🔄 Loops, Nested Loops, Functions, and Strings in C++

🔄 Loops

Loops in C++ allow for the execution of the same block of code multiple times. They are used for handling repetitions and iterating over data structures.

Types of Loops:

for loop:
Used when the number of iterations is known in advance.

#include <iostream>
using namespace std;

int main() {
    for (int i = 0; i < 5; i++) {
        cout << "Iteration: " << i << endl;
    }
    return 0;
}

while loop: Used when we do not know in advance how many iterations will be needed.

#include <iostream>
using namespace std;

int main() {
    int i = 0;
    while (i < 5) {
        cout << "Iteration: " << i << endl;
        i++;
    }
    return 0;
}

do-while loop: Guarantees the execution of the block at least once, even if the condition is false.

#include <iostream>
using namespace std;

int main() {
    int i = 0;
    do {
        cout << "Iteration: " << i << endl;
        i++;
    } while (i < 5);
    return 0;
}

🔁 Nested Loops

Nested loops are loops located inside other loops. They are useful for working with two-dimensional structures, such as matrices. Example:

#include <iostream>
using namespace std;

int main() {
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 3; j++) {
            cout << "i: " << i << ", j: " << j << endl;
        }
    }
    return 0;
}

Result:

i: 0, j: 0
i: 0, j: 1
i: 0, j: 2
...
i: 2, j: 2

🔧 Functions in C++

Functions allow for dividing the program into smaller modules, making the code more organized and easier to maintain.

Defining a Function:

#include <iostream>
using namespace std;

// Defining a function
int add(int a, int b) {
    return a + b;
}

int main() {
    int result = add(5, 7);
    cout << "Result: " << result << endl;
    return 0;
}

Types of Functions:

Functions without arguments and return value:

void greet() {
    cout << "Hello, world!" << endl;
}

Functions with arguments:

void printNumber(int num) {
    cout << "Number: " << num << endl;
}

Functions with a return value:

  int square(int x) {
      return x * x;
  }

📝 Strings

Strings are sequences of characters. In C++, there are two main types of strings:

C-style strings (character arrays):

#include <iostream>
using namespace std;

int main() {
    char str[] = "Hello";
    cout << str << endl;
    return 0;
}

Standard strings (std::string):

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

int main() {
    string greeting = "Hello, world!";
    cout << greeting << endl;
    return 0;
}

Basic string operations:

Concatenation:

string first = "Hello";
string second = " world";
string result = first + second;
cout << result << endl; // Hello world

String length:

string text = "Programming";
cout << "Length: " << text.length() << endl;

Accessing characters:

string word = "C++";
cout << word[0] << endl; // C

4. Range-based for loop (C++11)

A modern and convenient syntax for iterating over containers:

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

int main() {
    vector<int> numbers = {1, 2, 3, 4, 5};

    for (int num : numbers) {
        cout << num << " ";
    }
    cout << endl;

    // To modify elements, use a reference
    for (int& num : numbers) {
        num *= 2;
    }

    return 0;
}

5. break and continue

break: Terminates the execution of the loop
continue: Skips the current iteration and proceeds to the next one

// Example with break
for (int i = 1; i <= 10; i++) {
    if (i == 5) break; // Stops at i = 5
    cout << i << " ";
}
// Output: 1 2 3 4

// Example with continue
for (int i = 1; i <= 5; i++) {
    if (i == 3) continue; // Skips 3
    cout << i << " ";
}
// Output: 1 2 4 5

🎯 Practical Tasks with Loops

Sum of numbers from 1 to N

int sumToN(int n) {
    int sum = 0;
    for (int i = 1; i <= n; i++) {
        sum += i;
    }
    return sum;
    // Faster: return n * (n + 1) / 2;
}

Factorial

long long factorial(int n) {
    long long result = 1;
    for (int i = 2; i <= n; i++) {
        result *= i;
    }
    return result;
}

Fibonacci Numbers

void fibonacci(int n) {
    long long a = 0, b = 1;

    for (int i = 0; i < n; i++) {
        cout << a << " ";
        long long next = a + b;
        a = b;
        b = next;
    }
}

Palindrome Check (using loops)

bool isPalindrome(string s) {
    int left = 0, right = s.length() - 1;

    while (left < right) {
        if (s[left] != s[right]) {
            return false;
        }
        left++;
        right--;
    }
    return true;
}

🔁 Nested Loops - Advanced Applications

Pattern Printing

// Triangle of asterisks
void printTriangle(int n) {
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= i; j++) {
            cout << "* ";
        }
        cout << endl;
    }
}

// Diamond
void printDiamond(int n) {
    // Upper half
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n - i; j++) cout << " ";
        for (int j = 1; j <= 2 * i - 1; j++) cout << "*";
        cout << endl;
    }
    // Lower half
    for (int i = n - 1; i >= 1; i--) {
        for (int j = 1; j <= n - i; j++) cout << " ";
        for (int j = 1; j <= 2 * i - 1; j++) cout << "*";
        cout << endl;
    }
}

