1. Header file - Comment - namespace - standard input / output stream

1. Import header file

C++ The header file does not have to be .h ending , for example C Medium math.h,stdio.h stay C++ Named cmath,cstdio.

#include<cmath>
#include <cstdio>

int main() {
    double a=1.2;
    a=sin(a);
    printf("%lf\n",a);
    return 0;
}

2. notes

except c Multiline comments for ,c++ You can use a single line comment .

/*
 Multiline comment 
*/
#include<cmath>
#include <cstdio>

int main() {
    double a=1.2; // Define a variable a And initialize the assignment 1.2
    a=sin(a);
    printf("%lf\n",a);
    return 0;
}

3. Command space

To prevent naming conflicts （ The same name appears ）,C++ Introduce namespace ,（namespace）, adopt :: Operator defines which namespace a name belongs to .

give an example ：1 Xiao Minghe of class 2 Xiao Ming of Ben , The same name but not belonging to the same class .

#include <cstdio>
namespace f{
    int a;
}

namespace s{
    int a;
}
int main() {
    f::a=2;
    s::a=3;
    printf("%d\n%d",f::a,s::a);
    return 0;
}

There are usually three ways to use ：

• using namespace X; // Introduce the entire namespace
• using X::name; // Use a single name , After the introduction, write the qualifier
• X::name; // Add a namespace prefix to the program , Just introduce

4. I / O stream

cin >> Input stream and cout << Output stream , Both belong to the standard namespace std.

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
    double a;
    cout << "Please input a number" << endl; // endl Represents a newline character , And force output 
    cin >> a;
    a = sin(a);
    cout << a;
    return 0;
}