CSE1222: Lecture 9The Ohio State University1. Formatting Numbers for Output Number formatters are...

CSE1222: Lecture 9 The Ohio State University 1

Formatting Numbers for Output Number formatters are to be used in

conjunction with cout

For example,

double x = 6;cout << x << endl;

The above outputs 6, but x is declared a floating point number, so why isn’t 6.0000… displayed?


Formatting Numbers for Output (2) The decimal point is truncated for display, so we

must force it to be shown:

double x = 6;cout.setf(ios::fixed);cout << x << endl;

Notice the cout.setf(ios::fixed) command

It says “force cout to display the decimal point and 6 significant digits afterward.”

Now 6.000000 will be displayed


outputFormat1// format output#include <iostream>using namespace std;

int main(){ double x; cout << "Enter x: "; cin >> x;

cout << "x = " << x << endl; // general format, default cout.setf(ios::fixed); // use fixed decimal notation cout << "x = " << x << endl; cout.unsetf(ios::fixed); // unset fixed decimal notation flag cout.setf(ios::scientific); // use scientific notation cout << "x = " << x << endl; return 0;}


> outputFormat1.exeEnter x: 123.45x = 123.45x = 123.450000x = 1.234500e+02

> outputFormat1.exeEnter x: 1e15x = 1e+15x = 1000000000000000.000000x = 1.000000e+15


…

cout << "x = " << x << endl; // general format, default

cout.setf(ios::fixed); // use fixed decimal notation

cout << "x = " << x << endl;

cout.unsetf(ios::fixed); // unset fixed decimal notation flag

cout.setf(ios::scientific); // use scientific notation

cout << "x = " << x << endl;

…

> outputFormat1.exe

Enter x: 1e-15

x = 1e-15

x = 0.000000

x = 1.000000e-15

>


…



cout << "x = " << x << endl;



cout << "x = " << x << endl;

…

Setting Precision (1) Remember that fixed shows 6 decimal

places by default

But…

If our program dealt with printing out dollar amounts, do we really need to show 6 decimal places?

Another example: if our program dealt with some rigorous scientific simulation, is 6 decimal places enough?


Setting Precision (2) The cout.precision(n) formatter

Set the precisionWhere n is the number of decimal places to be

displayed

double x = 6;cout.precision(3);cout.setf(ios::fixed);cout << x << endl;

Now 6.000 is displayed


examplePrecision// example of setting precision…int main(){ cout.setf(ios::fixed); cout << "pi/100 = " << M_PI/100.0 << endl;

cout.precision(3); cout << "pi/100 = " << M_PI/100.0 << endl;


return 0;}


> examplePrecision.exepi/100 = 0.031416pi/100 = 0.031pi/100 = 0.031415926536

>


… cout.setf(ios::fixed); cout << "pi/100 = " << M_PI/100.0 << endl;


cout.precision(12); cout << "pi/100 = " << M_PI/100.0 << endl;…

examplePrecision2// example of setting precision…int main(){ cout.setf(ios::scientific); cout << "pi/100 = " << M_PI/100.0 << endl;



return 0;}


> examplePrecision2.exepi/100 = 3.141593e-02pi/100 = 3.142e-02pi/100 = 3.141592653590e-02

>


… cout.setf(ios::scientific); cout << "pi/100 = " << M_PI/100.0 << endl;


cout.precision(12); cout << "pi/100 = " << M_PI/100.0 << endl;…

examplePrecision3// example of setting precision…int main(){ cout.precision(30);

cout.setf(ios::fixed); cout << "pi/100 = " << M_PI/100.0 << endl; return 0;}


> examplePrecision3.exepi/100 = 0.031415926535897934

>

examplePrecision4// example of setting precision…int main(){ cout.precision(30);

cout.setf(ios::scientific); cout << "pi/100 = " << M_PI/100.0 << endl; return 0;}


> examplePrecision4.exepi/100 = 3.141592653589793394e-02

>

Double Precision

IEEE Standard for Floating-Point Arithmetic:

Double precision numbers have 16 significant digits


outputFormat2// format output#include <iostream>using namespace std;

int main(){ double x;

cout << "Enter x: "; cin >> x;

cout.precision(12); cout << "x = " << x << endl; // general format, default cout.setf(ios::fixed); // use fixed decimal

notation cout << "x = " << x << endl; cout.unsetf(ios::fixed); // unset fixed decimal

notation flag cout.setf(ios::scientific); // use scientific notation cout << "x = " << x << endl; return 0;}



> outputFormat2.exeEnter x: 1e15x = 1e+15x = 1000000000000000.000000000000x = 1.000000000000e+15


…

cout.precision(12);



cout << "x = " << x << endl;



cout << "x = " << x << endl;

…

> outputFormat2.exeEnter x: 123456789e-17x = 1.23456789e-09x = 0.000000001235x = 1.234567890000e-09

>


…

cout.precision(12);



cout << "x = " << x << endl;



cout << "x = " << x << endl;

…

Fixed vs. Scientific cout.precision(12);

cout.setf(ios::fixed);cout << “x = “ << x << endl;

Outputs 12 digits after the decimal place.

cout.precision(12);cout.setf(ios::scientific);cout << “x = “ << x << endl;

Outputs 13 significant digits.



