1. Lecture 8 String 1. Concept of strings String and pointers
Array of strings
String operations
String library

2. 1. Concepts of String
A string is a null-terminated character array. This means that after the last character, a null character (‘\0’) is stored to signify the end of the character array.
The general form of declaring a string is
char str[size];
For example if we write,
char str[] = “HELLO”;

3. Concepts of String
We are declaring a character array with 5 characters namely, H, E, L, L and O. Besides, a null character (‘\0’) is stored at the end of the string. So, the internal representation of the string becomes HELLO‘\0’.
Note that to store a string of length 5, we need locations (1 extra for the null character).
The name of the character array (or the string) is a pointer to the beginning of the string.
‘\octal_number’ or ‘\xhex_number, -- for character expression, the number has be in the char range
e.g. ‘a’ , ‘\141’, ‘\x61’, 97 are equivalent for char a

4. Example
char name[ ] = “cp264”; printf(“%s”, name); // will print cp264
name array has size 6. or
char name[] = {‘c’, ‘p’, ‘2’, ‘6’, ‘4’, ‘\0’};

5. Example use more space for string
char name[ 20 ];  //name hold a string of 19 characters.
name[0] = ‘c’; // or name[0] = 99;
name[1] = ‘p’;
name[2] = ‘2’;
name[3] = ‘6’;
name[4] = ‘4’;
name[5] = ‘\0’; // or name[5] = 0;
printf(“%s”, name);
Alternative declaration initialization
char name[20] = “cp264”;
char name[20] = {‘c’, ‘p’, ‘2’, ‘6’, ‘4’, ‘\0’};

6. 2. Pointers and Strings
Since string is char array, array pointers and operations apply to string
char name[20] = “cp264”;
char *p;
p = &name[0];
printf("%c\n", *p); // what this print?
printf("%c\n", *(p+3)); // what this print?
printf("%s\n", p+2); // what this print?

7. String pointer
char *p1 = "cp264"; printf("%s\n", p1); printf("%c\n", *(p1+2)); Note: if a string is declared like char *a = “cp264”; Then a can only be read. *(a+1) = ‘c’; is not allowed

8. Pointers and Strings #include<stdio.h> main()
Now, consider the following program that prints a text.
#include<stdio.h>
main()
{ char str[] = “Oxford”;
char *pstr = str;
printf(“\n The string is : ”);
while( *pstr != ‘\0’)
{ printf(“%c’, *pstr);
pstr++; }

9. 3. Arrays of Strings
Suppose there are 20 students in a class and we need a string that stores names of all the 20 students. How can this be done? Here, we need a string of strings or an array of strings. Such an array of strings would store 20 individual strings.
An array of strings is declared as: char names[20][30];
Here, the first index will specify how many strings are needed and the second index specifies the length of every individual string. So we allocate space for 20 names where each name can be maximum 30 characters long.

10. example
char name[5][40] = {"Data strucure II", "C programming language", "cp264"};
printf("\n%s", name[2]);
printf("\n%s", name[0]);
printf("\n%s", name[1]);
cp264
Data strucure II
C programming language
name[i] is char pointer to (i-1) th string printf("\n%c", *(name5[1]+2)); // what this print? *(name[1]+2) = ‘A’; // this is allowed

11. // array of char pointer, each pointer point to a string char
// array of char pointer, each pointer point to a string char *name[] = {"Data strucure II", "C programming language", "cp264"}; printf("\n%s", name[2]); printf("\n%s", name[0]); printf("\n%s", name[1]); // *(name[1]+2) = ‘A’; this is not allowed

12. Command Line Arguments
main( int argc, char *argv[] )
argc = # command line arguments
argv = command line arguments, array of strings
$ a.out argument1 argument2
argc = 3
argv[ 0 ] = “a.out”
argv[ 1 ] = “argument1”
argv[ 2 ] = “argument2”

13. 4. String operations Copy string Read string Write string
Length of a string
Change case
Concatenate/appending string
Compare string …

14. String copy by pointer
char a[] = “C programming language”; char c[30]; int i; // copy string for (i = 0; *(a+i) !='\0'; i++) *(c+i) = *(a+i); *(c+i) = ‘\0’; // add NULL to end //print string for (i = 0; c[i] !='\0'; i++) printf("%c", c[i]);
char d[30];
char *p1, *p2;
p1 = a;
p2 = d;
for (; *p1!='\0'; p1++,p2++)
*p2 = *p1;
*p2 = '\0'; // add NULL to end
printf("%s\n", d);

