precision.cpp

Review

Output streams save data from a program to a file, formatting it as they save it. The default format for floating point numbers, float, double, and long double, is decimal (base-10), six significant digits (aka significant figures or sig figs), and scientific notation for very large or very small (near zero) numbers. Stream objects maintain a set of 1-bit formatting flags. Each flag is a binary switch - on or off - controlling some output format. Programmers can change an output stream's floating point format by setting the switches. Although cout is permanently attached to the console, it is an otherwise typical output stream, suggesting that programmers may use the features described here with all output streams. The following example demonstrates some of the formatting options.

Feature Comment
ios::fixed A bitmask corresponding to the fixed flag. Setting the flag prevents displaying numbers in scientific notation regardless of their size.
ios::scientific A bitmask corresponding to the scientific flag. Setting the flag forces displaying numbers in scientific notation independent of their size.
  Setting both fixed and scientific flags "on" causes hexadecimal (base-16) output.
setf(f) The function, short for set flags, sets flags to 1 or "on."
unsetf(f) The function, short for unset flags, sets a flag to 0 or "off."
flags() Returns an integer whose bits correspond to the current formatting flags.
precision(n) Sets the number of significant digits in the output. The precision setting does not affect how integers are displayed.
Output stream formatting functions. For now, it's sufficient to know that ios::fixed and ios::scientific are symbolic constant names. ios, sort for input/output system, is the name of a class, and :: is the scope resolution operator. So, ios:: means that fixed and scientific are part of the ios class.

Compare to the fixed and scientific manipulators: notation.cpp.

Program Output Comments 
#define _USE_MATH_DEFINES
#include <iostream>
#include <cmath>
using namespace std;

int main()
{
    cout << "Default settings" << endl;
    if (cout.flags() & ios::fixed)
    	cout << "fixed flag set" << endl;
    else
    	cout << "fixed flag unset" << endl;
    if (cout.flags() & ios::scientific)
    	cout << "scientific flag set" << endl;
    else
    	cout << "scientific flag unset" << endl;
    cout << 123456789 << endl;
    cout << 123456789. << endl;
    cout << M_PI << endl;
    cout << endl;
    cout << 1.0/3.0 << endl;
    cout << 10.0/3.0 << endl;
    cout << 100.0/3.0 << endl;
    cout << 1.0/300000.0 << endl;
    cout << endl;
 







Default settings
fixed flag unset



scientific flag unset



123456789
1.23457e+08
3.14159

0.333333
3.33333
33.3333
3.33333e-06
 
 








True if fixed is set



True if scientific is set



Integer output
Automatic scientific notation
Six significant digits: 3, 1, 4, 1, 5, 9

Six significant digits: "333333"
Ditto
Ditto
Automatic scientific notation
 

    cout.precision(2);
    if (cout.flags() & ios::fixed)
    	cout << "fixed flag set" << endl;
    else
    	cout << "fixed flag unset" << endl;
    if (cout.flags() & ios::scientific)
    	cout << "scientific flag set" << endl;
    else
    	cout << "scientific flag unset" << endl;
    cout << 123456789 << endl;
    cout << 123456789. << endl;
    cout << M_PI << endl;
    cout << endl;
 

fixed flag unset



scientific flag unset



123456789
1.2e+08
3.1
 

Two significant digits in the output








Integer format unchanged
Two significant digits: 1 and 2
Two significant digits: 3 and 1
 
 
    cout.setf(ios::fixed);
    if (cout.flags() & ios::fixed)
    	cout << "fixed flag set" << endl;
    else
    	cout << "fixed flag unset" << endl;
    if (cout.flags() & ios::scientific)
    	cout << "scientific flag set" << endl;
    else
    	cout << "scientific flag unset" << endl;
    cout << 123456789 << endl;
    cout << 123456789. << endl;
    cout << M_PI << endl;
    cout << endl;
 

fixed flag set



scientific flag unset



123456789
123456789.00
3.14
 

Fixed point output (not scientific)









Large numbers not printed in scientific notation

 

    cout << "Hexadecimal output" << endl;
    cout.setf(ios::scientific);
    if (cout.flags() & ios::fixed)
    	cout << "fixed flag set" << endl;
    else
    	cout << "fixed flag unset" << endl;
    if (cout.flags() & ios::scientific)
    	cout << "scientific flag set" << endl;
    else
    	cout << "scientific flag unset" << endl;
    cout << 123456789 << endl;
    cout << 123456789. << endl;
    cout << M_PI << endl;
    cout << endl;
 
Hexadecimal output

fixed flag set



scientific flag set



123456789
0x1.d7p+26
0x1.92p+1
 
 
Both flags set, "fixed" set above










Hexadecimal (base-16) output
Ditto
 
 
    cout << "Decimal output" << endl;
    cout.unsetf(ios::fixed);
    cout.setf(ios::scientific);
    if (cout.flags() & ios::fixed)
    	cout << "fixed flag set" << endl;
    else
    	cout << "fixed flag unset" << endl;
    if (cout.flags() & ios::scientific)
    	cout << "scientific flag set" << endl;
    else
    	cout << "scientific flag unset" << endl;
    cout << 123456789 << endl;
    cout << 123456789. << endl;
    cout << M_PI << endl;

    return 0;
}
 
Decimal output


fixed flag unset



scientific flag set



123456789
1.23e+08
3.14e+00
 
 
 
 

Switches off (i.e., sets to 0) the fixed flag









Integer format unaffected
Scientific notation
Small numbers display in scientific notation
 
 
 
Formatting floating point numbers. An example program demonstrating stream class formatting functions.