Strings

In C, a string is a sequence of characters stored in contiguous memory, terminated by a special null character ('\0'). Unlike modern languages (like Python or Java) that have a built-in string type, C uses character arrays to represent strings.

Example:

code.c

char str[] = "Hello";

Memory representation:

Index	Value
0	H
1	e
2	l
3	l
4	o
5	\0

• Every string in C must end with '\0'.

Characteristics of Strings:

• Stored in contiguous memory.
• Terminated with null character.
• Can be manipulated using <string.h> library functions.
• Must allocate enough space for all characters plus one for '\0'.

Ways to Declare and Initialize Strings

String Literal

code.c

char str[] = "Hello";

• Compiler adds '\0' automatically.

Character Array

code.c

char str[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

Specifying Extra Size

code.c

char str[20] = "Hello"; // extra space available

• Always allocate an extra byte for ‘\0’.

Input and Output of Strings

Using printf and scanf

code.c

char name[20];
scanf("%s", name); // stops at space
printf("Name: %s", name);

Using gets()

code.c

gets(name);

Using fgets()

code.c

fgets(name, sizeof(name), stdin);

• Reads full line, including spaces.
• Safer than gets().

Remove trailing newline:

code.c

name[strcspn(name, "\n")] = '\0';

String Output

code.c

printf("%s", str);
puts(str); //adds newline automatically.

• Pointer points to string literal in read-only memory.
• Modifying this is undefined behavior.

String Handling Functions (<string.h>)

Function	Description
strlen(s)	Length of string (without '\0').
strcpy(dest, src)	Copy string.
strncpy()	Copy up to n characters.
strcat()	Append one string to another.
strcmp(s1, s2)	Compare two strings.
strchr()	Find first occurrence of a character.
strstr()	Find substring.

• Always include:

code.c

#include <string.h>

Examples of String Functions

String Length

code.c

#include <stdio.h>
#include <string.h>
int main() {
  char str[] = "C Programming";
  printf("Length: %zu", strlen(str));
  return 0;
}

Output

Length: 13

Copy String

code.c

char src[] = "Hello";
char dest[20];
strcpy(dest, src);

Concatenate Strings

code.c

char str1[20] = "Hello ";
char str2[] = "World";
strcat(str1, str2);

Compare Strings

code.c

if(strcmp("abc", "abc") == 0)
printf("Equal");

Manual Implementation of String Functions

strlen()

code.c

int my_strlen(char *str) {
  int length = 0;
  while(str[length] != '\0')
      length++;
  return length;
}

Strings vs Character Arrays

Aspect	String Literal	Character Array
Mutability	Cannot modify (read-only)	Can modify
Storage	Stored in text segment	Stored in stack or heap
Null Required	Yes	Yes

Advantages

• Efficient representation of text.
• Wide library support.

Disadvantages

• Manual memory management.
• Risk of buffer overflow if not careful.

Real-World Applications

• Text processing (editors, compilers).
• File paths handling.
• Command-line argument parsing.
• Network protocols (message parsing).

Example

code.c

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int main() {
  char str[100];
  int count = 0;
  printf("Enter a string: ");
  fgets(str, sizeof(str), stdin);

for(int i = 0; str[i] != '\0'; i++) {
    char ch = tolower(str[i]);
    if(ch=='a'||ch=='e'||ch=='i'||ch=='o'||ch=='u')
        count++;
}

printf("Vowel count: %d\n", count);
return 0;
}

Output

Enter a string: hello
Vowel count: 2