input and output - institut national de physique ...ipn · input and output (io) c/c++ io are based...

I. Hrivnacova @ Data Processing Course 2019 1

Input and Output

Data Processing Course,I. Hrivnacova, IPN Orsay

● Output to the Screen

● Input from the Keyboard

● IO Headers

● Output to a File

● Input from a File

● Formatting


C++ Program

● Including the necessary declarations for input and output ● Writing the string “Hello, World !” followed by std::endl symbol,

to the standard output

/* The first C++ program * - just outputs 'Hello, World !' */

#include <iostream>

using namespace std;

int main() { // print “Hello, World !” on the screen cout << “Hello, World !” << endl; }


Input and Output (IO)

● C/C++ IO are based on streams,which are sequence of bytesflowing in and out of the programs(just like water and oil flowingthrough a pipe).– In input operations, data bytes flow

from an input source (such askeyboard, file, network or anotherprogram) into the program.

– In output operations, data bytesflow from the program to an outputsink (such as console, file, networkor another program).

Chua Hock-Chuan: Programming Notes


Input and Output

● The input/output (I/O) functionality isprovided in components of standardlibrary:– #include <iostream>

● Standard channels to provideinput/output:– Generally assigned to the keyboard

(input) and the screen (output)

– Can be redirected by the operatingsystem to a file

● The standard library defines types andobjects specially designed for IO

● Objects:– cin, cout - input channel (the

keyboard), output channel (the screen)

– Global stream objects, they are alreadydeclared, and so available for use

● Types:– istream, ostream : defines the type for

objects that can be used for input, output

● Operators:– <<, >> operator: Send, receive data

t/fromo the output/an input channel


Output to the Screen

● We use the stream std::cout and the operator <<

#include <iostream>#include <string>


int main (){ int i = 1; float f = 2.3; string s = "abcd"; cout << " i=" << i << " f=" << f << " s=" << s << std::endl;}

The operator << is intelligent: itadapts to the type of the object to itsright


Output to the Screen

● We use the stream std::cout and the operator <<



int main (){ int i = 1; float f = 2.3; string s = "abcd"; cout << " i=" << i << " f=" << f << " s=" << s << std::endl;}

Program output:

> ./testOutput

i=1 f=2.3 s=abcd


Input from the Keyboard

● We use the stream std::cin and the operator >>#include <iostream>#include <string>


int main (){ int i; float f; string s; cin >> i >> f >> s;

cout << " i=" << i << " f=" << f << " s=" << s << endl;}

The entries in cascade can beseparated by any separationcharacter: space, TAB, CR


Input from the Keyboard

● We use the stream std::cin and the operator >>#include <iostream>#include <string>


int main (){ int i; float f; string s; cin >> i >> f >> s;

cout << " i=" << i << " f=" << f << " s=" << s << endl;}

Program output:

> ./testInput

1

2

Blablah

i=1 f=2 s=Blablah


Input from the Keyboard (2)

● We use the stream std::cin and the function std::getline()



int main (){ string line; getline(cin, line); cout << "Read: " << line << endl;

getline(cin, line); cout << "Read: " << line << endl; // ... }

● We read the entire line, even ifit contains separators (space,tab) until the character CR.

● We must analyse the linesourselves


Input from the Keyboard (2)

● We use the stream std::cin and the function std::getline()



int main (){ string line; getline(cin, line); cout << "Read: " << line << endl;

getline(cin, line); cout << "Read: " << line << endl; // ... }

Program output:

> ./testInput2

A text

Read: A text

Another text

Read: Another text


IO Header Files

Header File Function and Description<iostream> This file defines the cin, cout, cerr and clog objects,

which correspond to the standard input stream, thestandard output stream, the un-buffered and bufferedstandard error stream, respectively.

<fstream> This file declares services for user-controlled fileprocessing.

<iomanip> This file declares services useful for performing formattedI/O with so-called parameterized stream manipulators, suchas setw and setprecision.


