c++ setprecision用法详解

c++ setprecision用法详解

可以通过使用 setprecision 操作符来控制显示浮点数值的有效数字的数量。

#include 
#include  // Header file needed to use setprecision
using namespace std;
int main()
{
    double number1 = 132.364, number2 = 26.91;
    double quotient = number1 / number2;
    cout << quotient << endl;
    cout << setprecision(5) << quotient << endl;
    cout << setprecision(4) << quotient << endl;
    cout << setprecision(3) << quotient << endl;
    cout << setprecision(2) << quotient << endl;
    cout << setprecision(1) << quotient << endl;
    return 0;
}

程序输出结果：

4.91877
4.9188
4.919
4.92
4.9
5

请注意，如果有效数少于要显示的数字，则 setprecision 将舍入，而不是截断数字。另外还需要注意的是，末尾的零将被省略。因此，尽管指定了 setprecision(5)，但是 21.40 仍显示为 21.4。
setw 可以设置字段宽度。

// This program asks for sales figures for three days.
// The total sales are calculated and displayed in a table.
#include 
#include  // Header file needed to use stream manipulators
using namespace std;

int main()
{
    double day1, day2, day3, total;
    // Get the sales for each day
    cout << "Enter the sales for day 1: ";
    cin >> dayl;
    cout << "Enter the sales for day 2: ”;
    cin >> day2;
    cout << "Enter the sales for day 3: ”;
    cin >> day3;
    // Calculate total sales
    total = day1 + day2 + day3;
    // Display the sales figures
    cout << "\nSales Figures\n";
    cout << "-------------\n" ;
    cout << setprecision (5);
    cout << "Day 1: " << setw(8) << day1 << endl;
    cout << "Day 2: " << setw(8) << day2 << endl;
    cout << "Day 3: " << setw(8) << day3 << endl;
    cout << "Total: " << setw(8) << total << endl;
    return 0;
}

程序输出结果：

Enter the sales ；for day 1: 321.57 
Enter the sales for day 2: 269,60
Enter the sales for day 3: 307.00
Sales Figures
-------------
Day 1:   321.57
Day 2:    269.6
Day 3:      307
Total:   898.17

该程序创建的输出，按照指示，允许显示最多 5 个有效数，并以 8 个字符的字段宽度右对齐打印。