Character arrays

Introduction

Character arrays let you store and work with text in C. They hold letters, words, or sentences as a list of characters.

When you want to store a word or sentence in your program.

When you need to read text input from the user.

When you want to change or check individual letters in a word.

When you want to print text stored in your program.

When you want to compare two pieces of text.

Syntax

C

char array_name[size];

// Example:
char greeting[6];

The size is how many characters the array can hold.

Remember to leave space for the special '\0' character that marks the end of the text.

Examples

This array can hold a 4-letter word like "code" plus the end marker.

C

char word[5];
// Can hold 4 letters plus '\0' to end the string

The size is set automatically when you initialize with text.

C

char name[] = "Anna";
// Size is 5 automatically (4 letters + '\0')

This creates an empty string with space for 9 characters plus '\0'.

C

char empty[10] = {0};
// All characters start as '\0' (empty string)

Even one letter needs space for '\0' to mark the end.

C

char single_letter[2] = {'A', '\0'};
// Holds one letter and the end marker

Sample Program

This program creates a character array holding "Hello". It prints it, changes the first letter to 'J', then prints again.

C

#include <stdio.h>

int main() {
    char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};
    
    printf("Before change: %s\n", greeting);

    // Change 'H' to 'J'
    greeting[0] = 'J';

    printf("After change: %s\n", greeting);

    return 0;
}

Important Notes

Character arrays end with a special '\0' character to mark the end of the text.

Always leave space for '\0' when declaring the array size.

Time complexity to access or change a character is O(1) because you use the index directly.

Common mistake: forgetting the '\0' causes unexpected behavior when printing or using string functions.

Use character arrays when you need to work with text at a low level in C.

Summary

Character arrays store text as a list of characters ending with '\0'.

You can change letters by accessing array positions.

Always remember to include space for the '\0' end marker.

Practice

1. What is a character array in C used for?

easy

A. To store a sequence of characters, like a word or sentence

B. To store only numbers

C. To perform mathematical calculations

D. To create a list of integers

Solution

1. Step 1: Understand character arrays

   A character array in C is a collection of characters stored in contiguous memory locations.

2. Step 2: Identify its use

   It is mainly used to store text like words or sentences, not numbers or calculations.

3. Final Answer:

   To store a sequence of characters, like a word or sentence -> Option A

4. Quick Check:

   Character array = text storage [OK]

Hint: Character arrays hold text, not numbers or math [OK]

Common Mistakes:

• Thinking character arrays store numbers only
• Confusing character arrays with integer arrays
• Assuming character arrays perform calculations

2. Which of the following is the correct way to declare a character array to hold the word "hello" in C?

easy

A. char word[6] = "hello";

B. char word[5] = {'h', 'e', 'l', 'l', 'o'};

C. char word[] = {'h', 'e', 'l', 'l', 'o', '\0'};

D. char word[5] = "hello";

Solution

1. Step 1: Understand string termination in C

   Strings in C end with a special character '\0' to mark the end.

2. Step 2: Check array size and initialization

   char word[6] = "hello"; declares an array of size 6 (5 letters + '\0') and initializes it with "hello" correctly.

3. Final Answer:

   char word[6] = "hello"; -> Option A

4. Quick Check:

   String needs space for '\0' [OK]

Hint: Remember to include space for '\0' in string arrays [OK]

Common Mistakes:

• Not allocating space for the null terminator '\0'
• Using single quotes for strings
• Declaring array size too small

3. What will be the output of the following C code?

char str[] = "cat";
printf("%c", str[3]);

medium

A. t

B. 0 (null character, no visible output)

C. c

D. Compilation error

Solution

1. Step 1: Understand string indexing

   String "cat" has characters at indices 0:'c', 1:'a', 2:'t', and index 3 is the null terminator '\0'.

2. Step 2: Print character at index 3

   Printing str[3] prints the null character which is invisible, so no visible output appears.

3. Final Answer:

   0 (null character, no visible output) -> Option B

4. Quick Check:

   str[3] = '\0' (null) [OK]

Hint: Remember strings end with '\0' at last index [OK]

Common Mistakes:

• Expecting 't' at index 3 instead of '\0'
• Thinking it causes an error
• Confusing index positions

4. Identify the error in this code snippet:

char name[5];
strcpy(name, "hello world");

medium

A. No error, code works fine

B. Incorrect use of strcpy function

C. Missing #include <string.h>

D. Array size is too small for the string

Solution

1. Step 1: Check array size vs string length

   "hello world" has 11 characters plus '\0' = 12 total, but array size is only 5.

2. Step 2: Understand consequences

   Copying this string will overflow the array, causing undefined behavior.

3. Final Answer:

   Array size is too small for the string -> Option D

4. Quick Check:

   Array must fit string + '\0' [OK]

Hint: Array size must be >= string length + 1 [OK]

Common Mistakes:

• Ignoring array size limits
• Assuming strcpy checks size
• Forgetting null terminator space

5. Given the code below, what will be the output?

char arr[10] = "abc";
arr[3] = 'd';
arr[4] = '\0';
printf("%s", arr);

hard

A. abc\0d

B. abc

C. abcd

D. Runtime error

Solution

1. Step 1: Analyze initial array content

   arr is initialized with "abc" which is 'a','b','c','\0' plus extra space.

2. Step 2: Modify characters and null terminator

   arr[3] = 'd' adds 'd' after 'c', then arr[4] = '\0' sets new string end after 'd'.

3. Step 3: Print the string

   printf prints characters until '\0', so output is "abcd".

4. Final Answer:

   abcd -> Option C

5. Quick Check:

   String ends at arr[4] '\0' [OK]

Hint: Null terminator controls string end in arrays [OK]

Common Mistakes:

• Ignoring manual null terminator placement
• Expecting original string without changes
• Confusing character and string printing