C++ Formatting Output: setprecision, fixed and showpoint Manipulator

[qingquan-li]

Stream Manipulator	Description
setw(n)	Establishes a print field of n spaces.
fixed	Displays floating-point numbers in fixed-point notation.
showpoint	Causes a decimal point and trailing zeros to be displayed, even if there is no fractional part.
setprecision(n)	Sets the precision of floating-point numbers.
left	Causes subsequent output to be left-justified.
right	Causes subsequent output to be right-justified.

1. setprecision Manipulator

// This program demonstrates how setprecision rounds a floating-point value.

#include <iostream>
#include <iomanip> // Required for setprecision

using namespace std;

int main() {
    double num = 1.23456789;
    // Floating-point values may be rounded to a number of significant digits,
    // or precision, which is the total number of digits that
    // appear before and after the decimal point.
    cout << num << endl;                     // 1.23457
    // You can control the number of significant digits with which floating-point
    // values are displayed by using the setprecision manipulator.
    cout << setprecision(2) << num << endl;  // 1.2
    cout << setprecision(7) << num << endl;  // 1.234568
    cout << setprecision(10) << num << endl; // 1.23456789
    cout << setprecision(15) << num << endl; // 1.23456789
    cout << setprecision(17) << num << endl; // 1.2345678899999999
    return 0;
}

2. fixed Manipulator

#include <iostream>
#include <iomanip> // Required for fixed

using namespace std;

int main() {
    double gpa = 4.0;
    cout << gpa << endl; // 4
    // Stream manipulator, fixed, forces cout to print the digits
    // in fixed-point notation, or decimal.
    cout << fixed << gpa << endl; // 4.000000
    return 0;
}

// combine setprecision and fixed

#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    double x = 123.4567;
    cout << x << endl; // 123.457
    // The setprecision manipulator can sometimes surprise you in an undesirable way.
    // When the precision of a number is ser to a lower value,
    // numbers tend to be printed in scientific notation.
    cout << setprecision(2) << x << endl;          // 1.2e+02
    cout << setprecision(2) << fixed << x << endl; // 123.46
    return 0;
}

3. showpoint Manipulator

#include <iostream>
#include <iomanip> // Required for showpoint

using namespace std;

int main() {
    double x = 123.4, y = 456.0;
    // By default, floating-point numbers are not displayed with trailing zeros,
    // and floating-point numbers that do not have a fractional part
    // are not displayed with a decimal point.
    cout << x << endl; // 123.4
    cout << y << endl; // 456
    cout << showpoint;
    cout << x << endl; // 123.400
    cout << y << endl; // 456.000
    return 0;
}

// combine setprecision and showpoint

#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    double x = 123.4, y = 456.0;
    cout << x << endl; // 123.4
    cout << y << endl; // 456
    cout << setprecision(4) << showpoint << x << endl; // 123.4
    cout << y << endl;                                 // 456.0
    return 0;
}

// combine setprecision, fixed and showpoint

#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    cout << setprecision(2) << 25.00 << endl;               // 25
    cout << setprecision(2) << fixed << 25.00 << endl;      // 25.00
    cout << setprecision(0) << fixed << showpoint << 25.00; // 25.
    return 0;
}

4. Reset Manipulator (Restore Defaults)

#include <iostream>
#include <iomanip>

using namespace std;

int main() {
    const double BEEF_PRICE_PER_POUND = 3.14159265;
    double pound;
    double total_price;

    cout << "How many pounds of beef did you buy? ";
    cin >> pound;

    cout << "Each pound of beef sells for $" << setprecision(9)
         << BEEF_PRICE_PER_POUND << endl;

    total_price = BEEF_PRICE_PER_POUND * pound;
    cout << "The total price is: $"
         << setprecision(2) << fixed << showpoint
         << total_price << endl;

    // Reset iomanip:
    cout << setprecision(6) << resetiosflags( ios::fixed | ios::showpoint);
    cout << "\nYou have bought " << pound << " pound(s) of beef!" << endl;
    return 0;
}

Output:

How many pounds of beef did you buy? 3
Each pound of beef sells for $3.14159265
The total price is: $9.42

You have bought 3 pound(s) of beef!