Transcript Chapter 8 - Object Oriented Programming

Chapter 8:
Streams and Files
Problem Solving,
Abstraction, and Design using C++ 6e
by Frank L. Friedman and Elliot B. Koffman
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
8.1 Standard Input/Output
Streams
• A stream is a sequence of characters
• Standard input stream (cin) and standard output
stream (cout) of the iostream library
• Streams convert internal representations to
character streams or vice versa
• >> input (extraction) operator
• << output (insertion) operator
• Any character (printable or not)
• A stream has no fixed size
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Input Stream, >> Operator
• Last character read is kept track using an input
stream buffer pointer
• Each new input attempt begins at current pointer
position
• Leading white space (blanks, tab, nwln) skipped
until first non-white-space character is located
cin >> ch;
• For numeric value, all characters that are part of
the numeric value are processed until a character
that’s not legally part of a C++ number is read.
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Reading One Character at a
Time
• Can also read/write white space
– iostream functions get and put
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Listing 8.2
Processing individual characters in a stream
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Listing 8.2
Processing individual characters in a stream
(continued)
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Table 8.1
The File Manipulation Member Functions
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Using a Stream in a Condition
cout << “Enter a line or press “ << ENDFILE << “: “;
while (cin.get(next))
// get first char of new line
{
// insert loop body including inner while loop
...
cout << “Enter a line or press “ << ENDFILE << “: “;
} // end outer while
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8.2 External Files
• Interactive
– Expect user input from keyboard
– Ok for smaller programs
• Batch
–
–
–
–
–
–
Requires use of data files (save to disk)
Don’t expect input from keyboard, so no prompts
Often uses echo printing of input
Input file can be read many times
Can also write output to file
Output file from one program can be input to another
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Directory Names for External
Files
• Must know location of file within directory
structure (pathname)
• Directories are system dependent
• File names usually follow system and local
conventions
– how file is named
– use of extensions
• .cpp .dat .doc .txt
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Attaching Streams to External
Files
• Declaring a stream object
ifstream ins;
ofstream outs;
• External (physical) file name linked to internal
(logical) name by open function
#define inFile “InData.txt”
...
ins.open(inFile);
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Reading a File Name
string fileName;
cout << “Enter the input file name: “;
cin >> fileName;
ins.open(fileName.c_str( ));
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Case Study: Preparing a Payroll File
• Write a program that reads a data file
consisting of employee salary data, and
computes the employee’s gross salary. It
writes the employee’s name and gross
salary to an output file and accumulates the
gross salary amount in the total company
payroll, which it also displays.
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Case Study: Sample Files
• Input
Jim Baxter
35.5
7.25<nwln>
Adrian Cybriwsky
Ayisha Mertens
40.0
20.0
6.50<nwln>
8.00<nwln>
• Output
Jim Baxter
257.38<nwln>
Adrian Cybriwsky
Ayisha Mertens
260.00<nwln>
160.00<nwln><eof>
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Case Study: Problem Analysis
• Streams Used
– ifstream eds
– ofstream pds
// employee data info
// payroll data info
• Problem Input (from stream eds)
– for each employee
•
•
•
•
string firstName
string lastName
float hoursWorked
float hourlyRate
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Case Study: Problem Analysis
• Problem Output (to stream pds)
– for each employee
• string firstName
• string lastName
• float salary
• Problem Output (to stream cout)
– float totalPayroll
// total company payroll
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Case Study: Program Design
• Algorithm for function main
1. Prepare streams and associated files for
processing
2. Process all employees and compute payroll
total (function processEmp).
3. Display the payroll total.
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Case Study: Interface for
processEmp
• Input Arguments
– ifstream eds // input stream - employee data
– ofstream pds // output stream - payroll data
• Output Arguments
– none
• Function Return Value
– float totalPayroll
// total company payroll
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Case Study: Algorithm for
processEmp
1. Initialize payroll total to 0.0
2. While there are more employees
2.1 Read employee’s first and last names
and salary data from eds
2.2 Compute employee’s salary
2.3 Write employee’s first and last
names and salary to pds; add it to payroll
total
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Listing 8.5
Implementation of processEmp
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Listing 8.5
Implementation of processEmp (continued)
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8.4 More on Reading String Data
• getline (istream& ins, string& str)
• getline (istream& ins, string& str, char delimiter)
• ins.ignore (int n, char delimiter)
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Using getline
• getline - could be used to process an entire
line of data
• E.g. for data file containing
Jim Andrew Baxter#
Adrian Cybriwsky#
Ayisha W. Mertens#
35.5
40.0
20.0
7.25<nwln>
6.50<nwln>
8.00<nwln>
• Use # as a delimiter character
getline (eds, name, ‘#’);
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Using ignore
• getline does not skip leading white space,
so if a newline character is encountered at
the beginning of the characters to be
extracted, getline will stop immediately and
won’t perform the expected task
• E.g. data file contains
Jim Andrew Baxter<nwln>
35.5
7.25<nwln>
Adrian Cybriwsky <nwln>
40.0
6.50 <nwln>
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payroll = 0.0;
getline(eds, name);
while (!eds.eof( ))
{
eds >> hours >> rate;
salary = hours * rate;
pds << name << “ “ << salary << endl;
payroll += salary;
eds.ignore(100, ‘\n’);
getline(eds, name);
}
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Table 8.3
Input/Output Manipulators
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Manipulators Example
cout << “The value of x is “
<< fixed
<< showpoint
<< setprecision(4) << x << endl;
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8.6 Common Programming Errors
•
•
•
•
Connecting streams and external files
Using input and output streams incorrectly
Reading past the end of a file
Matching data in an input stream to input
data stores
• White space and input
• Proper handling of the newline character
• Input and output settings
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