C++ Program BASICS Revision

✅ First C++ Program

#include <iostream>  // Standard I/O library
using namespace std;

int main() {  // Main function
    cout << "Hello, World!";  // Output statement
    return 0;  // Return statement
}

🔹 Explanation:

• #include <iostream> → Header file for input/output.

• using namespace std; → Allows using standard namespace (removes need for std:: prefix).

• cout → Prints output to console.

• return 0; → Indicates successful execution.

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✅ Tokens in C++

Tokens are the smallest units in a C++ program.

🔹 Types of Tokens:

1. Keywords → Reserved words (e.g., int, return, if, else, while)

2. Identifiers → Names for variables, functions, classes (e.g., main, age, total)

3. Constants → Fixed values (5, 3.14, 'A')

4. Operators → Symbols for operations (+ , - , * , /)

5. Special Symbols → { } , ; ( ) [ ]

6. Strings → Text enclosed in "" (e.g., "Hello")

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✅ Functions in C++

A function is a block of code designed to perform a specific task.

🔹 Syntax:

return_type function_name(parameters) {
    // Function body
}

🔹 Example:

#include <iostream>
using namespace std;

void greet() {  // Function without return type
    cout << "Welcome to C++ Programming!\n";
}

int main() {
    greet();  // Function call
    return 0;
}

---

✅ Scope of Variables in C++

Scope determines where a variable is accessible.

🔹 Types:

1. Local Variable → Declared inside a function, accessible only there.

2. Global Variable → Declared outside functions, accessible everywhere.

3. Static Variable → Retains its value between function calls.

4. Register Variable → Stored in CPU registers (rarely used today).

🔹 Example:

#include <iostream>
using namespace std;

int globalVar = 10;  // Global Variable

void func() {
    int localVar = 5;  // Local Variable
    static int staticVar = 1;
    staticVar++;
    cout << "Local: " << localVar << ", Static: " << staticVar << "\n";
}

int main() {
    func();
    func();
    cout << "Global: " << globalVar << "\n";
    return 0;
}

---

✅ Conditional Statements in C++

Used for decision-making in programs.

🔹 if Statement

int num = 10;
if (num > 0) {
    cout << "Positive Number";
}

🔹 if-else Statement

int num = -5;
if (num > 0) {
    cout << "Positive";
} else {
    cout << "Negative";
}

🔹 else-if Ladder

int marks = 75;
if (marks >= 90) {
    cout << "Grade A";
} else if (marks >= 75) {
    cout << "Grade B";
} else {
    cout << "Grade C";
}

---

✅ Nested if and switch in C++

🔹 Nested if

int num = 5;
if (num > 0) {
    if (num % 2 == 0) {
        cout << "Even Positive";
    } else {
        cout << "Odd Positive";
    }
}

🔹 switch Statement

int choice = 2;
switch (choice) {
    case 1:
        cout << "Option 1";
        break;
    case 2:
        cout << "Option 2";
        break;
    default:
        cout << "Invalid Option";
}

---

✅ Increment and Decrement Operators in C++

Used to increase or decrease values by 1.

Operator	Meaning	Example
++	Increment by 1	a++ or ++a
--	Decrement by 1	a-- or --a

🔹 Example:

int a = 5;
cout << a++ << "\n";  // Post-increment → prints 5, then a = 6
cout << ++a << "\n";  // Pre-increment → a = 7, then prints 7

---

✅ Operators in C++

🔹 Types of Operators:

1. Arithmetic Operators → + , - , * , / , %

2. Relational Operators → == , != , > , < , >= , <=

3. Logical Operators → && , || , !

4. Assignment Operators → = , += , -= , *= , /=

5. Bitwise Operators → & , | , ^ , ~ , << , >>

6. Conditional (Ternary) Operator → condition ? true : false

7. Comma Operator → , used to separate expressions.

8. sizeof Operator → Returns the size of a variable.

🔹 Example:

int a = 10, b = 20;
cout << "Sum = " << a + b << "\n";  // Arithmetic
cout << (a > b) << "\n";        // Relational
cout << ((a < b) && (b > 15)) << "\n";  // Logical
a += 5;                         // Assignment
cout << "a = " << a << "\n";

---

✅ Loops in C++

Loops are used to execute a block of code multiple times.

🔹 Types of Loops:

1. for Loop

2. while Loop

3. do-while Loop

🔹 for Loop

#include <iostream>

using namespace std;

int main() {

    for (int i = 1; i <= 5; i++) {

        cout << i << " ";

    }

    return 0;

}

Output: 1 2 3 4 5

🔹 while Loop

#include <iostream>

using namespace std;

int main() {

    int i = 1;

    while (i <= 5) {

        cout << i << " ";

        i++;

    }

    return 0;

}

🔹 do-while Loop

#include <iostream>

using namespace std;

int main() {

    int i = 1;

    do {

        cout << i << " ";

        i++;

    } while (i <= 5);

    return 0;

}

----------------------------------------------------------------

✅ Arrays in C++

An array is a collection of similar data elements stored at contiguous memory locations.

🔹 Declaring an Array

int arr[5];  // Declares an array of size 5

🔹 Initializing an Array

int arr[5] = {1, 2, 3, 4, 5};

🔹 Accessing Elements
#include <iostream>
using namespace std;

int main() {
    int arr[5] = {10, 20, 30, 40, 50};
    cout << "First element: " << arr[0] << endl;
    return 0;
}

🔹 Looping Through an Array
#include <iostream>
using namespace std;

int main() {
    int arr[5] = {1, 2, 3, 4, 5};
    for (int i = 0; i < 5; i++) {
        cout << arr[i] << " ";
    }
    return 0;
}

✅ Working with 2D Arrays in C++

A 2D array is an array of arrays, commonly used to represent matrices.