Multiplication Table

void multiplicationTable(int n) {
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            cout << i * j << "\t";
        }
        cout << endl;
    }
}

🔧 Functions - Advanced Concepts

Pass by Value vs Pass by Reference

// By value - creates a copy
void incrementValue(int x) {
    x++; // Modifies only the copy
}

// By reference - works with the original
void incrementReference(int& x) {
    x++; // Modifies the original variable
}

int main() {
    int a = 5;
    incrementValue(a);
    cout << a << endl; // 5 (unchanged)

    incrementReference(a);
    cout << a << endl; // 6 (modified)
}

Functions with Default Values

int power(int base, int exp = 2) {
    int result = 1;
    for (int i = 0; i < exp; i++) {
        result *= base;
    }
    return result;
}

int main() {
    cout << power(5) << endl;    // 25 (5^2)
    cout << power(5, 3) << endl; // 125 (5^3)
}

Function Overloading

int add(int a, int b) {
    return a + b;
}

double add(double a, double b) {
    return a + b;
}

string add(string a, string b) {
    return a + b;
}

Recursive Functions - Basics

// Factorial recursively
int factorialRecursive(int n) {
    if (n <= 1) return 1;
    return n * factorialRecursive(n - 1);
}

// Exponentiation recursively
int powerRecursive(int base, int exp) {
    if (exp == 0) return 1;
    return base * powerRecursive(base, exp - 1);
}

📝 Strings - Advanced Operations

Iterating over a String

string text = "Programming";

// Classical way
for (int i = 0; i < text.length(); i++) {
    cout << text[i] << " ";
}

// Range-based for
for (char c : text) {
    cout << c << " ";
}

Comparing Strings

string s1 = "abc";
string s2 = "abd";

if (s1 == s2) cout << "Equal" << endl;
if (s1 < s2) cout << "s1 is lexicographically smaller" << endl;

Searching in a String

string text = "Programming in C++";

// Finding a substring
size_t pos = text.find("in");
if (pos != string::npos) {
    cout << "Found at position: " << pos << endl;
}

// Checking if starts/ends with a substring
if (text.substr(0, 4) == "Prog") {
    cout << "Starts with 'Prog'" << endl;
}

Modifying Strings

string s = "hello";

// Uppercase
for (char& c : s) {
    c = toupper(c);
}
// s = "HELLO"

// Lowercase
for (char& c : s) {
    c = tolower(c);
}

// Reversing
reverse(s.begin(), s.end());

// Insertion
s.insert(2, "XYZ"); // Inserts at position 2

// Deletion
s.erase(2, 3); // Deletes 3 characters from position 2

Tokenization

#include <sstream>

string text = "apple banana cherry";
stringstream ss(text);
string word;

while (ss >> word) {
    cout << word << endl;
}

Numbers and Strings

// String to number
string numStr = "12345";
int num = stoi(numStr);        // string to int
long long bigNum = stoll(numStr); // string to long long
double d = stod("3.14");       // string to double

// Number to string
int x = 42;
string str = to_string(x);

🎯 Practical Tasks with Strings

Counting Vowels and Consonants

int countVowels(string s) {
    int count = 0;
    string vowels = "aeiouAEIOU";

    for (char c : s) {
        if (vowels.find(c) != string::npos) {
            count++;
        }
    }
    return count;
}

Removing Spaces

string removeSpaces(string s) {
    string result = "";
    for (char c : s) {
        if (c != ' ') {
            result += c;
        }
    }
    return result;
}

Word Count

int countWords(string s) {
    int count = 0;
    bool inWord = false;

    for (char c : s) {
        if (c != ' ' && !inWord) {
            count++;
            inWord = true;
        } else if (c == ' ') {
            inWord = false;
        }
    }
    return count;
}

Anagram Check

bool areAnagrams(string s1, string s2) {
    if (s1.length() != s2.length()) return false;

    sort(s1.begin(), s1.end());
    sort(s2.begin(), s2.end());

    return s1 == s2;
}

💡 Efficiency Tips

Use the right loop: for for a known number of iterations, while for unknown.
Avoid redundant checks: Place conditions outside the loop where possible.
Be careful with nested loops: Complexity grows quickly (O(n²), O(n³)).
Pass strings by reference: void func(const string& s) to avoid copying.
Pre-reserve memory: string.reserve() if you know the approximate size.

🏁 Conclusion

Loops, functions, and strings are the fundamental building blocks of programming. Mastering these concepts is critical for solving algorithmic tasks. Practice regularly with different problems, experiment with various approaches, and gradually you will develop intuition for effectively using these tools! 🚀