Strings in C

A string is a sequence of characters such as a name, city, message, or sentence.

"Hello"
"India"
"Welcome to C"

In C, strings are stored inside character arrays.

A character represents a single character, while a string represents a sequence of characters.

char ch = 'A';

char name[] = "Amit";

Single quotes are commonly used for character constants, while double quotes are used for string literals.

A string can be declared using a character array.

char name[20];

This creates a character array capable of storing characters, including the terminating null character, within its declared size.

A string can be initialized directly when declaring the character array.

char name[] = "Rahul";

The compiler automatically reserves space for the characters and the terminating '\0'.

The null character '\0' marks the end of a C string.

char name[] = {'H', 'i', '\0'};

Functions that work with C strings use this character to determine where the string ends.

Each character in a string is stored at an array index.

char name[] = "Hello";

printf("%c", name[0]);
printf("%c", name[1]);

The first character is stored at index 0.

Individual characters can be accessed using their indexes.

char word[] = "Computer";

printf("%c", word[0]);
printf("%c", word[3]);

The index must be within the valid bounds of the character array.

Characters in a modifiable character array can be changed using an index.

char word[] = "Cat";

word[0] = 'B';

printf("%s", word);

The output becomes Bat.

The %s format specifier is commonly used with printf() to print a C string.

char name[] = "Amit";

printf("%s", name);

The characters are printed until the null character is reached.

A string can be printed together with other text.

char name[] = "Ravi";

printf("My name is %s", name);

This is useful for displaying formatted messages.

The %s format specifier can be used with scanf() to read a word.

char name[30];

scanf("%29s", name);

The array name is passed without & because it already represents the address of its first element in this context.

When scanf("%s", ...) is used, input stops at whitespace.

char name[30];

scanf("%29s", name);

If the user enters Raj Kumar, only Raj is read. Use a line-input method when you need to read spaces.

The fgets() function can read a line of text, including spaces.

char name[50];

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

It is generally safer than using an unbounded scanf("%s", ...) for a character array.

When the input fits in the buffer, fgets() may store the newline character entered by the user.

char name[50];

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

The newline can be removed when required before further processing.

The strlen() function is used to find the length of a C string. It is declared in <string.h>.

#include <string.h>

char word[] = "Hello";

printf("%zu", strlen(word));

The length does not include the terminating '\0'.

The strcpy() function can copy a string into another character array.

#include <string.h>

char source[] = "Hello";
char destination[20];

strcpy(destination, source);

printf("%s", destination);

The destination array must have enough space for the copied string and its terminating null character.

The strcmp() function can be used to compare two strings.

#include <string.h>

char a[] = "Apple";
char b[] = "Apple";

if(strcmp(a, b) == 0)
{
    printf("Strings are equal");
}

The strcat() function appends one string to another.

#include <string.h>

char first[30] = "Hello ";
char second[] = "World";

strcat(first, second);

printf("%s", first);

The destination array must have enough space for the resulting string.

Many standard string functions are declared in:

#include <string.h>

Common functions include:

• strlen()
• strcpy()
• strcat()
• strcmp()

A string can be processed character by character using a loop.

char word[] = "Hello";

for(int i = 0; word[i] != '\0'; i++)
{
    printf("%c\n", word[i]);
}

The loop stops when it reaches the null character.

A loop can be used to count characters in a string.

char word[] = "Computer";
int count = 0;

for(int i = 0; word[i] != '\0'; i++)
{
    count++;
}

printf("Length = %d", count);

You can check each character and count vowels.

char word[] = "education";
int count = 0;

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

printf("Vowels = %d", count);

Characters can be changed individually. For example, a lowercase letter can be changed to uppercase using the appropriate character conversion function.

#include <ctype.h>

char ch = 'a';

printf("%c", toupper((unsigned char)ch));

The <ctype.h> header provides functions such as toupper() and tolower().

A string literal can initialize a character array.

char city[] = "Patna";

The array contains the characters of the string followed by a null character. The array contents can be modified.

city[0] = 'R';

It is important to understand the difference between a character array and a string literal.

char name[] = "Amit";

Here, name is a modifiable character array initialized from the string literal.

Do not attempt to modify a string literal through a pointer.

const char *text = "Hello";

Always provide enough space for the characters and the terminating '\0'.

char name[6] = "Hello";

This is valid because five characters plus the null character require six elements.

An array that is too small cannot safely hold the complete string.

Do not use == to compare the contents of two C strings.

strcmp(first, second) == 0

The strcmp() function compares the characters of the strings.

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

int main()
{
    char name[50];

    printf("Enter your name: ");
    fgets(name, sizeof(name), stdin);

    printf("Name: %s", name);
    printf("Length: %zu", strlen(name));

    return 0;
}

This program reads a line of text and finds its length. If the input fits in the array, fgets() may also store the newline.

Strings are used in many real-world programs.

• Student names
• Usernames
• Passwords and login input
• Addresses
• Messages
• City and country names
• File names
• Search text
• Descriptions
• Text processing

Create a C program that:

1. Takes a student's name as input.
2. Displays the name.
3. Finds its length.
4. Counts the vowels.
5. Counts the consonants.
6. Displays each character on a separate line.

Use a character array, loops, and appropriate string/character functions.