📝 Lesson 6: Arrays and Strings

What Are Arrays?

Imagine you have 30 students in a class. Would you create 30 separate variables for their grades? That would be like having 30 separate lockers scattered around the school! Arrays are like a row of lockers - organized, numbered, and easy to access.

Declaring and Initializing Arrays

Array Declaration Examples

// Method 1: Declare then initialize
int scores[5];                    // Creates array of 5 integers
scores[0] = 85;
scores[1] = 92;
scores[2] = 78;
scores[3] = 95;
scores[4] = 88;

// Method 2: Declare and initialize together
int scores[5] = {85, 92, 78, 95, 88};

// Method 3: Let compiler count
int scores[] = {85, 92, 78, 95, 88};  // Size is 5

// Method 4: Partial initialization
int scores[5] = {85, 92};  // Rest are 0: {85, 92, 0, 0, 0}

// Method 5: Initialize all to zero
int scores[5] = {0};  // All elements are 0

💡 Modern C++ note: Raw arrays like int scores[5] still work everywhere, but modern C++ usually prefers std::array<int, 5> for a fixed-size collection (it knows its own size and plays nicely with range-based for and the standard algorithms) and std::vector<int> when the size can change. We start with raw arrays to learn the fundamentals, then meet std::vector at the end of the lesson.

Array Indexing: The Key to Access

Arrays use zero-based indexing - like floor numbers in some countries where the ground floor is 0!

Working with Arrays and Loops

Arrays and loops are best friends - they work together perfectly!

Common Array Operations

const int SIZE = 5;
int numbers[SIZE] = {5, 3, 8, 2, 7};

// 1. Find sum and average
int sum = 0;
for (int i = 0; i < SIZE; i++) {
    sum += numbers[i];
}
double average = static_cast<double>(sum) / SIZE;

// 2. Find maximum value
int max = numbers[0];
for (int i = 1; i < SIZE; i++) {
    if (numbers[i] > max) {
        max = numbers[i];
    }
}

// 3. Search for a value
int target = 8;
bool found = false;
for (int i = 0; i < SIZE; i++) {
    if (numbers[i] == target) {
        found = true;
        cout << "Found at index " << i << '\n';
        break;
    }
}

// 4. Reverse the array
for (int i = 0; i < SIZE/2; i++) {
    int temp = numbers[i];
    numbers[i] = numbers[SIZE-1-i];
    numbers[SIZE-1-i] = temp;
}

Multi-Dimensional Arrays: Tables and Grids

2D arrays are like spreadsheets or chess boards - they have rows and columns!

2D Array Examples

// Declare a 3x3 tic-tac-toe board
char board[3][3] = {
    {'X', 'O', 'X'},
    {'O', 'X', 'O'},
    {'X', 'X', 'O'}
};

// Access elements
cout << board[0][0];  // Prints 'X' (top-left)
cout << board[1][1];  // Prints 'X' (center)

// Process with nested loops
for (int row = 0; row < 3; row++) {
    for (int col = 0; col < 3; col++) {
        cout << board[row][col] << " ";
    }
    cout << '\n';
}

// Grade table for multiple students
int grades[4][6] = {
    {85, 92, 78, 95, 88, 91},  // Student 0
    {79, 87, 73, 89, 92, 85},  // Student 1
    {92, 95, 89, 97, 94, 96},  // Student 2
    {76, 82, 71, 85, 79, 80}   // Student 3
};

Character Arrays and C-Style Strings

Before the string class, C++ used character arrays - like spelling words letter by letter!

Working with C-Style Strings

// C-style string declaration
char name[20] = "Alice";     // Can hold up to 19 chars + '\0'
char city[] = "New York";    // Size determined automatically

// Important: Always leave room for null terminator!
char word[6] = "Hello";      // Needs 6 spaces: H-e-l-l-o-\0

// String input (be careful with buffer overflow!)
char username[50];
cout << "Enter username: ";
cin >> username;             // Reads a single word (stops at whitespace)
// To read a whole line (including spaces) instead, use:
// cin.getline(username, 50);

// String functions (need #include <cstring>)
char str1[20] = "Hello";
char str2[20] = "World";
char str3[40];

strlen(str1);                // Returns 5
strcpy(str3, str1);         // Copies str1 to str3
strcat(str3, " ");          // Appends space
strcat(str3, str2);         // str3 now contains "Hello World"
strcmp(str1, str2);         // Compares strings

The Modern Way: C++ String Class

The string class is like having a smart container that grows and shrinks as needed!