Formatted Writing to Files

Everything we can do with cout also applies to fout

Including I/O manipulation: setprecision(), setw(), etc.


writeFile2.cpp#include <cstdlib> // function exit() is in cstdlib#include <fstream> // class ofstream() is in fstream#include <iomanip>#include <iostream>using namespace std;

int main(){ ofstream fout; // declare an output file stream

fout.open("sample.txt" , ios::out); // open file file_name for output

if (!fout.is_open()) // check if file is opened for output

{ cerr << "Unable to open file sample.txt." << endl; exit(10); }


writeFile2.cpp (cont) cout << "Writing to file sample.txt." << endl;

// write to the file fout << "Test file output." << endl; fout << "100.0/3.0 = " << 100.0/3.0 << endl;

fout.precision(12); fout << "100.0/3.0 = " << 100.0/3.0 << endl; fout << "100.0/3.0 = " << fixed << 100.0/3.0 <<

endl;

// close file stream fout fout.close();

return 0; }



tempTable


> tempTable.exe

fahrenheit celsius kelvin -40 -40.00 233.15 -20 -28.89 244.26 0 -17.78 255.37 20 -6.67 266.48 40 4.44 277.59 60 15.56 288.71>

tempTableBad.cpp// Output with no formatting#include <iostream>using namespace std;

int main(){ cout << "fahrenheit celsius kelvin" << endl; for (int fahrenheit = -40; fahrenheit <= 60; fahrenheit += 20) { double celsius = (fahrenheit - 32.0) * (5.0/9.0); double kelvin = celsius + 273.15;

cout << " " << fahrenheit << " " << celsius << " " << kelvin << endl; }

return 0;}


tempTableBad


> tempTableBad.exefahrenheit celsius kelvin -40 -40 233.15 -20 -28.8889 244.261 0 -17.7778 255.372 20 -6.66667 266.483 40 4.44444 277.594 60 15.5556 288.706 >

cout << "fahrenheit celsius kelvin" << endl; for (int fahrenheit = -40; fahrenheit <= 60; fahrenheit += 20) { double celsius = (fahrenheit - 32.0) * (5.0/9.0); double kelvin = celsius + 273.15;


tempTableBad2.cpp. . .int main(){ cout.setf(ios::fixed); cout.precision(2);

cout << "fahrenheit celsius kelvin" << endl; for (int fahrenheit = -40; fahrenheit <= 60;

fahrenheit += 20) { double celsius = (fahrenheit - 32.0) * (5.0/9.0); double kelvin = celsius + 273.15;

cout << " " << fahrenheit << " " << celsius << " " << kelvin <<

endl; }

return 0;}


tempTableBad2


> tempTableBad2.exefahrenheit celsius kelvin -40 -40.00 233.15 -20 -28.89 244.26 0 -17.78 255.37 20 -6.67 266.48 40 4.44 277.59 60 15.56 288.71>

cout << "fahrenheit celsius kelvin" << endl; for (int fahrenheit = -40; fahrenheit <= 60; fahrenheit += 20) { double celsius = (fahrenheit - 32.0) * (5.0/9.0); double kelvin = celsius + 273.15;


Width Problem


> tempTableBad2.exefahrenheit celsius kelvin -40 -40.00 233.15 -20 -28.89 244.26 0 -17.78 255.37 20 -6.67 266.48 40 4.44 277.59 60 15.56 288.71>

cout << " " << fahrenheit << " " << celsius << " " << kelvin << endl;

Problem: Need to fix the width of each output field.

#include <iomanip>

Output can also be formatted using routines in iomanip

To use of these functions, include this file in your program header:

#include <iomanip>


setw The formatter setw(n) sets the width of the output to at least n

characters. String/value is right justified. Example:

cout << setw(10)<< “Age” << setw(10) << “Weight” << endl;cout << setw(10) << 4 << setw(10) << 32 << endl;cout << setw(10) << 22 << setw(10) << 130.5 << endl;cout << setw(10) << 101 << setw(10) << 120 << endl;

Output is: Age Weight 4 32 22 130.5 101 120


tempTable.cpp#include <iostream>#include <iomanip> // Note: Must include iomanip...int main(){ cout.setf(ios::fixed); cout.precision(2);

cout << "fahrenheit celsius kelvin" << endl; for (int fahrenheit = -40; fahrenheit <= 60;

fahrenheit += 20) { double celsius = (fahrenheit - 32.0) * (5.0/9.0); double kelvin = celsius + 273.15;

cout << setw(6) << fahrenheit << setw(15) << celsius << setw(10) << kelvin << endl;

