C++: المتغيرات وأنواع البيانات الأساسية

// Without variables, you can only output fixed text
std::cout << "Hello, World!" << std::endl;

// With variables, you can remember user input
std::string name;
std::cin >> name;
std::cout << "Hello, " << name << "!" << std::endl;

Type variableName;

int age; // Declare an integer variable named age
double price; // Declare a floating-point variable named price
char grade; // Declare a character variable named grade
bool isPassed; // Declare a boolean variable named isPassed
std::string name; // Declare a string variable named name

Type variableName = value;

int age = 25; // Declare age and assign 25
double price = 19.99; // Declare price and assign 19.99
char grade = 'A'; // Declare grade and assign 'A' (note: characters use single quotes)
bool isPassed = true; // Declare isPassed and assign true
std::string name = "MOTO"; // Declare name and assign "MOTO" (note: strings use double quotes)

int age = {25}; // Uniform initialization (recommended)
double price{19.99}; // Can omit the equals sign (recommended)

#include <iostream>

int main() {
 const double PI = 3.14159; // Constant (covered later)
 
 double radius;
 std::cout << "Please enter the radius of the circle：";
 std::cin >> radius;
 
 double area = PI * radius * radius;
 std::cout << "The area of the circle is：" << area << std::endl;
 
 return 0;
}

Please enter the radius of the circle：
The area of the circle is：

Please enter the radius of the circle：5
The area of the circle is：78.5397

int, double, char, bool, string, if, else, for, while, 
return, class, public, private, new, delete, ...

double circumference = 3.14159 * diameter; // This 3.14159 is a "magic number"
double area = 3.14159 * radius * radius; // If π is wrong, you have to fix it in two places

const double PI = 3.14159;
double circumference = PI * diameter; // Clear and easy to change
double area = PI * radius * radius;

const Type CONST_NAME = value;

const int DAYS_PER_WEEK = 7;
const double PI = 3.14159;
const std::string SCHOOL_NAME = "Meijia Vocational College";

const int MAX_SIZE; // ❌ Error: constant must be initialized

const int MAX_SIZE = 100;
MAX_SIZE = 200; // ❌ Error: cannot modify a constant's value

#include <iostream>
#include <string>

int main() {
 int age;
 std::string name;
 
 std::cout << "Please enter your name and age（separated by spaces）：";
 std::cin >> name >> age; // Read two variables at once
 
 std::cout << "Hello，" << name << "! You are " << age << " years old。" << std::endl;
 return 0;
}

Please enter your name and age（separated by spaces）：MOTO 35
Hello，MOTO! You are 35 years old。

#include <iostream>
#include <string>

int main() {
 std::string name;
 int age;
 double height;
 
 std::cout << "Please enter your name: ";
 std::cin >> name;
 
 std::cout << "Please enter your age: ";
 std::cin >> age;
 
 std::cout << "Please enter your height (meters): ";
 std::cin >> height;
 
 std::cout << "========== Personal Info ==========" << std::endl;
 std::cout << "Name: " << name << std::endl;
 std::cout << "Age: " << age << " years old" << std::endl;
 std::cout << "Height: " << height << " meters" << std::endl;
 
 return 0;
}

#include <iostream>

int main() {
 int x; // Declared x but didn't assign a value
 std::cout << x << std::endl; // ❌ x's value is "garbage" — undefined
 return 0;
}

int x = 0; // Initialize when declaring
std::cout << x << std::endl; // ✅ Outputs 0

int age;
std::cin >> age;
// If the user enters "abc", age will remain unchanged (or worse)

int age;
std::cin >> age;
if (std::cin.fail()) {
 std::cout << "InputError！Please enter a number." << std::endl;
}

#include <iostream>

int main() {
 double num1, num2;
 char op;
 
 std::cout << "Please enter the first number: ";
 std::cin >> num1;
 
 std::cout << "Please enter an operator (+ - * /): ";
 std::cin >> op;
 
 std::cout << "Please enter the second number: ";
 std::cin >> num2;
 
 double result;
 if (op == '+') {
 result = num1 + num2;
 } else if (op == '-') {
 result = num1 - num2;
 } else if (op == '*') {
 result = num1 * num2;
 } else if (op == '/') {
 if (num2 != 0) {
 result = num1 / num2;
 } else {
 std::cout << "Error: Divisor cannot be 0!" << std::endl;
 return 1; // Abnormal exit
 }
 } else {
 std::cout << "Error: Unsupported operator!" << std::endl;
 return 1;
 }
 
 std::cout << "Result: " << num1 << " " << op << " " << num2 << " = " << result << std::endl;
 return 0;
}

Please enter the first number: 
Please enter an operator (+ - * /): 
Please enter the second number: 
Error: Divisor cannot be 0!
Error: Unsupported operator!
Result:    = 

Please enter the first number: 10
Please enter an operator (+ - * /): *
Please enter the second number: 5
Result: 10 * 5 = 50

#include <iostream>

int main() {
    int a = 5;
    double b = 3.7;

    // Implicit conversion
    double c = a + b;
    std::cout << "Implicit conversion: " << c << std::endl;

    // Explicit conversion (C-style)
    int d = (int)b;
    std::cout << "C-style cast: " << d << std::endl;

    // C++-style conversion
    int e = static_cast<int>(b);
    std::cout << "C++-style cast: " << e << std::endl;

    return 0;
}

Implicit conversion: 
C-style cast: 
C++-style cast: 

My name is Alice，this year 20 years old，FinalScore 92.5 points。