Notable points in Strings, pointers, Structure, Union and Dynamic memory allocation, code snippets , interview tips , self-ex planation,

1. KEYPOINTS
STRING HANDLING & DYNAMIC
MEMORY ALLOCATION
PREPARED BY
AKILA K
ASSISTANT PROFESSOR
DEPT-CSE
RMKCET

2. STRING DECLARATIONS
char *p = "String";
char p2[] = "String";
char p3[5] = "String";
char arr[7]={‘s',‘t',‘r',‘i',‘n‘,‘g'};
scanf("%s",str); // to read the string
Printf(“%s”,str); // to print the string-

3. String Declaration
What’s the difference between:
char amessage[ ] = "message"
char *pmessage = "message"
Answer:
char amessage[ ] = "message" // single array
Cannot be reassigned to point to another string „
• The array of characters are located on the stack
• can be mutated i.e. you can change the string contents
char *pmessage = "message" // pointer and array
It is pointer that points to first location of the string
„Pointer can be reassigned to point to another string „
Cannot change the string contents (immutable) „The string is placed in a “read only” section

4. Strlib.h

5. Example
char str1[12] = "Hello";
char str2[12] = "World";
char str3[12];
int len ;
strcpy(str3, str1);
printf("strcpy( str3, str1) : %sn", str3 );
strcat( str1, str2);
printf("strcat( str1, str2): %sn", str1 );
len = strlen(str1);
printf("strlen(str1) : %dn", len );
char *a=strchr(str1, 'W');
printf("strchar: %s",a);
char *ab=strstr(str1, "lo");
printf("strchar: %s",ab);

6. Example
int strlen(char str[])
{
int i = 0;
while (str[i] != ‘0’)
{
i++;
}
return i;
}
int strlen(const char* str)
{
int i = 0;
while (*str != ‘0’)
{
i++;
str++;
}
return i;
}

7. String separation
#include <string.h>
#include <stdio.h>
int main ()
{ char str[80] = "This is - www.tutorialspoint.com - website";
const char s[2] = "-";
char *token;
token = strtok(str, s);
while( token != NULL )
{
printf( " %sn", token );
token = strtok(NULL, s);
}
return(0);
}

8. const keyword
const int x; // constant int
x = 2; // illegal - can't modify x
const int* pX; // changeable pointer to constant int
*pX = 3; // illegal - can't use pX to modify an int
pX = &someOtherIntVar; // legal - pX can point somewhere else
int* const pY; // constant pointer to changeable int
*pY = 4; // legal - can use pY to modify an int
pY = &someOtherIntVar; // illegal - can't make pY point anywhere else
const int* const pZ; // const pointer to const int
*pZ = 5; // illegal - can't use pZ to modify an int
pZ = &someOtherIntVar; // illegal - can't make pZ point anywhere else

9. Constant pointer - a constant pointer already pointing to an address, cannot
point to a new address.
Pointer to Constant -type of pointer cannot change the value at the address
pointed by it.
char ch, c;
char *ptr = &ch
ptr = &c
char ch = 'c';
char c = 'a';
char *const ptr = &ch; // A constant pointer
ptr = &c; // wrong
char ch = 'c';
char *ptr = &ch
*ptr = 'a';
char ch = 'c';
const char *ptr = &ch; // A constant pointer
*ptr = 'a';// WRONG!!!

10. Pointer to Pointer
char **p1 = NULL;
char *p2 = NULL;
char value = 'c';
p2 = &value;
p1 = &p2;
printf("n value = [%c]n",value);
printf("n *p2 = [%c]n",*p2);
printf("n p2 = [%lu]n",p2);
printf("n **p1 = [%c]n",**p1);
printf("n p1 = [%lu]n",p1);
Output:
value = [c]
*p2 = [c]
p2 = [8000]
**p1 = [c]
p1 = [5000]