}...


tempTable


> tempTable.exefahrenheit celsius kelvin -40 -40.00 233.15 -20 -28.89 244.26 0 -17.78 255.37 20 -6.67 266.48 40 4.44 277.59 60 15.56 288.71>

cout << setw(6) << fahrenheit << setw(15) << celsius << setw(10) << kelvin << endl;

trigTable


> trigTable.exe degrees radians sin cos -180 -3.14 -0.00 -1.00 -150 -2.62 -0.50 -0.87 -120 -2.09 -0.87 -0.50 -90 -1.57 -1.00 0.00 -60 -1.05 -0.87 0.50 -30 -0.52 -0.50 0.87 0 0.00 0.00 1.00 30 0.52 0.50 0.87 60 1.05 0.87 0.50 90 1.57 1.00 0.00 120 2.09 0.87 -0.50 150 2.62 0.50 -0.87 180 3.14 0.00 -1.00>

trigTable.cpp...#include <iomanip> // NOTE: Must include iomanip... cout.setf(ios::fixed); cout.precision(2);

cout << setw(8) << "degrees" << setw(8) << "radians" << setw(8) << "sin" << setw(8) << "cos" << endl;

for (int degrees = -180; degrees <= 180; degrees += 30)

{ double radians = degrees * M_PI/180.0;

cout << setw(8) << degrees << setw(8) << radians << setw(8) << sin(radians) << setw(8) <<

cos(radians) << endl; }...


trigTable


> trigTable.exe degrees radians sin cos -180 -3.14 -0.00 -1.00 -150 -2.62 -0.50 -0.87 -120 -2.09 -0.87 -0.50 -90 -1.57 -1.00 0.00 -60 -1.05 -0.87 0.50 -30 -0.52 -0.50 0.87 0 0.00 0.00 1.00 30 0.52 0.50 0.87 60 1.05 0.87 0.50 90 1.57 1.00 0.00 120 2.09 0.87 -0.50 150 2.62 0.50 -0.87 180 3.14 0.00 -1.00>

cout << setw(8) << "degrees" << setw(8) << "radians" << setw(8) << "sin" << setw(8) << "cos" << endl;

expTable


> expTable.exeEnter number of values: 10 x e^x 2^x 1 2.72 2.00 2 7.39 4.00 3 20.09 8.00 4 54.60 16.00 5 148.41 32.00 6 403.43 64.00 7 1096.63 128.00 8 2980.96 256.00 9 8103.08 512.00 10 22026.47 1024.00>

expTable.cpp...#include <iomanip> // NOTE: Must include iomanip... cout << "Enter number of values: "; cin >> n;

cout.setf(ios::fixed); cout.precision(2);

cout << setw(5) << "x" << setw(10) << "e^x" << setw(10) << "2^x" << endl;

for (int i = 1; i <= n; i++) { double x = i; cout << setw(5) < expTable.exeEnter number of values: 15 x e^x 2^x 1 2.72 2.00 2 7.39 4.00... 11 59874.14 2048.00 12 162754.79 4096.00 13 442413.39 8192.00 141202604.28 16384.00 153269017.37 32768.00>


expTable


> expTable.exeEnter number of values: 25 x e^x 2^x 1 2.72 2.00 2 7.39 4.00... 13 442413.39 8192.00 141202604.28 16384.00 153269017.37 32768.00... 239744803446.258388608.00 2426489122129.8416777216.00 2572004899337.3933554432.00 >


expTable2.cpp... cout << "Enter number of values: "; cin >> n;

cout.setf(ios::fixed); cout.precision(2);

cout << setw(5) << "x" << " " << setw(10) << "e^x" << " " << setw(10) << "2^x" << endl;

for (int i = 1; i <= n; i++) { double x = i; // Add spaces between fields. cout << setw(5) < expTable2.exeEnter number of values: 10 x e^x 2^x 1 2.72 2.00 2 7.39 4.00 3 20.09 8.00 4 54.60 16.00 5 148.41 32.00 6 403.43 64.00 7 1096.63 128.00 8 2980.96 256.00 9 8103.08 512.00 10 22026.47 1024.00>

// Add spaces between fields cout << setw(5) < expTable2.exeEnter number of values: 15 x e^x 2^x 1 2.72 2.00 2 7.39 4.00... 10 22026.47 1024.00 11 59874.14 2048.00 12 162754.79 4096.00 13 442413.39 8192.00 14 1202604.28 16384.00 15 3269017.37 32768.00>