Files

● The file input/output (I/O) functionalityis provided in components ofstandard library:– #include <fstream>

● Types:– ifstream, ofstream : define the

types for objects that can be used forinput, output

– They are of the same family as istream and ostream, and so theobjects of this type can be manipulatedthe same way

● Using operators: <<,>>

● Objects– There are no global file

stream objects (such as cin and cout), we have to definethem ourselves before we canuse them

– We choose the names of ourstream objects

ifstream inFile;ofstream outFile;


Open/Close a File

● The name of theinput/output filecan be definedeither at creatingthe file or at fileopenning

// create (open) a file a data file // “file.txt”ofstream outFile("file.txt");// do somethingoutFile.close();

// create a file streamofstream outFile;// create (open) a data file “file.txt”outFile.open("file.txt");// do somethingoutFile.close();


Output to a File#include <fstream>#include <iostream>#include <string>


ofstream outFile;

int main (){ int i = 1; float f = 2.3; string s = "abcd";

outFile.open("file.txt"); if ( ! outFile.is_open() ) { cerr << "Cannot open file" << endl; return 1; }

outFile << i << " " << f << " " << s << std::endl;

outFile.close();}

● The operation to open afile may fail– If the output file does not

exist, if one has no rightto write to the directoryfor the output file ...

– We have to test the resultof the open operation:

● The operator << is usedin the same way as tooutput to the screen


Output to a File

Program output:

> ./testFileOutput

> more file.txt

1 2.3 abcd

#include <fstream>#include <iostream>#include <string>


ofstream outFile;

int main (){ int i = 1; float f = 2.3; string s = "abcd";

outFile.open("file.txt"); if ( ! outFile.is_open() ) { cerr << "Cannot open file" << endl; return 1; }

outFile << i << " " << f << " " << s << std::endl;

outFile.close();}


Input from a File

● The operation to open afile may fail– The input file may not

exist

– We have to test the resultof the open operation:

● The operator >> is usedin the same way as toinput from the keyboard


using namespace srd;

ifstream inFile;

int main (){ int i; float f; std::string s;

inFile.open("file.txt"); if ( ! inFile.is_open() ) { cerr << "Cannot open file" << endl; return 1; }

inFile >> i >> f >> s;

std::cout << " i=" << i << " f=" << f << " s=" << s << std::endl;}


Input from a File

Program output:

> ./testFileInput

i=1 f=2.3 s=abcd


using namespace srd;

ifstream inFile;

int main (){ int i; float f; std::string s;

inFile.open("file.txt"); if ( ! inFile.is_open() ) { cerr << "Cannot open file" << endl; return 1; }

inFile >> i >> f >> s;

std::cout << " i=" << i << " f=" << f << " s=" << s << std::endl;}


Formatting Output

● The <iomanip> header provides so-called I/Omanipulators for formatting output (and input):

setw(int field-width) Set the field width for the next IO operation. setw() is non-sticky and must be issued prior to each IOoperation.

setfill(char fill-char) Set the filled character for padding to the field width.left|right|internal: set the alignment.

left|right|internal Set the alignment.

fixed|scientific Use fixed-point notation (e.g, 12.34) or scientific notation(e.g., 1.23e+006). For floating-point numbers.

setprecision(intnumDecimalDigits)

Specify the number of digits after the decimal point. Forfloating-point numbers.

boolalpha|noboolalpha display bool values as alphabetic string (true/false) or 1/0. Forbool.


Formatting Output#include <iostream>#include <iomanip> // Needed to do formatted I/Ousing namespace std; int main() { // Floating point numbers double pi = 3.14159265; cout << fixed << setprecision(4); // fixed format with 4 decimal places cout << pi << endl; cout << "|" << setw(8) << pi << "|" << setw(10) << pi << "|" << endl; // setw() is not sticky, only apply to the next operation. cout << setfill('-'); cout << "|" << setw(8) << pi << "|" << setw(10) << pi << "|" << endl; cout << scientific; // in scientific format with exponent cout << pi << endl; // booleans bool done = false; cout << done << endl; // print 0 (for false) or 1 (for true) cout << boolalpha; // print true or false cout << done << endl;}


Formatting Output#include <iostream>#include <iomanip> // Needed to do formatted I/Ousing namespace std; int main() { // Floating point numbers double pi = 3.14159265; cout << fixed << setprecision(4); // fixed format with 4 decimal places cout << pi << endl; cout << "|" << setw(8) << pi << "|" << setw(10) << pi << "|" << endl; // setw() is not sticky, only apply to the next operation. cout << setfill('-'); cout << "|" << setw(8) << pi << "|" << setw(10) << pi << "|" << endl; cout << scientific; // in scientific format with exponent cout << pi << endl; // booleans bool done = false; cout << done << endl; // print 0 (for false) or 1 (for true) cout << boolalpha; // print true or false cout << done << endl;}

Program output:> ./testFormatting3.1416| 3.1416| 3.1416||--3.1416|----3.1416|3.1416e+000false

input and output - institut national de physique ...ipn · input and output (io) c/c++ io are based...

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