Chapter 3
Input/Output
C++ Programming: Program Design Including Data Structures, Eighth Edition
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Objectives (1 of 2)
• In this chapter, you will:
• Learn what a stream is and examine input and output streams
• Explore how to read data from the standard input device
• Learn how to use predefined functions in a program
• Explore how to use the input stream functions get, ignore, putback, and peek
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Objectives (2 of 2)
• Become familiar with input failure
• Learn how to write data to the standard output device
• Discover how to use manipulators in a program to format output
• Learn how to perform input and output operations with the string data type
• Learn how to debug logic errors
• Become familiar with file input and output
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I/O Streams and Standard I/O Devices (1 of 3)
• I/O: sequence of bytes (stream of bytes) from source to destination
• Bytes are usually characters, unless program requires other types of information
• Stream: sequence of characters from the source to the destination
• Input stream: sequence of characters from an input device to the computer
• Output stream: sequence of characters from the computer to an output device
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I/O Streams and Standard I/O Devices (2 of 3)
• Use iostream header file to receive data from keyboard and send output to
the screen
• Contains definitions of two data types:
- istream: input stream
- ostream: output stream
• Has two variables:
- cin: stands for common input
- cout: stands for common output
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I/O Streams and Standard I/O Devices (3 of 3)
• Variable declaration is similar to:
• istream cin;
• ostream cout;
• To use cin and cout, the preprocessor directive #include <iostream>
must be used
• Input stream variables: type istream
• Output stream variables: type ostream
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cin and the Extraction Operator >> (1 of 7)
• The syntax of an input statement using cin and the extraction operator >> is
• The extraction operator >> is binary
• Left-side operand is an input stream variable
- Example: cin
• Right-side operand is a variable
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cin and the Extraction Operator >> (2 of 7)
• No difference between a single cin with multiple variables and multiple cin
statements with one variable in each statement
cin >> payRate >> hoursWorked;
cin >> payRate;
cin >> hoursWorked;
• When scanning, >> skips all whitespace
• Blanks and certain nonprintable characters
• >> distinguishes between character 2 and number 2 by the right-side operand
of >>
• If type char or int (or double), the 2 is treated as a character or as a number 2,
respectively
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cin and the Extraction Operator >> (3 of 7)
TABLE 3-1 Valid Input for a Variable of the Simple Data Type
Data Type of a Valid Input for a
char One printable character except the blank.
int An integer, possibly preceded by a + or - sign.
A decimal number, possibly preceded by a + or - sign. If the actual
double data input is an integer, the input is converted to a decimal number
with the zero decimal part.
• Entering a char value into an int or double variable causes serious errors,
called input failure
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cin and the Extraction Operator >> (4 of 7)
• When reading data into a char variable
• >> skips leading whitespace, finds and stores only the next character
• Reading stops after a single character
• To read data into an int or double variable
• >> skips leading whitespace, reads + or - sign (if any), reads the digits (including
decimal for floating-point variables)
• Reading stops on whitespace or a non-digit character
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cin and the Extraction Operator >> (5 of 7)
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cin and the Extraction Operator >> (6 of 7)
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cin and the Extraction Operator >> (7 of 7)
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Using Predefined Functions in a Program (1 of 3)
• A function (subprogram) is a set of instructions
• When activated, it accomplishes a task
• main executes when a program is run
• Other functions execute only when called
• C++ includes a wealth of functions
• Predefined functions are organized as a collection of libraries called header files
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Using Predefined Functions in a Program (2 of 3)
• Header file may contain several functions
• To use a predefined function, you need the name of the appropriate header file
• You also need to know:
- Function name
- Number of parameters required
- Type of each parameter
- What the function is going to do
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Using Predefined Functions in a Program (3 of 3)
• To use pow (power), include cmath
• Two numeric parameters
• Syntax: pow(x,y) = xy
- x and y are the arguments or parameters
• In pow(2,3), the parameters are 2 and 3
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cin and the get Function
• The get function
• Inputs next character (including whitespace)
• Stores in memory location indicated by its argument
• The syntax of cin and the get function
• varChar is a char variable
- It is the argument (or parameter) of the function
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cin and the ignore Function (1 of 2)
• ignore function
• Discards a portion of the input
• The syntax to use the function ignore is:
• intExp is an integer expression
• chExp 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
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cin and the ignore Function (2 of 2)
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The putback and peek Functions (1 of 2)
• putback function
• Places previous character extracted by the get function from an input stream back to
that stream
• peek function
• Returns next character from the input stream
