记录一下c++学习的步骤 : base on Course ‘CS205 C/C++ Program Design’ in 2021 Fall at Southern University of Science and Technology. | 视频

week03 : Branching and Looping Statements

基本的条件和循环跟python差不多, 只记录一下第一次见的

1.  // ? : 三元操作符, 跟单行的if else 类似
    // `condition` ? `(if True)返回值` : `(if False)返回值`
    factor = isPositive ? 1 : -1;
       
    //跳转的效率低下,可以转换为计算的话就转
    factor = (isPositive) * 2 - 1;
   

2.  //goto.cpp 错误处理常用,其他场景少...
    float mysquare(float value)
    {
        float result = 0.0f;
        if(value >= 1.0f || value <= 0)
        {
            cerr << "The input is out of range." << endl;
            goto EXIT_ERROR;
        }
        result = value * value;
        return result;
      EXIT_ERROR:
        //do sth such as closing files here
        return 0.0f;
    }
   

3.  //switch.cpp
    // 相当于goto, case 相当于一个label , Switch会跳转到label后按顺序往下执行(忽略case)
    // 所以要注意每个case 要break.
    switch (input_char)
    {
        case 'a':
        case 'A':
            cout << "Move left." << endl;
            break;
        case 'd':
        case 'D':
            cout << "Move right." << endl;
            break;
        default: 
            cout << "Undefined key." << endl;
            break;
    }
   

Lab3 Exercises

1.Basic Linux command

# Demonstrate two Linux commends which are introduced previously and randomly 
# selected by the SA for 

#creating a directory
mkdir new_folder

#changing to another directory, 
cd new_folder

#listing the subdirectories and files in a certain directory
ls folder
#or
ll folder

#removing files, 
rm -r new_folder

#copying and moving files.
cp lab3/new_folder lab3.5/new_folder

#moving files
mv lab3/new_folder lab3.5/new_folder

2.Compile the 3 source files one by one using “g++ - c”, then link the generated object files together to generate an execute file. Demonstrate to a SA to pass the test.

//main.cpp
#include <iostream>
#include "functions.h"
using namespace std;

int main(){
    print_hello();
    cout << "this is main:" << endl;
    cout << "this factorial of 5 is:" << factorial(5) << endl;
    return 0;
}

//factorial.cpp
#include "functions.h"

int factorial(int n)
{
    if (n==1)
        return 1;
    else
        return n * factorial(n - 1);
}

//helloworld.cpp
#include <iostream>
#include "functions.h"
using namespace std;

void print_hello()
{
    cout << "hello world!" << endl;
}

//functions.h
void print_hello();
int factorial(int n);

g++ -c main.cpp 
g++ -c factorial.cpp 
g++ -c printhello.cpp 
g++ factorial.o main.o printhello.o -o a.out
./a.out

3.Run the following source code and explain the result. You need to explain the reason to a SA to pass the test.

#include <iostream>
using namespace std;
int main()
{
	for(size_t n = 2; n >= 0; n--)   //size_t 是个无符号整数 n==0 时, 输出0, 此时n--, 由于无符号,此时n将会变成范围内最大的那个数
	cout << "n = " << n << " ";		 //故会一直循环打印整个size_t的范围
	return 0;
}

4.Run the following source code and explain the result. You need to explain the reason to a SA to pass the test

#include <iostream>
using namespace std;
int main()
{
    int n = 5;
    int sum;
    while(n >0){                           //由于n初始化为5,while循环内没有改变n的值,所以这里会恒为True(死循环)
        sum += n;
        cout << "n = " << n << " ";
        cout << "sum = " << sum << " ";    // 会一直加五输出
    }
return 0;
}

5. Run the following source code and explain the result. You need to explain the reason to a SA to pass the test.

#include <iostream>
using namespace std;
int main()
{
    int n,fa;
    // cout << "n = " << n << endl; --> 0
    // cout << "fa = " << fa << endl; --> 0
    do{
        fa *= n;
        n++;
    }while(n <= 10);
    
    cout << "fa = " << fa << endl;  // --> 0
    return 0;
}
//由于n,fa没有初始化, 我的机器初始化为0, 
//fa = fa * n
//不论后面n怎么变, fa都是0,所以输出是0
//btw, n会从 0 ~ 11, 即先做do再判断while条件