🔹 Declaring a 2D Array
int matrix[3][3];

🔹 Initializing a 2D Array
int matrix[2][2] = {{1, 2}, {3, 4}};

🔹 Accessing 2D Array Elements
#include <iostream>
using namespace std;

int main() {
    int matrix[2][2] = {{1, 2}, {3, 4}};
    cout << "Element at (1,1): " << matrix[1][1] << endl;
    return 0;
}

🔹 Looping Through a 2D Array
#include <iostream>
using namespace std;

int main() {
    int matrix[2][2] = {{1, 2}, {3, 4}};
    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 2; j++) {
            cout << matrix[i][j] << " ";
        }
        cout << endl;
    }
    return 0;
}


✅ Strings in C++

A string is a sequence of characters stored in a character array.

🔹 Declaring a String
char str[10];

🔹 Initializing a String
char str[] = "Hello";

🔹 Input and Output of Strings
#include <iostream>
using namespace std;

int main() {
    char name[20];
    cout << "Enter your name: ";
    cin >> name;  // Reads string until space
    cout << "Hello, " << name;
    return 0;
}


✅ String Library Functions in C++

C++ provides several built-in functions to work with strings (in string library).

Function
Description
strlen(str)              
Returns the length of the string
strcpy(dest, src)
Copies src to dest
strncpy(dest, src, n)
Copies first n characters of src to dest
strcat(str1, str2)
Concatenates str2 to str1
strncat(str1, str2, n)
Appends first n characters of str2 to str1
strcmp(str1, str2)
Compares two strings
strncmp(str1, str2, n)
Compares first n characters of two strings
strchr(str, ch)
Finds first occurrence of ch in str
strrchr(str, ch)
Finds last occurrence of ch in str
strstr(str1, str2)
Finds first occurrence of str2 in str1

🔹 Example: Using String Functions
#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char str1[] = "Hello";
    char str2[] = "World";
    char str3[20];

    cout << "Length of str1: " << strlen(str1) << endl;
    strcpy(str3, str1);
    cout << "Copied string: " << str3 << endl;
    strcat(str1, str2);
    cout << "Concatenated string: " << str1 << endl;
    cout << "Comparison result: " << strcmp("abc", "abd") << endl;
    
    cout << "First occurrence of 'l': " << strchr(str1, 'l') << endl;
    cout << "Last occurrence of 'l': " << strrchr(str1, 'l') << endl;
    cout << "Finding 'or' in 'World': " << strstr(str2, "or") << endl;

    return 0;
}

OUTPUT:
Length of str1: 5
Copied string: Hello
Concatenated string: HelloWorld
Comparison result: -1
First occurrence of 'l': lloWorld
Last occurrence of 'l': lWorld
Finding 'or' in 'World': or
--------------------------------------------------------------------------

Working with Structures in C++
A structure in C++ is a user-defined data type that groups together variables of different types.


Declaring and Using a Structure:
#include 
#include 
using namespace std;
struct Student {
 char name[50];
 int age;
 float marks;
};
int main() {
 Student student1;
 student1.age = 21;
 student1.marks = 85.5;
 strcpy(student1.name, "John Doe");
 cout << "Name: " << student1.name << endl;
 cout << "Age: " << student1.age << endl;
 cout << "Marks: " << student1.marks << endl;
 return 0;
}

Output:
Name: John Doe
Age: 21
Marks: 85.50
________________________________________________________________

Understanding Pointers in C++
A pointer in C++ is a variable that stores the memory address of another variable.

Example:
#include 
using namespace std;
int main() {
 int num = 10;
 int *ptr = #
 cout << "Value of num: " << num << endl;
 cout << "Address stored in ptr: " << ptr << endl;
 cout << "Value at ptr: " << *ptr << endl;
 return 0;
}

Output:
Value of num: 10
Address stored in ptr: [memory address]
Value at ptr: 10
----------------------------------------

Function Call in C++
A function in C++ is a block of code that performs a specific task.

Example:
#include 
using namespace std;
void greet() {
 cout << "Hello, World!" << endl;
}
int main() {
 greet(); // Function call
 return 0;
}

Output:
Hello, World!
______________________________________________________________

Files in C++
Files in C++ allow storing data permanently. File handling is done using `fstream`.

Example: Writing to a File

#include 
#include 
using namespace std;
int main() {
 ofstream file("example.txt");
 file << "Hello, File!" << endl;
 file.close();
 cout << "Data written to file." << endl;
 return 0;
}

Output:

Data written to file.
Example: Reading from a File

#include <iostream>
#include <fstream>  // Include fstream for file handling

using namespace std;

int main() {
    ifstream file("example.txt");  // Open the file for reading
    string content;

    if (file.is_open()) {  // Check if the file is successfully opened
        while (getline(file, content)) {  // Read file line by line
            cout << "Read from file: " << content << endl;
        }
        file.close();  // Close the file after reading
    } else {
        cout << "Error opening file!" << endl;  // Error message if file not found
    }

    return 0;
}


Output:
If example.txt contains: Hello, File!
Read from file: Hello, File!