2. Strings & Functions

Strings in C

In C, a string is functionally an array of characters ending with a special null character. It is primarily used to store words or text.

The String Terminator \0

Every string in C must be terminated by a null character (identifier: '\0'). The compiler uses this null character to determine the end of the string in memory.

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Initialization of Strings (4 Ways)

C provides multiple ways to declare and initialize a string:

1. Explicit character array with null terminator: char str[] = {'H', 'e', 'l', 'l', 'o', '\0'};
2. Double quotes (Implicit size and null terminator): char str[] = "Hello"; (Compiler automatically sets size to 6 and appends '\0')
3. Fixed size array with double quotes: char str[10] = "Hello"; (Fills unused spaces with '\0')
4. Using a Character Pointer: char *str = "Hello"; (Creates a read-only string literal in memory)

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String Header File <string.h>

To use predefined robust string manipulation functions, C provides the <string.h> header library. You must include this file using #include <string.h> at the top of your program.

Predefined String Functions

• strlen(s1): Returns the numeric length of the string s1 (excluding the null terminator).
• strcpy(s1, s2): Copies string s2 into string s1.
• strcat(s1, s2): Concatenates (appends) string s2 to the end of string s1.
• strcmp(s1, s2): Compares strings s1 and s2.
  • Returns 0 if they are exactly the same.
  • Returns <0 if s1 is lexicographically smaller than s2.
  • Returns >0 if s1 is lexicographically greater than s2.
• strchr(s1, ch): Returns a pointer to the first occurrence of character ch in string s1.
• strstr(s1, s2): Returns a pointer to the first occurrence of string s2 in string s1.

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Non-Standard String Functions

Some compilers (like Turbo C/Windows C libraries) provide non-standard extentions. (Note: These might not work in GCC/Linux environments without custom implementation).

• strrev(s1): Reverses the string s1.
• strlwr(s1): Converts the string s1 to lowercase.
• strupr(s1): Converts the string s1 to uppercase.

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The <ctype.h> Library

The <ctype.h> header provides functions primarily for checking and transforming individual characters (NOT strings, but chars).

Checking Functions (Return non-zero for true, 0 for false):

• isalpha(c): Checks if 'c' is an alphabet letter.
• isdigit(c): Checks if 'c' is a numeric digit.
• isalnum(c): Checks if 'c' is an alphabet string OR numeric digit.
• isspace(c): Checks if 'c' is a whitespace character (space, tab, newline).
• ispunct(c): Checks if 'c' is a punctuation character.

Transforming Functions:

• toupper(c): Converts lowercase letter to uppercase.
• tolower(c): Converts uppercase letter to lowercase.