变量与运算

变量声明

C++ 中的变量在使用前必须声明特定的数据类型。以下是基本数据类型：

基本变量声明

#include <iostream>
using namespace std;

int main() {
    // Integer types
    int age = 25;
    long population = 1000000L;
    short score = 95;
    
    // Floating point types
    float price = 19.99f;
    double pi = 3.14159265359;
    
    // Character types
    char grade = 'A';
    string name = "John Doe";
    
    // Boolean type
    bool isActive = true;
    
    cout << "Age: " << age << endl;
    cout << "Name: " << name << endl;
    cout << "Grade: " << grade << endl;
    cout << "Active: " << isActive << endl;
    
    return 0;
}

变量初始化

C++ 支持多种变量初始化方式：

初始化方法

#include <iostream>
using namespace std;

int main() {
    // Copy initialization
    int a = 10;
    
    // Direct initialization
    int b(20);
    
    // List initialization (C++11)
    int c{30};
    int d = {40};
    
    // Default initialization
    int e;  // Uninitialized (contains garbage value)
    
    // Multiple variable declaration
    int x = 1, y = 2, z = 3;
    
    cout << "a: " << a << ", b: " << b << ", c: " << c << ", d: " << d << endl;
    cout << "x: " << x << ", y: " << y << ", z: " << z << endl;
    
    return 0;
}

算术运算符

C++ 提供各种算术运算符进行数学计算：

算术运算

#include <iostream>
using namespace std;

int main() {
    int a = 15, b = 4;
    
    cout << "a = " << a << ", b = " << b << endl;
    cout << "Addition: a + b = " << (a + b) << endl;
    cout << "Subtraction: a - b = " << (a - b) << endl;
    cout << "Multiplication: a * b = " << (a * b) << endl;
    cout << "Division: a / b = " << (a / b) << endl;
    cout << "Modulus: a % b = " << (a % b) << endl;
    
    // Increment and decrement
    cout << "\nIncrement/Decrement:" << endl;
    cout << "a++ = " << a++ << " (post-increment)" << endl;
    cout << "a = " << a << " (after post-increment)" << endl;
    cout << "++a = " << ++a << " (pre-increment)" << endl;
    
    return 0;
}

赋值运算符

赋值运算符用于给变量赋值：

赋值运算

#include <iostream>
using namespace std;

int main() {
    int x = 10;
    
    cout << "Initial value: x = " << x << endl;
    
    x += 5;  // x = x + 5
    cout << "After x += 5: x = " << x << endl;
    
    x -= 3;  // x = x - 3
    cout << "After x -= 3: x = " << x << endl;
    
    x *= 2;  // x = x * 2
    cout << "After x *= 2: x = " << x << endl;
    
    x /= 4;  // x = x / 4
    cout << "After x /= 4: x = " << x << endl;
    
    x %= 3;  // x = x % 3
    cout << "After x %= 3: x = " << x << endl;
    
    return 0;
}

常量

常量是初始化后不能修改的值：

常量声明

#include <iostream>
using namespace std;

int main() {
    // Using const keyword
    const int MAX_SIZE = 100;
    const double PI = 3.14159;
    const char GRADE = 'A';
    
    // Using #define preprocessor
    #define SPEED_OF_LIGHT 299792458
    
    cout << "Max size: " << MAX_SIZE << endl;
    cout << "PI: " << PI << endl;
    cout << "Grade: " << GRADE << endl;
    cout << "Speed of light: " << SPEED_OF_LIGHT << " m/s" << endl;
    
    // This would cause a compilation error:
    // MAX_SIZE = 200;  // Error: cannot modify const variable
    
    return 0;
}