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

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”;

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 5 + 1 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

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

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’};

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?

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

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++; }

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.

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

// 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

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”

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

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);

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’) ;

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.

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';

Get string from Key board input c 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 cp264 17  

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);

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++; }

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

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

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 ?

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);

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 ?

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?