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C++中小數點輸出格式(實例代碼)

投稿：jingxian

在《算法競賽入門經典》一書中

習題1-5　打折　（discount）

一件衣服95元，若消費滿300元，可打八五折。輸入購買衣服件數，輸出需要支付的金額（單位：元），保留兩位小數。

我編寫的代碼為

#include<iostream>
#include<iomanip>
using namespace std;
int main(void){
  double s;
  cin>>s;
  
  if(s*95>=300){
    cout<<setiosflags(ios::fixed)<<setprecision(2)<<s*95*0.85<<endl;
  } 
  
  else{
    cout<<setiosflags(ios::fixed)<<setprecision(2)<<s*95<<endl;
  }
  
  
  return 0;
}

於是將C++中如何顯示不同格式的小數查了一下：

Floating point output can be changed with
<< setiosflags( ios::fixed ) : never use scientific notation //絕對不使用科學記數法。
<< setiosflags( ios::scientific ) : always use scientific notation //使用科學記數法。
<< resetiosflags( ios::floatfield ) : restores default (use fixed or scientific notation based on the precision)//重置為缺省。
<< (re)setiosflags( ios::showpoint ) : controls if the trailing zeros and decimal point will be output (default is no) //控制小數點後面的零是否顯示。
<< setprecision( digits ) : if fixed or scientific format is selected, controls the number of digits after the decimal place, otherwise controls the total number of significant digits// 如果fixed或者scientific已經確定了，則控制小數點後面的位數；否則，控制所有位數。

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

int main(void){
  double S = 0.000000123;
  double A = 456.7;
  double B = 8910000000000.0;

  cout << "default precision is "
     << cout.precision() << endl;  // 6

  cout << "  S = " << S << endl;  // 1.23e-07
  cout << "  A = " << A << endl;  // 456.7
  cout << "  B = " << B << endl;  // 8.91e+12

  cout << setiosflags(ios::showpoint)
     << "ios::showpoint ON" << endl;

  cout << "  S = " << S << endl;  // 1.23000e-07
  cout << "  A = " << A << endl;  // 456.700
  cout << "  B = " << B << endl;  // 8.91000e+12

  cout << resetiosflags(ios::showpoint)
     << "ios::showpoint OFF" << endl;
  cout << setiosflags(ios::fixed)
     << "ios::fixed ON" << endl;

  cout << "  S = " << S << endl;  // 0.000000
  cout << "  A = " << A << endl;  // 456.700000
  cout << "  B = " << B << endl;  // 8910000000000.000000

  cout << resetiosflags(ios::fixed)
     << setiosflags(ios::scientific)
     << "ios::scientific ON" << endl;

  cout << "  S = " << S << endl;  // 1.230000e-07
  cout << "  A = " << A << endl;  // 4.567000e+02
  cout << "  B = " << B << endl;  // 8.910000e+12
}

輸出為：

default precision is 6
S = 1.23e-007
A = 456.7
B = 8.91e+012
ios::showpoint ON
S = 1.23000e-007
A = 456.700
B = 8.91000e+012
ios::showpoint OFF
ios::fixed ON
S = 0.000000
A = 456.700000
B = 8910000000000.000000
ios::scientific ON
S = 1.230000e-007
A = 4.567000e+002
B = 8.910000e+012

--------------------------------

Process exited after 0.02482 seconds with return value 0

請按任意鍵繼續. . .

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