15. Main(){ char a[] = “C programming language”; char b[30]; copy_string(a, b); } void copy_string(char *from, char *to){ // version 1 for (; *from!='\0'; from++,to++) *to = *from; *to = '\0'; void copy_string(char *from, char *to){ // version 2 while ((*to = * from) != ‘\0’) { to++; from++; } void copy_string(char *from, char *to){ // version 3 while ((*to++ = *from++) != ‘\0’) ;

16. Reading Strings If we declare a string by writing char str[100];
Then str can be read from the user by using three ways
using scanf function
using gets() function
using getchar() function repeatedly
str can be read using scanf() by writing
scanf(“%s”, str);
str can be read by writing
gets(str);
gets() takes the starting address of the string which will hold the input. The string inputted using gets() is automatically terminated with a null character.

17. Reading Strings
str can also be read by calling the getchar() function repeatedly to read a sequence of single characters (unless a terminating character is entered) and simultaneously storing it in a character array.
i=0;
ch = getchar ();
while(ch != '*’)
{ str[i] = ch;
i++;
ch = getchar;
} str[i] = '\0';

18. Get string from Key board inputc p 2 6 4 1 7
<enter>
char name[20];
scanf(“%s”, name); name has value cp264
fgets(name, sizeof(name), stdin);
name has value cp  

19. Writing Strings Strings can be displayed on screen using three ways
using printf() function
using puts() function
using putchar() function repeatedly
str can be displayed using printf() by writing
printf(“%s”, str);
str can be displayed by writing
puts(str);

20. Writing Strings
str can also be written by calling the putchar() repeatedly to print a sequence of single characters
i=0;
while(str[i] != '\0’) {
putchar(str[i]);
i++; }

21. Finding Length of a String
The number of characters in a string constitutes the length of the string.
For example, LENGTH(“C PROGRAMMING IS FUN”) will return 20. Note that even blank spaces are counted as characters in the string.
ALGORITHM TO CALCULATE THE LENGTH OF A STRING
Step 1: [INITIALIZE] SET I = 0
Step 2: Repeat Step 3 while STR[I] != NULL
Step 3: SET I = I + 1
[END OF LOOP]
Step 4: SET LENGTH = I
Step 5: END

22. Converting Characters of a String into Upper Case
ALGORITHM TO CONVERT THE CHARACTERS OF STRING INTO UPPER CASE
Step1: [Initialize] SET I=0
Step 2: Repeat Step 3 while STR[I] != NULL
Step 3: IF STR[1] >= ‘a’ AND STR[I] <= ‘z’
SET Upperstr[I] = STR[I] - 32
ELSE
SET Upperstr[I] = STR[I]
[END OF IF]
[END OF LOOP]
Step 4: SET Upperstr[I] = NULL
Step 5: EXIT

23. 4. String library #include <string.h> String copy function
char *strcpy( char *destination, char *source );
e.g.
char name[20];
strcpy( name, “cp264 data structures” );
2. String length function
int strlen( const char *str ); /* returns string length */
printf(“%d”, sizeof(name)); // what value ?
printf(“%d”, strlen(name)); // what value ?

24. const char src[50] = " cp264 data structures "; char dest[50];
3. Memory copy function
void *memcpy(void *str1, const void *str2, size_t n);
// copies n characters from memory area str2 to memory area str1.
e..g
const char src[50] = " cp264 data structures ";
char dest[50];
memcpy(dest, src, strlen(src)+1);

25. 4. String library #include <string.h>
char *strcpy( char *destination, char *source );
e.g.
char name[20];
strcpy( name, “cp264 data structures” );
const char src[50] = "memory copy function"; char dest[50]; printf("Before memcpy dest = %s\n", dest); memcpy(dest, src, strlen(src)+1);
Get string lenghth
int strlen( const char *str ); /* returns string length */
printf(“%d”, sizeof(name)); // what value ?
printf(“%d”, strlen(name)); // what value ?

26. Concatenate two strings
char *strcat( char *destination, char *source );
Source is appended to destination char s1[20] = “hello “; char s2[20] = “world.”; strcat(s1, s2); // s1 has value hello world.
Compare two strings
int strcmp( const char *first, const char *second ); /* ASCII order */
value = 0 if two strings are identical
value = -1 if first is less than second
value = 1 if first is greater than second
char s1[20] = “hello “; char s2[20] = “world.”;
printf(“%d”, strcmp(s1, s2)); // what’s the output?