Slide set 6 Strings and pointers.pdf

Pointers and Strings
COMPUTER
PROGRAMMING

Reading, writing and manipulating Strings,
Understanding computer memory, accessing via
pointers, pointers to arrays, dynamic
allocation, drawback of pointers.
Topics to be covered

String definition
• String is a 1-d array of characters terminated
by a null (‘0’ )
• char C[] = {'A','B','C','0'};
• char S[] = "ABC";
• Both C and S have length = 3
• You can also have NULL instead of ‘0’ defined
in stdio.h and string.h

String input including blanks
main(){
char name[25] ;
printf ( "Enter name: " ) ;
scanf ( "%s", name ) ;
printf("hello %s",name);
}

Another way
main(){
char C[20];
gets(C); // accepts blank spaces
puts(C);
}

NULL and strlen()
main(){
int i=0;
char C[10];
gets(C);
while(C[i]!=NULL) printf("%c",C[i++]);
//OR while(i<strlen(C)) printf("%c",C[i++]);
}

Few Inbuilt functions
main( ) {
char A[] = "wxYZ", B[]="78",C[20];
printf("length of %s = %d",A,strlen(A));
if(strcmp(A,B)==0) printf("nA,B same srings");
printf("nC = %s",strcpy(C,A));
printf("nUppercase of A = %s",strupr(A));
printf("nConcat A+C = %s",strcat(C,A));
printf("n Reverse A = %s",strrev(A));
}

2-d character array
char Names[6][10] = {"akshay",
"parag",
"raman"};
puts(Names[0]); //akshay

• Reading, writing and
manipulating Strings,
Understanding computer
memory, accessing via pointers,
pointers to arrays, dynamic
allocation, drawback of
pointers.

Advantages of pointers
• Dynamic memory allocation
• System level programming
• To modify a variable in another function
• Illusion of multiple returns
• Passing and returning arrays

& - address of operator
main( ){
int i = 3 ;
printf ( "nAddress of i = %u", &i ) ;
printf ( "nValue of i = %d", i ) ;
}
//%u means unsigned integer, what is %p,%i,%e

* is value at address
main( ) {
int i = 3 ;
printf ( "nAddress of i = %u", &i ) ;
printf ( "nValue of i = %d", i ) ;
printf ( "nValue of i = %d", *( &i ) ) ;
}
// output: some address, 3 and 3

int *j ;
• This declaration tells the compiler that j will
be used to store the address of an integer
value.

Problem 1
main( ) {
int i = 3 ,*j ;
j = &i ;
printf ( "nAddr of i = %u", &i ) ;
printf ( "nAddr of i = %u", j ) ;
printf ( "nAddr of j = %u", &j ) ;
printf ( "n j = %u", j ) ; printf ( "n i = %d", i ) ;
printf ( "n i = %d", *( &i ) ) ; printf ( "n i = %d", *j ) ;
}

Answer
Addr of i = 65524
Addr of i = 65524
Addr of j = 65522
j = 65524
i = 3
i = 3
Value of i = 3

Pointers for other datatypes
• int *alpha ;
• char *ch ;
• float *s ;
Pointers are variables that contain addresses,
and since addresses are whole numbers,
pointers would always contain whole
numbers.

char *ch ;
• *ch will contain a character
• ch is an unsigned int containing the address

main(){
inti=3,*j,**k;
j=&i; k=&j;
printf("naddr of i=%u",&i);
printf("naddr of i=%u",j);
printf("naddr of i=%u",*k);
printf("naddr of j=%u",&j);
printf("naddr of j=%u",k);
printf("naddr of k=%u",&k);
printf("nj=%u",j); printf("nk=%u",k);
printf("ni=%d",i); printf("ni=%d",*(&i));
printf("ni=%d",*j); printf("ni=%d",**k);
}
Problem 2

