STL String in C++ Explained — Creation, Methods and Common Mistakes

#include <iostream>
#include <string>       // Required for std::string
using namespace std;

int main() {
    // Method 1: Empty string — useful when you'll build it up later
    string emptyGreeting;
    emptyGreeting = "Hello";

    // Method 2: Direct initialisation — most common approach
    string playerName = "Aria";

    // Method 3: Copy from another string
    string backupName = playerName;   // backupName gets its own copy of "Aria"

    // Method 4: Repeat a character N times — e.g. a divider line
    string dividerLine(20, '-');      // Creates "--------------------"

    // Method 5: Initialise from part of a string literal
    string city("New York City", 8);  // Takes first 8 chars → "New York"

    // Printing them all out
    cout << "Greeting  : " << emptyGreeting << "\n";
    cout << "Player    : " << playerName    << "\n";
    cout << "Backup    : " << backupName    << "\n";
    cout << "Divider   : " << dividerLine   << "\n";
    cout << "City      : " << city          << "\n";

    // std::string knows its own length — no need to count manually
    cout << "Name length: " << playerName.length() << " characters\n";

    return 0;
}

#include <iostream>
#include <string>
#include <cctype>    // for toupper(), tolower()
using namespace std;

// Converts every character in a string to uppercase
string toUpperCase(string inputText) {
    for (int i = 0; i < (int)inputText.length(); i++) {
        inputText[i] = toupper(inputText[i]);  // toupper works on single chars
    }
    return inputText;
}

int main() {
    string username = "  aria_42  ";  // Simulating raw user input with spaces

    // --- 1. Check if it's empty ---
    if (username.empty()) {
        cout << "Username cannot be blank.\n";
        return 1;
    }

    // --- 2. length() — how long is the input? ---
    cout << "Raw input length : " << username.length() << "\n";  // 10

    // --- 3. find() — does it contain a forbidden word? ---
    string forbidden = "admin";
    size_t foundPosition = username.find(forbidden);  // size_t is the right type for positions
    if (foundPosition != string::npos) {              // npos means "not found"
        cout << "Username contains forbidden word at position: " << foundPosition << "\n";
    } else {
        cout << "No forbidden words found.\n";
    }

    // --- 4. erase() — strip leading/trailing spaces manually ---
    // Find first non-space character
    size_t startPos = username.find_first_not_of(' ');
    size_t endPos   = username.find_last_not_of(' ');
    string trimmedUsername = username.substr(startPos, endPos - startPos + 1);
    cout << "Trimmed username : \"" << trimmedUsername << "\"\n";  // "aria_42"

    // --- 5. substr() — grab the part before the underscore ---
    size_t underscorePos = trimmedUsername.find('_');
    if (underscorePos != string::npos) {
        string displayName = trimmedUsername.substr(0, underscorePos);  // from 0, take 4 chars
        cout << "Display name     : " << displayName << "\n";           // "aria"
    }

    // --- 6. replace() — mask part of the username for a log ---
    string logEntry = trimmedUsername;          // copy so we don't alter the original
    logEntry.replace(0, 4, "****");             // replace first 4 chars with ****
    cout << "Masked log entry : " << logEntry << "\n";  // "****_42"

    // --- 7. append() / += — build a full greeting ---
    string greeting = "Welcome back, ";
    greeting += trimmedUsername;                // += is shorthand for append()
    greeting.append("!");                       // append() for a single char or string
    cout << greeting << "\n";

    // --- 8. at() — safe character access (throws if out of range) ---
    cout << "First char (safe): " << trimmedUsername.at(0) << "\n";  // 'a'

    // --- 9. Convert to uppercase ---
    cout << "Uppercased       : " << toUpperCase(trimmedUsername) << "\n";

    return 0;
}

#include <iostream>
#include <string>
#include <algorithm>  // for sort() and transform()
#include <vector>
#include <cctype>      // for tolower()
using namespace std;

// Returns a lowercase copy of any string — used for case-insensitive comparison
string toLower(string inputText) {
    transform(inputText.begin(), inputText.end(), inputText.begin(), ::tolower);
    return inputText;
}

int main() {
    string enteredPassword = "OpenSesame";
    string correctPassword = "OpenSesame";
    string wrongPassword   = "opensesame";  // same letters, different case