// Add spaces between fieldscout << setw(5) < expTable2.exeEnter number of values: 25 x e^x 2^x 1 2.72 2.00 2 7.39 4.00... 20 485165195.41 1048576.00 21 1318815734.48 2097152.00 22 3584912846.13 4194304.00 23 9744803446.25 8388608.00 24 26489122129.84 16777216.00 25 72004899337.39 33554432.00>

// Add spaces between fieldscout << setw(5) << i << " " << setw(10) << exp(x) << " " << setw(10) << pow(2.0,x) << endl;


// DO NOT SET FIXED OUTPUT FORMAT // cout.setf(ios::fixed); cout.precision(2);

cout << setw(5) << "x" << setw(10) << "e^x" << setw(10) << "2^x" << endl;

for (int i = 1; i <= n; i++) { double x = i; cout << setw(5) < expTable3.exeEnter number of values: 25 1 2.7 2 2 7.4 4... 20 4.9e+08 1e+06 21 1.3e+09 2.1e+06 22 3.6e+09 4.2e+06 23 9.7e+09 8.4e+06 24 2.6e+10 1.7e+07 25 7.2e+10 3.4e+07>


expTable3


> expTable3.exeEnter number of values: 10 x e^x 2^x 1 2.7 2 2 7.4 4 3 20 8 4 55 16 5 1.5e+02 32 6 4e+02 64 7 1.1e+03 1.3e+02 8 3e+03 2.6e+02 9 8.1e+03 5.1e+02 10 2.2e+04 1e+03 >


Other Formatters Iomanip contains many other formatters, including:

fixed Force show a decimal point with 6 trailing decimal places

scientific Outputs floating point number in scientific notation

hex Outputs integer in hexadecimal (base16)

oct Outputs integer in octal (base 8)

showpos Force positive numbers to be shown with a leading ‘+’ symbol

setprecision(n) Set floating point precision to n places

setw(n) Set the field width to n characters


outputFormat3// format output#include <iostream>#include <iomanip>using namespace std;

int main(){ double x;

cout << "Enter x: "; cin >> x;

// general format, default, 12 significant digits cout << "x = " << setprecision(12) << x << endl; // fixed decimal notation, 12 digits after decimal

point cout << "x = " << fixed << setprecision(12) << x <<

endl; // scientific notation, 12 digits after decimal point cout << "x = " << scientific << setprecision(12) << x

<< endl; return 0;}



> outputFormat3.exeEnter x: 123456789e-17x = 1.23456789e-09x = 0.000000001235x = 1.234567890000e-09


…

// general format, default, 12 significant digits

cout << "x = " << setprecision(12) << x << endl;

// fixed decimal notation, 12 digits after decimal point

cout << "x = " << fixed << setprecision(12) << x << endl;

// scientific notation, 12 digits after decimal point

cout << "x = " << scientific << setprecision(12) << x << endl;

…

Rounding Output// example of output rounding

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

int main(){ double x = 1.0/7.0;

cout << "x = " << setprecision(12) << x << endl; cout << "x = " << setprecision(7) << x << endl; cout << "x = " << setprecision(5) << x << endl; return 0;}


• C++ rounds output

> roundOutput.exex = 0.142857142857x = 0.1428571x = 0.14286

>


…

double x = 1.0/7.0;




…

expTable



expTable4


> expTable4.exeEnter number of values: 10 x e^x 2^x 1 2.72 2 2 7.39 4 3 20.09 8 4 54.60 16 5 148.41 32 6 403.43 64 7 1096.63 128 8 2980.96 256 9 8103.08 512 10 22026.47 1024>


cout.setf(ios::fixed);

cout << setw(5) << "x" << " " << setw(10) << "e^x" << " " << setw(8) << "2^x" << endl;

for (int i = 1; i <= n; i++) { double x = i; // Use precision 2 for e^x and precision 0

(integer) for 2^x. cout << setw(5) < expTable4.exeEnter number of values: 10 x e^x 2^x 1 2.72 2 2 7.39 4 3 20.09 8 4 54.60 16 5 148.41 32 6 403.43 64 7 1096.63 128 8 2980.96 256 9 8103.08 512 10 22026.47 1024>

// Use precision 2 for e^x and precision 0 (integer) for 2^x.cout << setw(5) << i << " " << setw(10) << setprecision(2) << exp(x) << " " << setw(8) << setprecision(0) << pow(2.0,x) << endl;

Summary cout.setf(ios::fixed); // use fixed decimal notation cout.setf(ios::scientific); // use scientific notation cout.precision(k); // print k digits after decimal

iomanip: #include <iomanip> // include IO manipulators(formatters) cout << setw(k) << ...; // set next field width to k cout << setprecision(k) << ...; // print k digits after decimal cout << fixed << ...; // switch to fixed decimal notation cout << scientific << ...; // switch to scientific notation

Date post:	30-Dec-2015
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CSE1222: Lecture 9The Ohio State University1. Formatting Numbers for Output Number formatters are...

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