed.\n");
return 1;
}
}
Review
Allocating memory @run-time
char *ptr;
ptr = malloc(100);
if (ptr == NULL) printf(..!!);
malloc allocates contiguous block of
memory of at least 100 bytes and
returns the address of the first byte
Returns NULL if the allocation fails.
Review
De-allocating memory
free(ptr);
De-allocates the block of memory
pointed to by ptr.
After calling free, ptr is uninitialized
and using ptr will result in error
The lifetime of an allocated block is
determined by malloc/calloc/realloc
and free; other functions have no
effect on its existence.
calloc()
Allocates continuous space for an array
of elements.
Requires two arguments,
calloc(n, el_size), allocate space for n
elements each of el_size
space shall be all initialized to 0 bits
If successful, returns a void pointer else
NULL
#include <stdio.h>
#include <stdlib.h> /* required for the malloc and free
functions */
int main() {
int number, i;
int *ptr;
printf("How many ints would you like store? ");
scanf("%d", &number);
/* allocate memory */
ptr = malloc(number*sizeof(int));
if(ptr!=NULL) {
for(i=0 ; i<number ; i++)
scanf("%d",(ptr+i));
#include <stdio.h>
#include <stdlib.h> /* required for the malloc and free
functions */
int main() {
int number, i;
int *ptr;
printf("How many ints would you like store? ");
scanf("%d", &number);
/* allocate memory */
ptr = calloc(number,sizeof(int));
if(ptr!=NULL) {
for(i=0 ; i<number ; i++)
scanf("%d",(ptr+i));
malloc Vs. calloc
Allocates memory Allocates as a
in bytes sequence of bytes
Allocated space is Allocated space
not initialized initialized to zero.
realloc()
Allows to allocate more memory space
without losing data.
Requires two arguments,
realloc(*ptr, el_size),
First is the pointer referencing memory
Second is the total no. of bytes you want to
reallocate.
If successful, returns a void pointer else
NULL
#include<stdio.h>
#include <stdlib.h>
int main() {
int *ptr;
int i;
ptr = calloc(5, sizeof(int));
if(ptr!=NULL) {
*ptr = 1;
*(ptr+1) = 2;
ptr[2] = 4;
ptr[3] = 8;
ptr[4] = 16;
ptr = realloc(ptr, 7*sizeof(int));
if(ptr!=NULL) {
printf("Now allocating more memory... \n");
ptr[5] = 32; /* now it's legal! */
ptr[6] = 64;
for(i=0 ; i<7 ; i++) {
printf("ptr[%d] holds %d\n", i, ptr[i]);
}
realloc(ptr,0); /* same as free(ptr); - just fancier! */
return 0;
}
}
Using const qualifier with pointers
const qualifier means variable cannot
be changed
int *const ptr1;
points to a constant memory location
must be initialized when defined
*const int ptr2;
ptr2 points to a memory location that stores
constant integers.
#include <stdio.h>
void xyz(int *number) {
*number = *number/2;
}
void xyz2(int number) {
number = number/2;
}
int main() {
int i = 8;
int *ptr = &i;
xyz(ptr);
printf("The value of i is %d\n", i);
xyz(&i);
printf("The value of i is %d\n", i);
xyz2(i);
printf("The value of i is %d\n", i);
return 0;
}
Character Array versus Strings
A character array
char array1[4] = {d','e','e',r};
A string constant
char array2[7]
={c','o','f','f','e','e','\0'};
Strings
A string is an array of characters
terminated by the end of string sentinel
\0 or null character.
String constants are written within
double quotes ().
Conversion parameter %s causes the
value of the corresponding string
argument to be printed.
Initializing & Printing Strings
char str1[] = Hello World; // Compiler
counts the number of characters
char *str2 = Hello World;
printf(%s, str1);
printf(%s, str2);
printf(First Character = %c, str1[0]);
printf(First Character = %c, *str2);
Declarations and Initializations
char str1[] = "Hello World!";
char *str2 = "Hello World";
Expression Value
printf(" %s", str1+1); ello World!
printf(" %s", str2+1); ello World
printf(%c, str1[2]); l
printf(%c, *(str2+2)); l
printf(%c, *(str2)+2); J
