算数运算

    Constant numbers 常数

    1. 整数类型
    2. 95 // decimal 10进制
    3. 0137// octal 8进制
    4. 0x5F // hexadecimal 16进制
    6. 95 // int 
    7. 95u // unsigned int 
    8. 95l // long 
    9. 95ul // unsigned long 
    10. 95lu // unsigned long
    1. 浮点数类型
    2. 3.14159 // 3.14159 
    3. 6.02e23 // 6.02 x 10^23 
    4. 1.6e-19 // 1.6 x 10^-19 
    5. 3.0 // 3.0 
    7. 6.02e23L // long double 
    8. 6.02e23f // float 
    9. 6.02e23 // double

    const type qualifier：const类型限定符

    1. const float PI = 3.1415926f;
    2. PI += 1; //error!
    • If a variable/object is const-qualified, it cannot be modified. 变量被const修饰后，其值不可更改
    • It must be initialized when you define it. const变量定义时就要初始化，可以替代宏定义

    auto (since C++11)

    • auto is placeholder type specifier.
    • The type of the variable will be deduced from its initializer. 数据类型由变量初始化的时候自动识别
      1. auto a = 2; // type of a is int
      2. auto bc = 2.3; // type of b is double
      3. auto c ; //valid in C, error in C++
      4. auto d = a * 1.2;  // double
      Question: ```cpp auto a = 2; // type of a is int

    // will a be converted to a double type variable? a = 2.3; // No! 2.3 will be converted to a int 2, then assigned to a

    1. 变量的类型一旦在初始化的阶段确定之后，就不会在变
    3. Arithmetic operators：算数操作符<br />![image.png](https://cdn.nlark.com/yuque/0/2021/png/353587/1635055485952-4399da06-4b66-445a-9b6e-6b4c5bdc5c50.png#clientId=u8869f747-6d34-4&from=paste&height=336&id=u821321eb&margin=%5Bobject%20Object%5D&name=image.png&originHeight=552&originWidth=376&originalType=binary&ratio=1&size=68373&status=done&style=none&taskId=uaa7aba52-f242-4d0c-b4e1-ae72238b692&width=229)
    5. Operator Precedence 运算符优先级<br />If you cannot remember the precedence, use parentheses(括号)!
    7. 1. a++
    8. 1. ++a
    9. 1. *   /
    10. 1. +   -
    11. 1. <<   >>
    13. Assignment Operators 赋值操作
    15. | a = b | a += b | a -= b | a *= b | a /= b | a %= b |
    16. | --- | --- | --- | --- | --- | --- |
    17. | a &= b | a &#124;= b | a ^= b | a <<= b | a >>= b |  |
    19. Increment/decrement
    21. | a++ | ++a | a-- | --a |
    22. | --- | --- | --- | --- |
    24. ```cpp
    25. int a = 3;
    26. int b = a++; // What's the value of b? ->  b = 3, a = 4，先取值在自增
    27. int c = ++a; // What's the value of c? -> c = 5, 先自增，再取值

    Data type conversions

    #include <iostream>
using namespace std;

int main()
{
    int num_int1 = 9; // assigning an int value to num_int1
    int num_int2 = 'C'; // 'C' is a char, implicit conversion to int type
    int num_int3 = (int)'C'; // explicit conversion, C-style
    int num_int4 = int('C'); // explicit conversion, function style
    int num_int5 = 2.8; //implicit conversion
    float num_float = 2.3; // implicit conversion from double to float, may loss precision
    short num_short = 650000; 

    cout << "num_short = " << num_short << endl;

    return 0;
}

    DANGER:

    • The source code can be compiled successfully (even with warning messages) when the data types do not match.
    • Please use explicit conversion if possible

    Data loss
    image.png
    上图中从上到下，数据存储范围从长到短，如果从上面类型转换到下面类型，数据会损失精度。

    #include <iostream>
using namespace std;

int main()
{
    int num_int1 = 0x7ABCDEF0;
    float num_int_float = num_int1;
    int num_int2 = (int)(num_int_float);

    cout << "num_int1 = " << num_int1 << endl;
    cout << "num_int_float = " << num_int_float << endl;
    cout << "num_int2 = " << num_int2 << endl;

    return 0;
}

    Will num_int2 be the same with num_int1? Nope

    num_int1 = 2059198192
num_int_float = 2.0592e+09
num_int2 = 2059198208

    Operator Associativity 结合率

    • Left-to-right associativity or a right-to-left associativity

    • The following two lines are equivalent.

      int i = 17 / 5 * 5;
int i = (17 / 5) * 5;

      Divisions

    • Both operands are integers

      • Perform integer division 整数除法
      • Any fractional part of the answer is discarded to make the result an integer
        float f = 17 / 5;  // f will be 3.f, not 3.4f.

    • One or both operands are floating-point numbers

      • Perform floating-point division
        float f = 17 / 5.f; // f will be 3.4f.

    Distinct Operations for Different Types

    • int, long, float, double: four kinds of operations

    • If the operands are not the four types, automatic convert their types

      unsigned char a = 255;
unsigned char b = 1;
int c = a + b; // c = 256

      如果两个unsigned char类型的变量相加，只能转换成int类型相加。

    • The operands will be converted to one of the four types without losing data: int, long, float, double

    C/C++ Supposes

    • You (the programmer) are smart enough!
    • You know what exactly the source code means!

    C++ 非常灵活，很容易出bug，所以一定要谨慎，明晰语句作用和原理，对每个操作、符号的原理进行深刻理解