Input/Output

ObjectivesIn this chapter you will:Learn what a stream is and examine input

and output streamsExplore how to use the input stream

functions get, ignore, fill, putback, and peek

Become familiar with file input and outputBecome familiar with input failure

2

Input/Output StreamsI/O: sequence of bytes (stream of bytes) from

source to destinationBytes are usually characters, unless program

requires other types of information Stream: sequence of characters from source

to destinationInput Stream: sequence of characters from

an input device to the computerOutput Stream: sequence of characters from

the computer to an output device

3

Standard I/O DevicesUse iostream to extract (receive) data from

keyboard and send output to the screeniostream contains definitions of two types

istream - input streamostream - output stream

iostream has two variablescin - stands for common inputcout - stands for common output

4

Using cin and coutTo use cin and cout, the preprocessor

directive #include <iostream> must be used

The declaration is similar to the following C++ statements: istream cin;

ostream cout;

Input stream variables: type istream Output stream variables: type ostream

5

cin and the Extraction Operator >>The syntax of an input statement using cin

and the extraction operator >> iscin >> variable >> variable...;

The extraction operator >> is binaryThe left-hand operand is an input stream

variable such as cinThe right-hand operand is a variable of a

simple data type

6

Standard InputEvery occurrence of >> extracts the next

data item from the input streamTwo variables can be read using a single cin statement

No difference between a single cin with multiple variables and multiple cin statements with one variable

When scanning, >> skips all whitespaceWhitespace characters consist of blanks

and certain nonprintable characters

7

Data Type of Input>> distinguishes between character 2 and

number 2 by the right hand operand of >>If it is of type char, the 2 is treated as

character 2 If it is of the type int (or double) the 2 is

treated as the number 2

8

9

Reading DataWhen reading data into a char variable

Extraction operator >> skips leading whitespace, finds and stores only the next character

Reading stops after a single character

10

Reading Data (Continued)To read data into an int or double variable

Extraction operator >> skips leading whitespace, reads plus or minus sign (if any), reads the digits (including decimal)

Reading stops on whitespace non-digit character

11

Example 3-1 int a, b;double z;char ch, ch1, ch2;

Statement Input Value Stored in Memory

1. cin >> ch; A ch = 'A‘

2. cin >> ch; AB ch = 'A', 'B' is held for later

input

3. cin >> a; 48 a = 48

4. cin >> a; 46.35 a = 46, .35 is held for later input

5. cin >> z; 74.35 z = 74.35

6. cin >> z; 39 z = 39.0

7. cin >> z >> a; 65.78 38 z = 65.78, a = 3812

Statement Input Value Stored in Memory

8. cin >> a >> b; 4 60 a = 4, b = 60

9. cin >> a >> ch >> z; 57 A 26.9 a = 57, ch = 'A', z = 26.9

10.cin >> a >> ch >> z; 57 A26.9 a = 57, ch = 'A', z = 26.9

11.cin >> a >> ch >> z; 57 A26.9 a = 57, ch = 'A', z = 26.9

12.cin >> a >> ch >> z; 57A26.9 a = 57, ch = 'A', z = 26.9

13.cin >> z >> ch >> a; 36.78B34 z = 36.78, ch = 'B', a = 34

14.cin >> z >> ch >> a; 36.78 B34 z = 36.78, ch = 'B', a = 34

15.cin >> a >> b >> z; 11 34 a = 11, b = 34, computer waits for

the next number13

Statement Input Value Stored in Memory

16.cin >> a >> z; 46 32.4 68 a = 46, z = 32.4,

68 is held for later input

17.cin >> a >> z; 78.49 a = 78, z = 0.49

18.cin >> ch >> a; 256 ch = '2', a = 56

19.cin >> a >> ch; 256 a = 256, computer waits for the

input value for ch

20.cin >> ch1 >> ch2; A B ch1 = 'A', ch2 = 'B'

14

cin and the get FunctionThe get function

Inputs next character (including whitespace)Stores character location indicated by its

argumentThe syntax of cin and the get function: cin.get(varChar);varCharIs a char variableIs the argument (parameter) of the function

15

16

The get() function can be used to read a single character.

get() obtains the very next character from the input stream without skipping any leading whitespace characters

Another Way to Read char Data

17

char first; char middle; char last;

cin.get(first); cin.get(middle); cin.get(last);

NOTE: The file reading marker is left pointing to the space after the ‘B’ in the input stream

first middle last

At keyboard you type: A[space]B[space]C[Enter]

first middle last

‘A’ ‘ ’ ‘B’

cin and the ignore Functionignore: discards a portion of the inputThe syntax to use the function ignore is:cin.ignore(intExp, chExp);

intExp is an integer expressionchExp is a char expression

If intExp is a value m, the statement says to ignore the next m characters or all characters until the character specified by chExp

18

19

Use function ignore() to skip charactersThe ignore() function is used to skip(read and discard) characters in the input stream The callcin.ignore(howMany, whatChar);

will skip over up to howMany characters or until whatChar has been read, whichever comes first

