1. Lecture 5A
File processing
Richard Gesick

2. Outline Objectives Defining File Streams Reading Data Files
Generating a Data File
Problem Solving Applied: Data Filters – Modifying an HTML File
Error Checking
Numerical Technique: Linear Modeling*
Problem Solving Applied: Ozone Measurements*

3. Objectives Develop 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.

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Defining File Streams
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5. Standard Input and Output
C++ defines the cin and cout objects to 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.

6. Other kinds of Streams
Standard C++ library also defines special kinds of streams for file input and output.
#include <ifstream>
#include <ofstream>

7. Disk Files for I/O #include <fstream> your variable
(of type ifstream)
(of type ofstream)
disk file
“myInfile.dat”
“myOut.dat”
executing
program
input data
output data

8. Disk I/O To use disk I/O Access #include <fstream>
Choose valid identifiers for your file streams and declare them
Open the files and associate them with disk names

9. Disk I/O, cont...
Use your file stream identifiers in your I/O statements(using >> and << , manipulators, get, ignore)
Close the files

10. Stream Object Hierarchy
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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.
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12. Opening a File Opening a file
Places a file reading marker at the very beginning of the file, pointing to the first character in the file

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Input File Streams
Each data files used for input must have an ifstream object associated with it:
ifstream sensor1;
sensor1.open(“sensor1.dat”);
Or just –
ifstream sensor1(“sensor1.dat”);
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Avoiding Bugs
If opening the named file fails, the fail error 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.
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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 }
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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.
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Output File Streams
Each data files used for output must have an ofstream object associated with it:
ofstream sensor1;
sensor1.open(“balloon.dat”);
Or just –
ofstream sensor1(“balloon.dat”);
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Output File Modes
By default, opening a file for output in 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);
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More on File Objects
The close() methods for both input and 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.
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Reading Data Files
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File Formats
To read a file, some knowledge about the 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.
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22. Specified Number of Lines
sensor1.dat
//open input file
ifstream sensor1(“sensor1.dat”);
//read the number of data entries
int numEntries;
sensor1 >> numEntries;
//read every row
double t, y;
for (int i = 0; i < numEntries; i++) {
sensor1 >> t >> y; //do something with the data }
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23. Trailer/Sentinel Signal
sensor2.dat
//open input file
ifstream sensor2(“sensor2.dat”);
//read every row
double t, y;
do {
sensor1 >> t >> y; if (t < 0 || y < 0) break;
//do something with the data } while (! sensor1.eof());
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24. Run Time File Name Entry
#include <string> // Contains conversion function c_str ifstream inFile; string fileName; // Prompt: cout << “Enter input file name: “ << endl; cin >> fileName; // Convert string fileName to a C string type inFile.open(fileName.c_str());

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EOF-based File Input
sensor3.dat
//open input file
ifstream sensor3(“sensor3.dat”);
//read every row
double t, y;
while (! sensor3.eof()) {
sensor1 >> t >> y;
if (sensor3.fail()) break; //do something with the data }
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Generating a Data File
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Writing Files
After opening an output file, writing to a 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.
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Error Checking
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29. Opening a File Opening a file
Associates the C++ identifier for your file with the physical(disk) name for the file
If the input file does not exist on disk, open is not successful
If the output file does not exist on disk, a new file with that name is created
If the output file already exists, it is erased

30. Stream Fail State When 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

31. Stream Fail State 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

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Error Checking
In addition to indicating if an error occurs 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.
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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.
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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).
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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 1
Failure to open a file
Unexpected data encountered
End of file encountered
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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.
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37. Problem Solving Applied: Data Filters – Modifying an HTML File
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38. Problem Solving Applied: Data Filters – Modifying an HTML File
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39. Problem Solving Applied: Data Filters – Modifying an HTML File
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40. Problem Solving Applied: Data Filters – Modifying an HTML File
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41. Problem Solving Applied: Data Filters – Modifying an HTML File
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42. Numerical Technique: Linear Modeling*
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Linear Modeling
Linear modeling is the name given to the 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.
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Example
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45. Solving for Linear Model Parameters
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46. Problem Solving Applied: Ozone Measurements*
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47. Problem Solving Applied: Ozone Measurements*
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48. Problem Solving Applied: Ozone Measurements*
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49. Problem Solving Applied: Ozone Measurements*
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50. Problem Solving Applied: Ozone Measurements*
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