11. Array of Pointers
char *p1 = "Himanshu";
char *p2 = "Arora";
char *p3 = "India";
char *arr[3];
arr[0] = p1;
arr[1] = p2;
arr[2] = p3;
printf("n p1 = [%s] n",p1);
printf("n p2 = [%s] n",p2);
printf("n p3 = [%s] n",p3);
printf("n arr[0] = [%s] n",arr[0]);
printf("n arr[1] = [%s] n",arr[1]);
printf("n arr[2] = [%s] n",arr[2]);

12. Function Pointers
int func (int a, int b)
{
printf("n a = %dn“,a);
printf("n b = %dn“,b);
return 0;
}
int main(void)
{
int(*fptr)(int,int); // Function pointer
fptr = func; // Assign address to function pointer func(2,3);
fptr(2,3);
return 0;
}

13. ASCII VALUES
v

14. VOWELS & CONSONANTS
scanf("%s",str);
while(str[i]!='0')
{
if (str[i]=='a' || str[i]=='e' || str[i]=='i'
|| str[i]=='o' || str[i]=='u' )
{
c=str[i];
c++;
s[i]=(char)c;
}
else if(str[i]>=98 && str[i]<=100)
{
s[i]='e';
}
else if(str[i]>=102 && str[i]<=104)
{
s[i]='i';
}
else if(str[i]>=105 && str[i]<=109)
{
s[i]='o';
}
else if(str[i]>=111 &&
str[i]<=115)
{
s[i]='u';
}
else
{
s[i]='a';
}
i++;
}
printf("%s",s);
return 0;
}

15. Encryption
char st[100];
int x,y,i=0;
fgets(st,100,stdin);
scanf("%d %d",&x,&y);
for(i=0;st[i]!='0';i++)
{
if (st[i]>=97 && st[i]<=122)
printf("%c",st[i]+x);
else if (st[i]>=48 && st[i]<=57)
printf("%c",st[i]+y);
else
printf("%c",st[i]);
}
Input:
hello 101fm
1
1
Output:
ifmmp 212gn

16. lexicographical order
char str[10][50], temp[50];
printf("Enter 10 words:n");
for(i=0; i<10; ++i)
scanf("%s[^n]",str[i])
for(i=0; i<9; i++)
for(j=i+1; j<9 ; j++)
{
if(strcmp(str[i], str[j])>0)
{
strcpy(temp, str[i]);
strcpy(str[i], str[j]);
strcpy(str[j], temp);
}}
printf("nIn lexicographical order: n");
for(i=0; i<10; ++i) { puts(str[i]); }

17. DYNAMIC MEMORY ALLOCATION
malloc() allocates single block of requested memory.
Initializes the allocated memory with garbage values.
(cast_type *)malloc(Size_in_bytes); // 1 argument
calloc() allocates multiple block of requested memory.
initializes the allocated memory with 0 value.
(cast_type *)calloc(blocks , size_of_block); // 2 arguments
realloc() reallocates the memory occupied by malloc() or calloc()
functions.
free() frees the dynamically allocated memory
4 functions of stdlib.h

18. int *x;
x = (int*)malloc(50 * sizeof(int));
free(x)
struct employee {
char *name;
int salary; };
typedef struct employee emp;
emp *e1;
e1 = (emp*)calloc(30,sizeof(emp));
int *x;
x = (int*)malloc(50 * sizeof(int));
x = (int*)realloc(x,100);

19. STRUCTURE AND UNION
• `

20. #include <stdio.h>
union unionJob
{
char name[32]; float salary; int workerNo;
} uJob;
struct structJob
{ char name[32]; float salary;
int workerNo;
} sJob;
int main() {
printf("size of union = %d", sizeof(uJob));
printf("nsize of structure = %d", sizeof(sJob));
return 0; }

21. #include <stdio.h>
union job {
char name[32];
float salary;
int workerNo; } job1;
int main() {
printf("Enter name:n");
scanf("%s", &job1.name);
printf("Enter salary: n");
scanf("%f", &job1.salary);
printf("DisplayingnName :%sn", job1.name);
printf("Salary: %.1f", job1.salary); return 0; }
Enter name Hillary
Enter salary 1234.23
Displaying
Name: f%Bary
Salary: 1234.2
You may get different garbage value or empty string for the name.