Chapter 6 Strings - PDF

Chapter 6 Strings 1 Chapter Outcomes By the end of this chapter, the students will be able to: declare and initialize a string variable read and write a string variable. Use standard string library functions 2  S...

Chapter 6 Strings 1 Chapter Outcomes By the end of this chapter, the students will be able to: declare and initialize a string variable read and write a string variable. Use standard string library functions 2  Strings in C Strings are defined as an array of characters terminated with a null character ‘\0’. When the compiler encounters a sequence of characters enclosed in the double quotes (“), it appends a null character \0 at the end by default. Beginning End of string of string delimter "A String" A S t r i n g \0 String literal values are represented by sequences of characters between double quotes (“)  String vs character String “”; char ‘H’; String “H”; Empty string H H \0 \0 ‘H’ is a single character value (stored in 1 byte) as the ASCII value for H “H” is an array with two characters, the first is H, the second is the character value \0. String – end of Array --- no string character end of string H E L L O \0 H E L L O The difference between a character array and a string is 4 the string is terminated with a special character ‘\0’.  Declaration of a string Declaring a string is as simple as declaring a one dimensional array: char str_name[size]; str_name: is a name given to the string variable size: is used to define the length of the string, i.e the number of characters the string can store. Example: char s; 5  Initialization of a string You can initialize strings in a number of ways. Examples: 1. char S[] = "abcd"; By string 2. char S = "abcd"; literal 3. char S[] = {'a', 'b', 'c', 'd', '\0'}; By char array 4. char S = {'a', 'b', 'c', 'd', '\0'}; Index starts from 0 S S S S S a b c d \0 In example (1) and (2), the C compiler automatically places the '\0' at 6 the end of the string,  Initialization of a string Differences between char array and literal string: ✓ We need to add the '\0' at the end of the array explicitly whereas, it is appended implicitly by the compiler in the case of the literal string. Example1: 7 String Input/Output Use %s field specification in scanf to read string Example 2: Even though Ali Ben Amor was entered in the above program, only “Ali" was stored in the name string. program will ignore Ben Amor because, scanf() function stops reading when if finds a 8 white space.  String Input/Output C Can use the width value in the field specification to limit the number of characters to be read. Example 3: Enter name: Ahmed Your name is Ahm. Strings shorter than the field specification are read normally, but if it is longer, like in this example, it will ignore the rest. Using name without square brackets ‘[‘ and ‘]’ will give the base address of this string. That’s why we have not used ‘&’ in this case as 9 we are already providing the base address of the string to scanf. * However, you still can use ‘&’ before string name.  String Input/Output Use %s field specification in printf. Example 4: For %10s, it added 5 spaces before printing Ahmed, because the length of entered string is 5 and the total length is 10 , 10 For %-10s, it is left aligned, because we use (-), and the extra spaces (if any) will be added after the name.  Read and write a whole line - gets and puts There are predefined functions gets() and puts() in C language to read and display a line of string. Example 5: gets() reads only one string at a time. 11 It stops when the newline character is read. Traversing a String Difference between traversing an integer array and a string: To traverse an integer array, we need to know the length of the array, In the case of string, we may use the null character to identify the end of the string and terminate the loop or using the predefined function (strlen(s)). Hence, there are two ways to traverse a string: ✓By using the length of string (strlen(str)) ✓By using the null character (‘\0’). 12  Traversing String Using the null character Using the length of string Example 6: Example 7: 13  Exercise 1 Write a C program/ algorithm to find the Algorithm ex1 frequency of a character in a string. var str:string ch:character i,frequecy: integer begin 1- write(‘’enter a string’’) 2- read(str) 3- write(‘’enter a character to find the frequency’’) 4- read(ch) 5- frequency0, for(i=0, str[i]!=‘\0’,i++) if(ch=str[i]) frequency  frequency +1 End for 6- write(‘’frequency of ‘’,ch, ‘’=‘’, frequency) 14 end.  String functions Each string literal in a C program is stored at a different location. So even if the string literals contain the same string, they are not equal (in the == sense) Example: char string1 = “hello”; char string2 = “hello”; → but string1 does not equal string2 (they are stored at different locations) string1=string2; → Compile error C provides a wide range of string functions for performing different string tasks, Functions come from the utility library string.h 15 #include to use  String functions strlen(s1) → Returns the length of string s1. Output: 16 String functions strlwr(s1) → converts all the uppercase characters in s1 to lowercase characters. strupr(s1) → converts all the lowercase characters in s1 to uppercase characters. Output: 17 String functions strcpy(s1,s2) → Copies string s2 into string s1. Output: 18 String functions strcat(s1,s2) → Concatenates string s2 onto the end of string s1. 19 Output:  String functions strcmp(s1,s2) → Returns 0, if s1 and s2 are the same; less than 0, if s1s2. Note : the strcmp function compares two strings lexicographically based on their ASCII code. 20 Output: String functions sprintf(s, string format,...) → sends formatted output to a string pointed to, by s. 21 Output:  String functions sscanf(s, c,...) → reads formatted input from a string (s). Output: 22 String functions atoi(s) → converts the string argument s to an integer. atof(s) → converts the string argument s to a float number. #include to use 23 String functions Output: 24  Exercise 2 Write a C program to remove all characters in a string except alphabet Output: 25