Character arrays in C++

Introduction

Character arrays let you store and work with text in C++. They are simple ways to hold words or sentences as a list of letters.

When you want to store a word or sentence in a fixed-size container.

When you need to manipulate text letter by letter.

When you want to pass text to functions that expect C-style strings.

When you want to learn how text is stored in memory at a low level.

When you want to avoid using more complex string classes for simple tasks.

Syntax

C++

char arrayName[size];

// Example:
char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

The size must be enough to hold all characters plus the special '\0' character that marks the end of the text.

The '\0' character is called the null terminator and tells C++ where the text ends.

Examples

This array holds 5 letters but is missing the '\0' at the end, so it is not a proper C-style string.

C++

char word[5] = {'H', 'e', 'l', 'l', 'o'}; // No null terminator, not a proper string

This array holds the word "Hello" and ends with '\0' so C++ knows where the text ends.

C++

char word[6] = {'H', 'e', 'l', 'l', 'o', '\0'}; // Proper string with null terminator

This array represents an empty string because it only contains the null terminator.

C++

char empty[1] = {'\0'}; // Empty string with only null terminator

This is a shortcut to create a character array with the text "Hi!" and the null terminator.

C++

char greeting[] = "Hi!"; // Size is automatically set to 4 (3 letters + '\0')

Sample Program

This program creates a character array holding "Hello". It prints the word, changes the first letter to 'Y', then prints the new word "Yello". Finally, it prints each letter separately.

C++

#include <iostream>

int main() {
    // Create a character array to hold the word "Hello"
    char greeting[6] = {'H', 'e', 'l', 'l', 'o', '\0'};

    // Print the greeting
    std::cout << "Greeting: " << greeting << std::endl;

    // Change the first letter
    greeting[0] = 'Y';

    // Print the changed greeting
    std::cout << "Changed Greeting: " << greeting << std::endl;

    // Show each character one by one
    std::cout << "Characters: ";
    for (int i = 0; greeting[i] != '\0'; i++) {
        std::cout << greeting[i] << ' ';
    }
    std::cout << std::endl;

    return 0;
}

Important Notes

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

Space complexity is fixed by the array size you declare.

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

Use character arrays when you want simple, fixed-size text storage without extra features of std::string.

Summary

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

Always leave space for the null terminator to mark the end of the text.

You can change characters by accessing array positions directly.

Practice

1. What is the purpose of the null terminator '\0' in a character array in C++?

easy

A. It marks the end of the string stored in the array.

B. It initializes all characters to zero.

C. It increases the size of the array by one.

D. It converts characters to uppercase.

Solution

1. Step 1: Understand the role of '\0' in character arrays

   The null terminator '\0' is a special character used to mark the end of a string in C++ character arrays.

2. Step 2: Identify its effect on string handling

   Functions that work with strings stop reading characters when they encounter '\0', so it tells where the string ends.

3. Final Answer:

   It marks the end of the string stored in the array. -> Option A

4. Quick Check:

   Null terminator = end of string [OK]

Hint: Remember '\0' ends the string, like a period ends a sentence. [OK]

Common Mistakes:

• Thinking '\0' initializes characters to zero
• Confusing '\0' with increasing array size
• Assuming '\0' changes character case

2. Which of the following is the correct way to declare a character array to hold the word "code" including the null terminator?

easy

A. char word[3] = {'c', 'o', 'd', '\0'};

B. char word[4] = {'c', 'o', 'd', 'e'};

C. char word[4] = "code";

D. char word[5] = {'c', 'o', 'd', 'e', '\0'};

Solution

1. Step 1: Count characters and null terminator

   The word "code" has 4 letters plus 1 null terminator '\0', so the array size must be 5.

2. Step 2: Check each option's array size and initialization

   char word[5] = {'c', 'o', 'd', 'e', '\0'}; correctly declares a char array of size 5 and includes the null terminator explicitly.

3. Final Answer:

   char word[5] = {'c', 'o', 'd', 'e', '\0'}; -> Option D

4. Quick Check:

   Array size = letters + 1 for '\0' [OK]

Hint: Array size = number of chars + 1 for '\0'. [OK]

Common Mistakes:

• Forgetting to add space for the null terminator
• Using incorrect array size
• Not including '\0' explicitly when needed

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

char arr[6] = "hello";
arr[0] = 'H';
std::cout << arr << std::endl;

medium

A. hello

B. Hello

C. H

D. Compilation error

Solution

1. Step 1: Understand the initial array content

   The array is initialized with "hello" which includes the null terminator automatically, so arr = {'h','e','l','l','o','\0'}.

2. Step 2: Modify the first character and print

   Changing arr[0] to 'H' updates the first letter. Printing arr outputs the string until '\0', so output is "Hello".

3. Final Answer:

   Hello -> Option B

4. Quick Check:

   Changing arr[0] changes first letter [OK]

Hint: Changing arr[0] changes first letter; output prints until '\0'. [OK]

Common Mistakes:

• Expecting output to be lowercase 'hello'
• Thinking only one character prints
• Assuming compilation error due to modification

4. Identify the error in the following code snippet:

char name[4] = "John";
std::cout << name << std::endl;

medium

A. Incorrect use of double quotes for character array.

B. Missing semicolon after declaration.

C. Array size is too small to hold the string and null terminator.

D. Cannot print character arrays with std::cout.

Solution

1. Step 1: Check array size vs string length

   The string "John" has 4 characters plus a null terminator, so it needs an array of size 5.

2. Step 2: Understand consequences of small array

   Declaring name[4] leaves no space for '\0', causing undefined behavior when printing.

3. Final Answer:

   Array size is too small to hold the string and null terminator. -> Option C

4. Quick Check:

   Array size must include space for '\0' [OK]

Hint: Always add 1 to string length for '\0' in array size. [OK]

Common Mistakes:

• Ignoring null terminator space
• Thinking printing char arrays is invalid
• Confusing syntax errors with size errors

5. You want to store the phrase "Hi!" in a character array and then replace the exclamation mark with a question mark. Which code correctly does this?

hard

A. char phrase[4] = "Hi!";
phrase[2] = '?';
std::cout << phrase << std::endl;

B. char phrase[3] = {'H', 'i', '?'};
std::cout << phrase << std::endl;

C. char phrase[4] = "Hi?";
phrase[3] = '!';
std::cout << phrase << std::endl;

D. char phrase[3] = "Hi!";
phrase[3] = '?';
std::cout << phrase << std::endl;

Solution

1. Step 1: Declare array with correct size for "Hi!"

   The phrase "Hi!" has 3 characters plus 1 for '\0', so array size must be 4.

2. Step 2: Replace '!' with '?' at correct index

   The '!' is at index 2, so phrase[2] = '?' correctly replaces it.

3. Step 3: Print the modified phrase

   Printing phrase outputs "Hi?" as expected.

4. Final Answer:

   char phrase[4] = "Hi!"; phrase[2] = '?'; std::cout << phrase << std::endl; -> Option A

5. Quick Check:

   Array size = 4; replace index 2 [OK]

Hint: Array size = chars + 1; replace character by index. [OK]

Common Mistakes:

• Using array size too small to hold string and '\0'
• Replacing wrong index character
• Confusing initialization with replacement