Unit 9: Strings

1. Defining and Initializing Strings
2. Reading and Writing Strings
3. Processing of Strings
4. String Library Functions
5. Exam Focus Enhancements

1. Defining and Initializing Strings

In C programming, a string is defined as a one-dimensional array of characters terminated by a null character (\0). The null character is essential because it marks the end of the string in memory.

Declaration

char string_name[size];

When declaring a string, the size must be large enough to hold the characters plus the null terminator. For example, to store "HELLO", you need an array of at least 6 characters.

Initialization Methods

Character-by-character: char str[] = {'H', 'e', 'l', 'l', 'o', '\0'};
String Literal: char str[] = "Hello"; (The compiler automatically adds \0 at the end).
With Size: char str[10] = "Hello"; (The remaining 4 indices are filled with null characters).

2. Reading and Writing Strings

C provides several ways to perform Input and Output (I/O) operations on strings.

Reading Strings

scanf(): scanf("%s", str); - Reads a string but stops at the first whitespace (space, tab, or newline).
gets(): gets(str); - Reads a full line of text including spaces until the Enter key is pressed. (Note: Be careful as it does not check for buffer overflow).

Writing Strings

printf(): printf("%s", str); - Displays the string on the screen.
puts(): puts(str); - Displays the string and automatically adds a newline character at the end.

3. Processing of Strings

Processing involves manipulating the characters within the string using loops or pointers. Since a string is an array, you can access individual characters using their index.

Example: Counting Vowels
To process a string, you typically use a loop that runs until it encounters the '\0' character.

 for(int i = 0; str[i] != '\0'; i++) { // Process each character str[i] }

4. String Library Functions

The <string.h> header file provides a powerful set of functions to simplify string operations.

Function	Purpose	Example Usage
strlen()	Returns the length of the string (excluding \0).	len = strlen("Hello"); // Result: 5
strcpy()	Copies one string into another.	strcpy(dest, src);
strcat()	Concatenates (joins) two strings.	strcat(s1, s2);
strcmp()	Compares two strings alphabetically.	if(strcmp(s1, s2) == 0) // Identical
strrev()	Reverses the string characters.	strrev(str);

5. Exam Focus Enhancements

Exam Tips

The +1 Rule: Always remember to allocate space for the null terminator. If your string has 5 characters, your array size must be at least 6.
Header File: Never forget to include #include <string.h> when using functions like strlen or strcpy.
strcmp Return Values: 0 means equal, positive means s1 > s2, and negative means s1 < s2.

Common Mistakes

No Null Terminator: Treating a character array without a '\0' as a string. Functions like printf will keep reading memory until they find a zero, causing "garbage" output.
scanf Limitation: Using scanf("%s", name) to read a name with a space (e.g., "John Doe"). It will only store "John".
String Assignment: Attempting str1 = str2; to copy strings. In C, you must use strcpy() for this operation.

Frequently Asked Questions

Q: What is the difference between 'A' and "A"?
A: 'A' is a single character (1 byte), while "A" is a string containing 'A' and '\0' (2 bytes).

Q: Is the null character included in the length returned by strlen()?
A: No, strlen() only counts the visible characters.

Q: Why is gets() considered dangerous?
A: It does not check the size of the array, so if a user enters more characters than the array can hold, it overwrites other memory (buffer overflow).

Knowlet