String Class Examples

#include <iostream>
#include <string>
using namespace std;

// Declaration and initialization
string name = "Alice";
string empty;                    // Empty string
string greeting("Hello");        // Alternative syntax

// String operations
string first = "John";
string last = "Doe";
string full = first + " " + last;  // "John Doe"

// String methods
string text = "Hello World";
cout << text.length() << '\n';     // 11
cout << text.size() << '\n';       // 11 (same as length)
cout << text[0] << '\n';           // 'H'
cout << text.at(6) << '\n';        // 'W' (with bounds checking)

// String modification
text.append("!");                   // "Hello World!"
text.insert(5, ",");               // "Hello, World!"
text.erase(5, 1);                  // Remove the comma
text.replace(0, 5, "Hi");          // "Hi World!"

// Finding substrings
size_t pos = text.find("World");   // Returns position or string::npos
if (pos != string::npos) {
    cout << "Found at position: " << pos << '\n';
}

// Substring extraction
string sub = text.substr(3, 5);    // Extract 5 chars starting at pos 3

Array Bounds and Safety

Going outside array bounds is like trying to access locker #31 when there are only 30 lockers!

Practice Exercise: Grade Management System

#include <iostream>
#include <string>
using namespace std;

int main() {
    const int NUM_STUDENTS = 5;
    const int NUM_TESTS = 3;

    string names[NUM_STUDENTS];
    int grades[NUM_STUDENTS][NUM_TESTS];

    // TODO: Input student names and grades
    // TODO: Calculate averages
    // TODO: Find highest and lowest
    // TODO: Display report

    return 0;
}

// Input phase
for (int i = 0; i < NUM_STUDENTS; i++) {
    cout << "Enter name for student " << i+1 << ": ";
    cin >> names[i];

    cout << "Enter 3 test grades: ";
    for (int j = 0; j < NUM_TESTS; j++) {
        cin >> grades[i][j];
    }
}

// Calculate averages
double averages[NUM_STUDENTS];
for (int i = 0; i < NUM_STUDENTS; i++) {
    int sum = 0;
    for (int j = 0; j < NUM_TESTS; j++) {
        sum += grades[i][j];
    }
    averages[i] = static_cast<double>(sum) / NUM_TESTS;
}

Common Array Algorithms

String Manipulation Examples

// Common string operations
string sentence = "The quick brown fox";

// Convert to uppercase (range-based for is cleaner than an index loop)
// Note: cast to unsigned char first — passing a negative char to toupper()
// is undefined behavior. Needs #include <cctype>.
for (char& c : sentence) {
    c = toupper(static_cast<unsigned char>(c));
}

// Count words
int wordCount = 0;
bool inWord = false;
for (int i = 0; i < sentence.length(); i++) {
    if (sentence[i] != ' ' && !inWord) {
        wordCount++;
        inWord = true;
    } else if (sentence[i] == ' ') {
        inWord = false;
    }
}

// Reverse a string
string reversed = "";
for (int i = sentence.length() - 1; i >= 0; i--) {
    reversed += sentence[i];
}

// Check if palindrome
string word = "racecar";
bool isPalindrome = true;
for (int i = 0; i < word.length() / 2; i++) {
    if (word[i] != word[word.length() - 1 - i]) {
        isPalindrome = false;
        break;
    }
}

Challenge Exercise: Text Analysis

Arrays vs Vectors (Preview)

C++ also offers dynamic arrays called vectors - they're like arrays that can grow!

🎯 Quick Quiz

Summary

📚 Additional Resources

• cppreference — Array declaration
• cppreference — std::basic_string
• LearnCpp — Arrays (Part I)

🚀 What's Next?

You can now store and process whole collections of data with arrays and strings. Next, in Lesson 7: Functions, you'll learn to package logic into reusable blocks — including how to pass those very arrays into functions to keep your grade-tracking code organized and DRY.