Haste makes waste

Nano01(自動運転)-【C++】-Lesson03-Arithmetic Operations算数运算符

Posted on Jan 01, 2019 By lijun

0. 小结

本节非常简单，C++的非常基础的知识。

3. Arithmetic Operations

#include <cmath>

std::pow(base,exponent)

answer = std::pow(3,5)  //3**5

//pi is equal to M_PI
//M_PI = 3.14159265358979323846
area = M_PI * r * r


/*Goal: practice arithmetic operations in C++
**Write a program that calculates the volumes of:
**a cube, sphere, cone.
**Cube Volume = side^3
**Sphere Volume = (4/3) * pi * radius^3
**Cone Volume = pi * radius^2 * (height/3)
**Write the values to the console.
*/

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

int main()
{
    //Dimension of the cube
    float cubeSide = 5.4;
    //Dimension of sphere
    float sphereRadius = 2.33;
    //Dimensions of cone
    float coneRadius = 7.65;
    float coneHeight = 14;
    
    float volCube, volSphere, volCone = 0;
    
    volCube = pow(cubeSide,3);
    volSphere = (4.0/3.0) * M_PI * pow(sphereRadius,3);
    volCone = M_PI * pow(coneRadius,2) * (coneHeight/3);
    
    cout << volCube << endl;
    cout << volSphere << endl;
    cout << volCone << endl;
    return 0;
}

输出:

157.464
52.9854
857.985

注意上面使用(4.0/3.0)，如果是(4/3)的话，结果为1.

6. Variable Assignment

  1. The type of every variable must be known at compile time.
  2. C++ allows for some implicit conversions 隐式转换.
/*Goal: understand C++ implicit variable assignments. 
**Run the program to see implicit variable assignment at work. 
 */


#include<iostream>

int main()
{
    
    int a = 65;
    char charA = 65;
    char charB = 'B';
    float answer = 0;
    char charC = 67;
    int integer = 80;
    float floatNumber = 0.0;
    
    std::cout<<"a = "<<a<<"\n";
    std::cout<<"charA = "<<charA<<"\n";
    std::cout<<"charB = "<<charB<<"\n";
    
    //we can assign an integer to a float
    floatNumber = integer;
    std::cout<<"integer = "<<integer<<"\n\n";
    std::cout<<"floatNumber = integer = "<<floatNumber<<"\n";
    
    //we can assign a char to a float
    floatNumber = charB;
    std::cout<<"floatNumber = charB = "<<floatNumber<<"\n";  
    
    answer = floatNumber/4;
    std::cout<<"answer = floatNumber/4 = "<<answer<<"\n"; 
    
    //But assigning a float to a char doesn't quite work
    charC = answer;
    std::cout<<"charC = answer = "<<charC<<"\n";
    
    //assigning a float to an interger, results in the float being truncated
    integer = answer;
    std::cout<<"integer = floatNumber = "<<integer<<"\n";   
    return 0;
}


a = 65
charA = A
charB = B
integer = 80

floatNumber = integer = 80
floatNumber = charB = 66
answer = floatNumber/4 = 16.5
charC = answer = 
integer = floatNumber = 16

  #include <iostream>

     int main()
     {
          float width = 4.5;
          float height = 5.5;

          int area = width * height;
          std::cout << "area = " << area;
          return 0;
     }

如果是float输出area = 24.75，但是这里因为是int，则输出area = 24。

注意下面的代码是错的：

#include <iostream>
int main(void)
{
    int numerator = 4;
    int denominator = 5;

    float answer = numerator / denominator;
    std::cout<<"answer = "<<answer;
    return 0;
}

输出结果仍然是answer = 0，这里涉及到低精度到高精度转换的问题，需要修改为如下，结果为answer = 0.8：

  #include <iostream>
     int main(void)
     {
         int numerator = 4;
         float denominator = 5;
         float answer = 0;

         answer = numerator / denominator;
         std::cout<<"answer = "<<answer;
         return 0;
     }

11. PreFix and PostFix

Incrementing：

  • prefix: ++a
  • postfix: a++

Decrementing：

  • prefix: –a
  • postfix: a–
  1. Prefix operators increment the value of the variable, then return the reference to the variable.
  2. Postfix operators create a copy of the variable and increments the value of the variable. Then it returns a copy from BEFORE the increment.

以前知道这两者的运算结果不同，但是不理解原因，原来如此，prefix的时候是直接返回该变量的引用，所以得到的结果与加法后的一样。 但是postfix是先生成了一个copy再加法运算，再返回一个copy(未加法的)。

/*Goal: Practice PreFix and PostFix
**
*/

#include<iostream>

using namespace std;

int main()
{
    int a, b = 0;
    int post, pre = 0;
    cout<<"Inital values: \t\t\tpost = "<<post<<" pre= "<<pre<<"\n";
    post = a++; // post = 0, a = 1
    pre = ++b; // pre = 1, b = 1
    cout<<"After one postfix and prefix: \tpost = "<<post<<" pre= "<<pre<<"\n";
    cout<<"After one postfix and prefix: \ta = "<<a <<" b= "<<b<<"\n";
    post = a++; // post = 1, a = 2
    pre = ++b; // pre = 2, b = 2
    cout<<"After two postfix and prefix: \tpost = "<<post<<" pre= "<<pre<<"\n"; 
    cout<<"After one postfix and prefix: \ta = "<<a <<" b= "<<b<<"\n";
    return 0;
}

结果：

Inital values: 			post = 0 pre= 0
After one postfix and prefix: 	post = 0 pre= 1
After one postfix and prefix: 	a = 1 b= 1
After two postfix and prefix: 	post = 1 pre= 2
After one postfix and prefix: 	a = 2 b= 2

13. Variable Assignment Operators

image

/*Goal: understand variable assignment operators*/

#include<iostream>

int main()
{
    int a = 0;
    std::cout<<"Variable\t\tOperation\tResult\n";
    std::cout<<"a = "<<a;
    a += 2;
    std::cout<<"\t\t\ta += 2 \t\t a = "<<a<<"\n"; // 2

    std::cout<<"a = "<<a<<" : ";
    a -= 4;
    std::cout<<"\t\ta -= 4 \t\t a = "<<a<<"\n"; // -2 
    
    int b = 3;
    std::cout<<"a = "<<a<<", b = "<<b<<" : "; // -2,3 
    a *= b;
    std::cout<<"\ta *= b \t\t a = "<<a<<"\n"; // -6
    
    std::cout<<"a = "<<a<<", b = "<<b<<" : "; // -6, 3
    a /= b;
    std::cout<<"\ta /= b \t\t a = "<<a<<"\n"; // -2
        
    
    return 0;
}

输出：

Variable		Operation	Result
a = 0			a += 2 		 a = 2
a = 2 : 		a -= 4 		 a = -2
a = -2, b = 3 : 	a *= b 		 a = -6
a = -6, b = 3 : 	a /= b 		 a = -2