20

An Example Using cin.ignore()

a b c

a b c

a b c

a b c

957 34

957 34 128

957 34

NOTE: shows the location of the file reading marker STATEMENTS CONTENTS MARKER

POSITION int a; 957 34 1235\nint b; 128 96\nint c;cin >> a >> b; 957 34 1235\n

128 96\n

cin.ignore(100, ‘\n’); 957 34 1235\n128 96\n

cin >> c; 957 34 1235\n128 96\n

21

Another Example Using cin.ignore()

i ch

957 34

957 34

957 34i ch

i ch

i ch16 ‘A’

‘A’

‘A’

NOTE: shows the location of the file reading marker STATEMENTS CONTENTS MARKER

POSITION int i; A 22 B 16 C 19\n char ch;

cin >> ch; A 22 B 16 C 19\n

cin.ignore(100, ‘B’); A 22 B 16 C 19\n

cin >> i; A 22 B 16 C 19\n

putback and peek Functionsputback function

Places previous character extracted by the get function from an input stream back to that stream

peek functionReturns next character from the input streamDoes not remove the character from that

stream

22

putback and peek Functions (continued)The syntax for putback:

istreamVar.putback(ch);istreamVar - an input stream variable, such as cin

ch is a char variableThe syntax for peek:

ch = istreamVar.peek();istreamVar is an input stream variable (cin)ch is a char variable

23

Dot NotationIn the statement cin.get(ch);

cin and get are two separate identifiers separated by a dot

Dot separates the input stream variable name from the member, or function, name

In C++, dot is the member access operator

24

Disk Files for I/O

your variable

(of type ifstream)

your variable

(of type ofstream)

disk file“myInfile.dat”

disk file“myOut.dat”

executingprogram

input data output data

#include <fstream>

25

Disk I/OTo use disk I/O

Access #include <fstream>Choose valid identifiers for your filestreams

and declare themOpen the files and associate them with disk

namesUse your filestream identifiers in your I/O

statements(using >> and << , manipulators, get, ignore)

Close the files26

Disk I/O Statements#include <fstream>

ifstream myInfile; // Declarations

ofstream myOutfile;

myInfile.open(“myIn.dat”); // Open filesmyOutfile.open(“myOut.dat”);

myInfile.close(); // Close filesmyOutfile.close();

27

Opening a FileOpening a file

Associates the C++ identifier for your file with the physical(disk) name for the fileIf the input file does not exist on disk, open is

not successfulIf the output file does not exist on disk, a new

file with that name is createdIf the output file already exists, it is erased

Places a file reading marker at the very beginning of the file, pointing to the first character in the file

28

Stream Fail StateWhen a stream enters the fail state,

Further I/O operations using that stream have no effect at all

The computer does not automatically halt the program or give any error message

Possible reasons for entering fail state include Invalid input data (often the wrong type) Opening an input file that doesn’t exist Opening an output file on a disk that is

already full or is write-protected

29

Input FailureThings can go wrong during executionIf input data does not match the

corresponding variables, the program may run into problems

Trying to read a letter into an int or double variable would result in an input failure

If an error occurs when reading dataInput stream enters the fail state

30

Input Failure (continued)Once in a fail state, all further I/O statements

using that stream are ignored The program continues to execute with

whatever values are stored in variables This causes incorrect resultsThe clear function restores input stream to a

working stateistreamVar.clear();

31

Run Time File Name Entry#include <string>// Contains conversion function c_str

ifstream inFile;string fileName;

cout << “Enter input file name: “ << endl; // Promptcin >> fileName;

// Convert string fileName to a C string typeinFile.open(fileName.c_str());

32

33

AlgorithmMain Module Level 0

Open files Get social security numberGet nameWrite data in proper formatsClose files

Open Files Level 1inData.open("name.dat")outData.open("name.out")

34

Get Name

Get first nameGet middle name or initialGet last name

Write Data in Proper FormatsWrite first name, blank, middle name, blank, last name, blank, social security numberWrite last name, comma, first name, blank, middle name, blank, social security numberWrite last name, comma, blank, first name, blank, middle initial, period, blank,

social security numberWrite first name, blank, middle initial, period, blank, last name

35

Middle initial Level 2

Set initial to middleName.substr(0, 1) + period

Close filesinData.close()outData.close()

36

C++ Program//*************************************************************// Format Names program// This program reads in a social security number, a first name// a middle name or initial, and a last name from file inData. // The name is written to file outData in three formats: // 1. First name, middle name, last name, and social security // number.// 2. last name, first name, middle name, and social // security number// 3. last name, first name, middle initial, and social// security number// 4. First name, middle initial, last name//*************************************************************

37

#include <fstream> // Access ofstream#include <string> // Access stringusing namespace std;

int main(){ // Declare and open files ifstream inData; ofstream outData; inData.open("name.dat"); outData.open("name.out"); // Declare variables string socialNum; // Social security number string firstName; // First name string lastName; // Last name string middleName; // Middle name string initial; // Middle initial

38

// Read in data from file inData inData >> socialNum >> firstName >> middleName >> lastName; // Access middle initial and append a period initial = middleName.substr(0, 1) + '.'; // Output information in required formats outData << firstName << ' ' << middleName << ' ' << lastName << ' ' << socialNum << endl; outData << lastName << ", " << firstName << ' ' << middleName << ' ' << socialNum << endl; outData << lastName << ", " << firstName << ' ' << initial << ' ' << socialNum << endl; outData << firstName << ' ' << initial << ' ' << lastName; // Close files inData.close(); outData.close(); return 0;

}