copyright © 2012 pearson education, inc. chapter 5 working with data files

Copyright © 2012 Pearson Education, Inc.

Chapter 5

Working with Data Files


OutlineObjectives

1. Defining File Streams

2. Reading Data Files

3. Generating a Data File

4. Problem Solving Applied: Data Filters – Modifying an HTML File

5. Error Checking

6. Numerical Technique: Linear Modeling*

7. Problem Solving Applied: Ozone Measurements*


ObjectivesDevelop problem solutions in C++ that:

• Open and close data files for input and output.

• Read data from files using common looping structures.

• Check the state of an input stream.

• Recover from input stream errors.

• Apply the numerical technique of linear modeling.

Defining File Streams


Standard Input and Output

• C++ defines the cin and cout objectsto represent the standard input (keyboard) and standard output (console).

• C++ also defines the standard error stream, cerr.

• Standard input is a kind of general input stream class; standard output and error streams are special kinds of general output stream.


Other kinds of Streams

• Standard C++ library also definesspecial kinds of streams for file input and output.– #include <ifstream>– #include <ofstream>


Stream Object Hierarchy


The ifstream Class• The ifstream (input file

stream) class is derived from istream (input stream class).– Thus ifstream inherits the

input operator and member functions eof() and fail() defined in istream.

– The ifstream class also defined specialized methods specific to working with files.


Input File Streams

• Each data files used for input musthave an ifstream object associated with it:

ifstream sensor1;sensor1.open(“sensor1.dat”);

- Or just –

ifstream sensor1(“sensor1.dat”);Copyright © 2012 Pearson Education, Inc.

Avoiding Bugs

• If opening the named file fails, the failerror bit is set, and all statements to read from the file will be ignored.– NO error message will be generated but the

program will continue to execute.– Check to be sure that the open was

successful.• fail() method of istream returns false.


Input File Example

ifstream sensor1;sensor1.open("sensor1.dat");if ( sensor1.fail() ) //open failed{ cerr << "File sensor1.dat could not be opened"; exit(1); //end execution of the program}


The ofstream Class• The ofstream (output file

stream) class is derived from ostream (output stream class).– Thus ofstream inherits the

output operator and member functions ostream.

– The ofstream class also defined specialized methods specific to working with files.


Output File Streams

• Each data files used for output musthave an ofstream object associated with it:

ofstream sensor1;sensor1.open(“balloon.dat”);

- Or just –

ofstream sensor1(“balloon.dat”);Copyright © 2012 Pearson Education, Inc.

Output File Modes

• By default, opening a file for outputin this way will either create the file if it doesn’t already exist or overwrite a previously existing file.

• If you wish to simply append new content to the previously existing file, you may open the file in append mode:

sensor1.open(“balloon.dat”, ios::append);


More on File Objects

• The close() methods for both inputand output stream objects should be called when the program is done with the file.– The method(s) will be called automatically when

the program exits.

• When you use a string class to represent the file name, you must use the c_str() method of string to provide the appropriate type for use in input and output open/constructor methods.


Reading Data Files


File Formats

• To read a file, some knowledge aboutthe contents of the file are needed:– Name of the file.– Order and data types of the values stored in the file.

• Three common structures:– First-line in file contains number of lines/records in the

file.– Trailer signal/sentinel signal used to indicate the last

line/record in the file.– End-of-file used directly to indicate last line/record in

the file.Copyright © 2012 Pearson Education, Inc.

Specified Number of Lines

sensor1.dat100.0 132.50.1 147.20.2 148.30.3 157.30.4 163.20.5 158.20.6 169.30.7 148.20.8 137.6


//open input fileifstream sensor1(“sensor1.dat”);//read the number of data entriesint numEntries;sensor1 >> numEntries;//read every rowdouble t, y;for (int i = 0; I < numEntries; i++) {

sensor1 >> t >> y;//do something with the data

}

Trailer/Sentinel Signal

sensor2.dat0.0 132.50.1 147.20.2 148.30.3 157.30.4 163.20.5 158.20.6 169.30.7 148.20.8 137.6-99 -99


//open input fileifstream sensor2(“sensor2.dat”);//read every rowdouble t, y;do {

sensor1 >> t >> y;if (t < 0 || y < 0) break;//do something with the data

} while (! sensor1.eof());

EOF-based File Input

sensor3.dat0.0 132.50.1 147.20.2 148.30.3 157.30.4 163.20.5 158.20.6 169.30.7 148.20.8 137.6


//open input fileifstream sensor3(“sensor3.dat”);//read every rowdouble t, y;while (! sensor3.eof()) {

sensor1 >> t >> y;if (sensor3.fail()) break;//do something with the data

}

Generating a Data File


Writing Files

• After opening an output file, writing toa file is no different from writing to standard output.

• Must decide on a file format.– Sentinels can be hard to choose to avoid conflict with

valid data.– Knowing in advance how many lines/records are in

the file is sometimes difficult or impractical.

– Usually best to use eof-style file format where file structure contains only valid information.


Problem Solving Applied: Data Filters – Modifying

an HTML File


Error Checking


Error Checking

• In addition to indicating if an erroroccurs when opening a file, fail() may also be used to detect other types of errors.– Errors can occur when trying to read from an

input stream when the data on the input stream isn’t of the appropriate type.


Whitespace

• Whitespace is defined as blank, tab, newline, form feed, and carriage return characters.

• Used to separate data when reading from an input stream.

• For example, to read 2 integers from an input stream, there must be whitespace between the two numbers.– C++ ignores the whitespace to read the numbers.– If any other characters are encountered, the error (fail)

state is set.


Input Stream Errors

• Do not generate notification of errors!– Only set the error state.

• Reads will not affect the state of the input buffer or alter variables.– However the program will continue executing!

• Must “clear” the error state before additional input may be read (using istream’s clear() method).


Stream State

• Indicated by a set of state flags:– badbit, failbit, eofbit, goodbit.

• Events may alter the stream state:


Event badbit failbit eofbit goodbit

Initialization of a stream

0 0 0 1

Failure to open a file 0 1 0 0

Unexpected data encountered

0 1 0 0

End of file encountered

0 1 1 0

Stream Class Methods


Method Description

bool bad() Returns true iff badbit is set.

bool eof() Returns true iff eofbit is set.

bool fail() Returns true iff failbit is set.

bool good() Returns true iff goodbit is set.

void clear(iostate flag = goodbit) Sets the state flags.

iostate rdstate() Returns the value of the state flags.

Numerical Technique: Linear Modeling*


Linear Modeling

• Linear modeling is the name given tothe process that determines the linear equation (y = mx + b) that is the best fit to a set of data points.– Linear regression does this by minimizing the

squared distance between the line and the data points.


Example


Solving for Linear Model Parameters


Problem Solving Applied: Ozone Measurements*


copyright © 2012 pearson education, inc. chapter 5 working with data files

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