• Does not remove the character from that stream
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The putback and peek Functions (2 of 2)
• Syntax for putback
• istreamVar: an input stream variable (such as cin)
• ch is a char variable
• Syntax for peek
• istreamVar: an input stream variable (such as cin)
• ch is a char variable
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The Dot Notation between I/O Stream Variables and I/O Functions: A
Precaution
• In the statement
cin.get(ch);
cin and get are two separate identifiers separated by a dot
• Called the dot notation, the dot separates the input stream variable name from
the member, or function, name
• In C++, the dot is the member access operator
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Input Failure
• Things can go wrong during execution
• If input data does not match corresponding variables, the program may run into
problems
• Trying to read a letter into an int or double variable will result in an input
failure
• If an error occurs when reading data
• Input stream enters the fail state
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The clear Function
• 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 results
• The clear function restores the input stream to a working state
• The syntax of the function clear is:
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Output and Formatting Output
• Syntax of cout when used with <<
• expression is evaluated
• value is printed
• manipulator is used to format the output
• Example: endl
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setprecision Manipulator
• Syntax
• Outputs decimal numbers with up to n decimal places
• Must include the header file iomanip
• #include <iomanip>
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fixed Manipulator
• fixed outputs floating-point numbers in a fixed decimal format
• Example: cout << fixed;
• Disable by using the stream member function unsetf
- Example: cout.unsetf(ios::fixed);
• scientific manipulator outputs floating-point numbers in scientific format
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showpoint Manipulator
• showpoint forces output to show the decimal point and trailing zeros
• Examples
• cout << showpoint;
• cout << fixed << showpoint;
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C++14 Digit Separator
• Reading and writing of long numbers can be error prone
• In C++, commas cannot be used to separate the digits of a number
• C++14 introduces digit separator ' (single-quote character)
• Example: 87523872918 can be represented as 87'523'872'918
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setw
• Outputs the value of an expression in a specified number of columns
• cout << setw(5) << x << endl;
• If number of columns exceeds the number of columns required by the
expression
• Output of the expression is right-justified
• Unused columns to the left are filled with spaces
• Must include the header file iomanip
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Additional Output Formatting Tools
• Additional formatting tools that give you more control over your output:
• setfill manipulator
• left and right manipulators
• unsetf manipulator
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setfill Manipulator
• Output stream variables can use setfill to fill unused columns with a
character
• Example:
• cout << setfill('#');
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left and right Manipulators
• left manipulator left-justifies the output
• Disable left by using unsetf
• right manipulator right-justifies the output
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Types of Manipulators
• Two types of manipulators
• Those with parameters
• Those without parameters
• Parameterized stream manipulators require the iomanip header
• setprecision, setw, and setfill
• Manipulators without parameters require the iostream header
• endl, fixed, scientific, showpoint, and left
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Input/Output and the string Type
• An input stream variable (such as cin) and >> operator can read a string into
a variable of the data type string
• The extraction operator:
• Skips any leading whitespace characters
• Stops reading at a whitespace character
• The function getline reads until end of the current line
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Debugging: Understanding Logic Errors and Debugging with cout
statements
• Syntax errors are reported by the compiler
• Logic errors are typically not caught by the compiler
• Spot and correct using cout statements
- Temporarily insert an output statement
• Correct the problem
• Remove output statement
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File Input/Output
• A file is an area in secondary storage to hold info
• File I/O is a five-step process
1. Include fstream header
2. Declare file stream variables
3. Associate the file stream variables with the input/output sources – referred to as
opening the files
4. Use the file stream variables with >>, <<, or other input/output functions
5. Close the files
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Quick Review (1 of 3)
• Stream: infinite sequence of characters from a source to a destination
• Input stream: from a source to a computer
• Output stream: from a computer to a destination
• cin: common input
• cout: common output
• To use cin and cout, include iostream header
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Quick Review (2 of 3)
• get reads data character-by-character
• ignore skips data in a line
• putback puts last character retrieved by get back to the input stream
• peek returns next character from input stream, but does not remove it
• Attempting to read invalid data into a variable causes the input stream to enter
the fail state
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Quick Review (3 of 3)
• The manipulators setprecision, fixed, showpoint, setw, setfill,
left, and right can be used for formatting output
• Include iomanip for the manipulators setprecision, setw, and
setfill
• Header fstream contains the definitions of ifstream and ofstream
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