Ｌｉｎｕｘ下程式开发２ 变量数据类型的代码以及编译&运算符与表达式

变量数据类型的代码以及编译&运算符与表达式

实例１

[root@localhost code]# vim float.cpp
[root@localhost code]# g++ float.cpp
float.cpp: In function `int main()':
float.cpp:14: parse error before `(' token
[root@localhost code]#

修改如下：

#include <iostream>
#include<iomanip>
using namespace std;
int main (void)
{
  float f;
  double d;
  long double  ld;
  f=1.23456789f;
  d=1.23456657675677657;
  ld=1.23454356465656565464555364L;
  cout <<"effect number characters"<<endl;
  cout<<"float f="<<setprecision(10)<<f<<endl;
  cout<<"double d="<<setprecision(20)<<d<<endl;
  cout<<"long double ld="<<setprecision(29)<<ld<<endl;
  f=1e36f;
  d=1e307;
  ld=1e4930L;
  cout <<endl<<"Exponent"<<endl;
  cout<<setiosflags(ios::scientific);
  cout<<"float f="<<f<<endl;
  cout<<"double d="<<d<<endl;
  cout<<"long double ld="<<ld<<endl;

[root@localhost code]# g++ float.cpp  -o float.out
[root@localhost code]# ./float.out
effect number characters
float f=1.234567881
double d=1.2345665767567766
long double ld=1.2345435646565656547
Exponent
float f=9.99999961690316245e+35
double d=9.99999999999999986e+306
long double ld=1.00000000000000000003e+4930

教科书中，说c++ 存储浮常数也就是ｆｌｏａｔ这一类，预设类型为ｄｏｕｂｌｅ，另外，爱将某浮点数值指定为ｆｌｏａｔ存储时候，必须在数值字未加ｆ，Ｆ；

ｆｌｏａｔ有效精度７位

还有，使用控制精度，需加＃ｉｎｌｃｕｄｅ＜ｉｏｍａｎｉｐ＞

实例２

#include<iostream>
using namespace std;
int main(void)
{
float f=1e36f;
if (abs(f-1e36)<1e30)
        {  f=f+5e３5;
           cout<<"float f="<<f<<endl;
        }
return 0;
}

[root@localhost code]# g++ compare.cpp
[root@localhost code]# g++ compare.cpp -o compare.out
[root@localhost code]# ./compare.out
float f=1.5e+36

实例３

#include<iostream>
using namespace std;
int main (void)
{
char ch1='A';char ch2=65;
  cout<<"ch1= '"<<chl<<"'"<<endl;
  cout<<"ch2= '"<<ch2<<"'"<<endl;
  return 0;
}

一开始报错如下：

char.cpp: In function `int main()':
char.cpp:5: parse error before `char'
char.cpp:6: `chl' undeclared (first use this function)
char.cpp:6: (Each undeclared identifier is reported only once for each function
   it appears in.)
char.cpp:7: `ch2' undeclared (first use this function)

原因如下：  char ch1='A';char ch2=65; 中间的；用逗号

修改后还是报错：

char.cpp: In function `int main()':
char.cpp:6: `chl' undeclared (first use this function)
char.cpp:6: (Each undeclared identifier is reported only once for each function
   it appears in.)

原因非常明确：  cout<<"ch1= '"<<chl<<"'"<<endl; 应该改为  cout<<"ch1= '"<<ch１<<"'"<<endl; 显然，１和ｌ经常录错．

这回正确了

教科书说：Ａ 的ＡＳＣＩＩ码为６５，所以，直接用６５ 结果是一样的．

实例４ 转义序列

＃include

匈牙利命名法

变量的有效范围

#include <iostream>
using namespace std;
int main(void)
{
  int i=1;
  {
        int j=2
        cout<<"i="<<i<<",j="<<j<<endl;
  }
cout<< "j="<<j<endl;
return 0;
}

验证错误:

[root@localhost code]# g++ bloack.cpp
bloack.cpp: In function `int main()':
bloack.cpp:8: parse error before `<<' token
bloack.cpp:10: `j' undeclared (first use this function)
bloack.cpp:10: (Each undeclared identifier is reported only once for each
   function it appears in.)

当广域变量遇上区域变量的时候:

广域变量暂时被区域变量盖住,若爱指定存取广域变量时候,可以运用::这个解析运算符.

代码:

#include<iostream>
using namespace std;
int i=1;
int main(void)
{
    int i=2;
    cout<<"i="<<i<<endl;
    cout<<"::i="<<::i<<endl;
return 0;
}

运行结果

[root@localhost code]# ./loc_glo.out
i=2
::i=1

常数,自定义数,和const常数和#define:

#include <iostream>
using namespace std;
#define HI cout<<"hello!fleetwgx"<<endl;
int main(void)
{ const int a=100;
HI cout<< "\n" << endl; HI
cout<<"\na="<<a<<endl;
return 0;

[root@localhost code]# ./hi.out
hello!fleetwgx
hello!fleetwgx
a=100

读入语句cin;

代码:

#include<iostream>
using  namespace std;
int main(void)
{
  double  input_double;
  int input_int;
cout<<"please input a float_point number!"<<endl;
cin>>input_double;
cout<<input_double<<"is inputed.\n"<<endl;
cout<<"please input a float-point number as well as an integer number!"<<endl;
cin>>input_double>>input_int;
cout<<input_double<<"and"<<input_int<<"is inputed ."<<endl;
return 0;
}

运行结果:

[root@localhost code]# ./cin.out
please input a float_point number!
3.1415926789096456803486034344546578675075679657853875843728075
3.14159is inputed.
please input a float-point number as well as an integer number!
3.14159 3
3.14159and3is inputed .
[root@localhost code]#