Answer
addr of i=6487628
addr of i=6487628
addr of i=6487628
addr of j=6487616
addr of j=6487616
addr of k=6487608
j=6487628
k=6487616
i=3
i=3
i=3
i=3

Problem 3
main() {
int Var = 10;
int *ptr = &Var;
printf("Var = %dn", *ptr);
printf("Var Addr = %un", ptr);
*ptr = 20;
printf("Var = %dn", Var);
}

Answer
Var = 10
Var Addr = 6487620
Var = 20

Problem 4
main() {
int v[3] = {10, 100, 200},i;
int *ptr;
ptr = v;
for(i = 0; i < 3; i++) {
printf("*ptr = %dn", *ptr);
printf("ptr = %un", ptr);
ptr++;
}
}

Answer
*ptr = 10
ptr = 6487600
*ptr = 100
ptr = 6487604
*ptr = 200
ptr = 6487608

main( ) {
int i=3,*x; float j=1.5,*y; char k='c',*z ;
printf("n i=%d, j=%f, k=%c",i,j,k) ;
x = &i ; y = &j ; z = &k ;
printf("n Addr of x,y,z = %u, %u, %u",x,y,z);
x++; y++; z++;
printf("n New Addr of x,y,z = %u, %u, %u",x,y,z);
}
Problem 5

Answer
• i=3, j=1.500000, k=c
• Addr of x,y,z = 6487604, 6487600, 6487599
• New Addr of x,y,z = 6487608, 6487604,
6487600

Problem 6
main( ) {
int arr[] = { 10, 20, 30, 45, 67, 56, 74 };
int *i, *j;
i = &arr[1]; j = &arr[5];
printf("%d %d", j-i, *j-*i);
}

Problem 7
main(){
int arr[]={10,20,36,72,45,36};
int*j,*k;
j=&arr[4];
k=(arr+4);
if(j==k)
printf("Same locations");
else printf("Different locations");
}

Problem 8
main(){
int a = 10, b = 20 ;
swapv ( a, b ) ;
printf ( "na = %d b = %d", a, b ) ;
}
swapv ( int x, int y ) {
int t = x ;
x = y ; y = t ;
printf ( "nx = %d y = %d", x, y ) ;
}

Answer
x = 20 y = 10
a = 10 b = 20

Problem 9: Pointer swap
main( ) {
int a = 10, b = 20 ;
swapr ( &a, &b ) ;
printf ( "na = %d, b = %d", a, b ) ;
}
swapr( int *x, int *y ) {
int t = *x ; *x = *y ; *y = t ;
}

Problem 10: One * for 1-d
main( ) {
int a[3]={1,2,3},*b,i;
b=a;
for(i=0;i<3;i++) {
printf("n %d %d",a[i],b[i]);
}}

Output
• 1 1
• 2 2
• 3 3

Problem 11: Multiple returns
AreaPeri(int r, float *A, float *P){
*A = 3.14*r*r; *P = 2*3.14*r;
}
main( ){
int r; float A,P;
printf("nEnter radius: " ); scanf("%d",&r);
AreaPeri(r,&A,&P); printf("Area = %f, Peri = %f",A,P);
}

Same things
• name[i]
• *( name + i )
• *( i + name )
• i[name]

Problem 15: Dynamic allocation
main(){
int *ptr, n,i;
printf("Size of array? "); scanf("%d",&n);
ptr = (int*)malloc(n*sizeof(int));
for(i=0;i<n;i++) ptr[i]=i;
for(i=0;i<n;i++) printf(" %d",ptr[i]);
free (ptr);
}

Output
Size of array? 10
0 1 2 3 4 5 6 7 8 9

Drawback of pointers
• Complicated to use and debug
• Uninitialized pointers might cause
segmentation fault
• Dynamically allocated block needs to be freed
explicitly. Otherwise, it would lead to memory
leak (occupied but unused memory)
• If pointers are updated with incorrect values,
it might lead to memory corruption

Slide set 6 Strings and pointers.pdf

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