    // --- Direct equality check --- 
    if (enteredPassword == correctPassword) {
        cout << "Access granted!\n";          // This prints
    }

    // --- Case-sensitive mismatch ---
    if (enteredPassword != wrongPassword) {
        cout << "Passwords differ (case-sensitive check).\n";  // This prints
    }

    // --- Case-insensitive comparison ---
    if (toLower(enteredPassword) == toLower(wrongPassword)) {
        cout << "Passwords match (case-insensitive check).\n"; // This also prints
    }

    // --- Lexicographic ordering with < and > ---
    string fruitA = "apple";
    string fruitB = "banana";
    string fruitC = "apple";  // identical to fruitA

    cout << "\n--- Dictionary Order Checks ---\n";
    cout << fruitA << " < " << fruitB << " ? " << (fruitA < fruitB ? "Yes" : "No") << "\n"; // Yes
    cout << fruitA << " > " << fruitB << " ? " << (fruitA > fruitB ? "Yes" : "No") << "\n"; // No
    cout << fruitA << " == " << fruitC << " ? " << (fruitA == fruitC ? "Yes" : "No") << "\n"; // Yes

    // --- Sorting a list of strings alphabetically ---
    vector<string> cityList = {"Tokyo", "Amsterdam", "Mumbai", "Berlin"};
    sort(cityList.begin(), cityList.end());  // std::sort uses < under the hood

    cout << "\nCities in alphabetical order:\n";
    for (const string& city : cityList) {   // const reference avoids copying each string
        cout << "  " << city << "\n";
    }

    return 0;
}

#include <iostream>
#include <string>
#include <stdexcept>   // for std::invalid_argument exception
using namespace std;

int main() {
    // ========================================================
    // PART 1: Reading input safely
    // ========================================================

    int playerAge;
    string playerFirstName;
    string playerFullName;

    cout << "Enter your age: ";
    cin >> playerAge;          // reads the number, leaves '\n' in the buffer

    cin.ignore();              // CRITICAL: discard that leftover newline character

    cout << "Enter your first name: ";
    cin >> playerFirstName;    // single word — cin >> is fine here
    cin.ignore();              // flush again before getline

    cout << "Enter your full name: ";
    getline(cin, playerFullName);  // reads the ENTIRE line including spaces

    cout << "\nAge       : " << playerAge       << "\n";
    cout << "First name: " << playerFirstName << "\n";
    cout << "Full name : " << playerFullName  << "\n";

    // ========================================================
    // PART 2: Number <-> String conversions
    // ========================================================

    // to_string() — turn a number into a string
    int    scoreValue   = 9450;
    double temperatureC = 36.6;
    string scoreText    = to_string(scoreValue);    // "9450"
    string tempText     = to_string(temperatureC);  // "36.600000"

    cout << "\nScore as string     : " << scoreText << "\n";
    cout << "Temperature as string: " << tempText  << "\n";

    // String concatenation with number conversion
    string leaderboardEntry = playerFirstName + " scored " + to_string(scoreValue) + " points!";
    cout << leaderboardEntry << "\n";

    // stoi() — parse a string back to an integer
    string rawInput  = "1024";     // imagine this came from a config file
    int    parsedInt = stoi(rawInput);  // safe because "1024" is a valid int
    cout << "Parsed int : " << parsedInt << "\n";
    cout << "Doubled    : " << parsedInt * 2 << "\n";

    // stod() — parse a decimal string to double
    string precisionInput = "3.14159";
    double parsedDouble   = stod(precisionInput);
    cout << "Parsed pi  : " << parsedDouble << "\n";

    // Safe parsing with exception handling
    string badInput = "abc123";
    try {
        int result = stoi(badInput);  // This will throw — "abc123" is not a pure integer
        cout << result << "\n";       // Never reaches here
    } catch (const invalid_argument& e) {
        cout << "Could not convert \"" << badInput << "\" to an integer.\n";
    }

    return 0;
}