1. C++ Programming Lecture 8 File ProcessingBy
Ghada Al-Mashaqbeh
The Hashemite University
Computer Engineering Department

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Outline
Introduction.
The Data Hierarchy.
Files and Streams.
Creating a Sequential Access File.
Reading Data from a Sequential Access File.
Updating Sequential Access Files.
Input/Output Manipulations.
Examples.
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Introduction
Data files can be created, updated, and processed by C++ programs
Files are used for permanent storage of large amounts of data
Storage of data in variables and arrays is only temporary
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The Data Hierarchy I
Bit - smallest data item
value of 0 or 1
Byte – 8 bits
used to store a character
Decimal digits, letters, and special symbols
Field - group of characters conveying meaning
Example: your name
Record – group of related fields
Represented as a struct or a class
Example: In a payroll system, a record for a particular employee that contained his/her identification number, name, address, etc.
File – group of related records
Example: payroll file
Database – group of related files
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The Data Hierarchy II 1
Judy
Green
Sally
Black
Tom
Blue
Iris
Orange
Randy
Red
File
Record
Field
Byte (ASCII character J)
Bit
Record key
identifies a record to facilitate the retrieval of specific records from a file
Sequential file
records typically sorted by key
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Files and Streams
C++ views each file as a sequence of bytes
File ends with the end-of-file marker
Stream is created when a file is opened
File processing
Headers <iostream> and <fstream>
class ifstream - input
class ofstream - output
class fstream - either input or output
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7. Creating a Sequential Access File
Files are opened by creating objects of stream classes ifstream, ofstream or fstream
File stream member functions for object file:
file.open(“Filename”, fileOpenMode);
file.close();
closes file
File open modes:
Makes a "line of communication" with the object and the file.
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Example I
#include<iostream>
#include<fstream>
using namespace std;
int main() {
ofstream out_file("HU.txt", ios::out); //or HU.dat
if(out_file == 0)
cout <<"Unable to open the file\n";
exit(1); }
for(int i = 0; i < 9; i++)
out_file << i << endl;
out_file.close();
return 0;
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9. Example II – Storing Data Entered By the User
#include<iostream>
#include<fstream>
using namespace std;
int main() {
ofstream out_file("HU.txt", ios::in); //or HU.dat
int a;
if(out_file == 0)
cout <<"Unable to open the file\n";
exit(1); }
cout <<"Enter an integer to save on file\n";
cin >> a;
while(a != -1) //end when -1 is entered {
out_file << a << endl;
cout <<"Enter an integer to save on file\n";
cin >> a; }
out_file.close();
return 0;
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Notes
You will find the created text file in the same directory in which you save the source code unless you specify the full path of where you want to save the created text file.
The extension of the created file could be .txt or .dat
Pay attention to how the data is written on the file (wether there are formatted, only one value is stored per line or multiple ones, etc.) to read the data correctly.
All attributes associated with cin object (stopping at enetr or white spaces, read vales based on the data type of the holder, etc.) are applied to files reading.
ofstream are opened for output by default.so, in the previous example writing ofstream out_file("HU.txt"); is the same as ofstream out_file("HU.txt", ios::out);
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11. Reading Data from a Sequential Access File
File stream member functions for repositioning file position pointer:
seekg (seek get) for istream and seekp (seek put) for ostream
// position to the nth byte of fileObject
// assumes ios::beg fileObject.seekg( n );
// position n bytes forward in fileObject fileObject.seekg( n, ios::cur );
// position y bytes back from end of fileObject fileObject.seekg( y, ios::end );
// position at end of fileObject fileObject.seekg( 0, ios::end );
tellg and tellp – return current location of pointer
location = fileObject.tellg() //returns long int
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Example
#include<iostream>
#include<fstream>
using namespace std;
int main() {
ifstream in_file("HU.txt", ios::in); //or HU.dat
int a;
if(in_file == 0)
cout <<"Unable to open the file\n";
exit(1); }
for(int i = 0; i < 9; i++)
in_file >> a;
cout << a << endl;
in_file.close();
return 0;
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Example Output
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Note
The file that you read from must be stored in the same directory in which you store the source code file, otherwise you must specify the full path of where you store the file.
The extension of the created file could be .txt or .dat
ifstream are opened for input by default. so, in the previous example writing ifstream in_file("HU.txt"); is the same as ifstream in_file("HU.txt", ios::in);
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15. Updating Sequential Access Files
Cannot be modified without the risk of destroying other data
Formatted text which is output to a file is very different than the internal representation
300 White Jones (old data in file)
if we want to change White's name to Worthington,
300 White Jones 32.87
300 Worthington 0.00ones 32.87
300 Worthington 0.00
Data gets overwritten
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16. Input/Output Manipulations
C++ provides various stream manipulators that perform formatting tasks.
Till now we have dealt with screen output using cout. So, all these manipulators will format the text that appears on the screen.
Examples:
Setting the precision of the displayed floating point numbers.
Setting the field width.
Showing trailing zeros and decimal point in floating point numbers.
Filling field with specific characters.
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17. Floating-Point Precision (setprecision)
sets number of digits to the right of decimal point (with rounding)
parameterized stream manipulator
Like all parameterized stream manipulators, <iomanip> required
specify precision:
cout << setprecision(2) << x;
Changes last until a different value is set.
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Field Width(setw)
setw stream manipulator
sets field width (number of character positions a value should be output or number of characters that should be input).
returns to previous width after the implementation (so it is applied to the next insertion or extraction only).
If values processed are smaller than width, fill characters inserted as padding.
Large values are not truncated - full number printed.
Displayed output is right justified.
Examples:
cin >> setw(5) >> number;
cout << setw(5) << number;
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19. Trailing Zeros and Decimal Points (showpoint)
ios::showpoint
forces a float with an integer value to be printed with its decimal point and trailing zeros
cout << showpoint << 79.0;
will print
number of zeros determined by precision settings
Applied for all subsequent output operations.
To reset this option apply the following command:
cout << noshowpoint;
Remember trailing zeros are applied to floating point numbers only and not for integers.
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Padding (setfill)
If you specify the field width to 10 and you print 56 then this number will be preceded by white spaces.
White space here is the fill character.
You can change this fill character to any other character you want.
Applied for all subsequent cout statements. To remove it set the fill character to white space to return to the default settings.
setfill manipulator
sets fill character
cout << setfill ('*');
cout << setw(5) << 3; //output is ****3
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21. Floating-Point Numbers; Scientific Notation (scientific, fixed)
forces output of a floating point number in scientific notation:
e+009
Only one digit in the integer part of the floating point number and n digits after the floating point (depends on the precision value) and put the suitable power after the e exponent.
E.g.: cout << scientific << ;
fixed
forces floating point numbers to display a specific number of digits to the right of the decimal (specified with precision) and the whole integral part.
E.g.: cout << fixed << 234.5e-1;
Applied for all subsequent cout statements.
The default setting in C++ is the fixed formatting or depends on the original value of the floating point number.
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Additional Notes
This lecture covers the following material from the textbook:
Chapter 14: Sections 14.1 – 14.6
Chapter 12: Sub-Sections , , , ,
This is the last lecture included in the first exam material.
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