Transcript Document

Today’s Lecture
 I/O Streams
 Console I/O
 File I/O
 Tools for File I/O
 Sequential Access & Random Access to
Files
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Introduction
 Streams
 Special objects
 Deliver program input and output
 In C++
 Console I/O is working with streams
 File I/O is also working with streams
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Streams
 Stream: A flow of data
 Input stream
 Flow into program
 Can come from keyboard
 Can come from file
 Output stream
 Flow out of program
 Can go to screen
 Can go to file
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Streams Usage
 We’ve used streams already
 cin
 Input stream object connected to keyboard
 cout
 Output stream object connected to screen
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Stream Example
int num1, num2, total;
cout << "First number: ";
cin >> num1;
cout << "Second number: ";
cin >> num2;
total = num1 + num2;
cout << “total = " << total << endl;
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Streams Usage
 We’ve used streams already
 cin
 Input stream object connected to keyboard
 cout
 Output stream object connected to screen
 Can define other streams
 To or from files
 Used similarly as cin, cout
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Streams Usage Examples: cin,
cout
 Consider:
 Using the pre-defined stream cin
int
cin
Number;
>> Number;
 Reads value from stream (keyboard), assigned to
Number
 Using the pre-defined stream cout
cout
<<
Number;
 Writes value to stream (monitor)
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Streams Usage Like cin, cout
 Consider:
 Given program defines stream inStream
that comes from some file:
int
Number;
inStream
>>
Number;
 Reads value from stream (file), assigned to
theNumber
 Program defines stream outStream that goes
to some file
outStream
<<
theNumber;
 Writes value to stream, which goes to file
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Files
 Reading from file
 When program takes input
 Writing to file
 When program sends output
 Start at beginning of file (Sequential
access)
 Other methods available
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File Connection
 Must first create a file stream object
 Then connect the file to the file stream object
 For input only:
 File  ifstream object
 For output only:
 File  ofstream object
 Classes ifstream and ofstream
 Defined in library <fstream>
 Named in std namespace
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File I/O Libraries
 To allow both file input and output in
your program:
#include <fstream>
using namespace std;
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Declaring Streams Objects
(Variables)
 Stream must be declared like any other
class variable:
ifstream input_file_stream;
ofstream out_file_stream;
 Must then "connect" to file:
input_file_stream.open("infile.txt");
 Called "opening the file"
 Uses member function open
 Can specify complete pathname
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Streams Usage
 Once declared and connected, use it like cin and
cout!
int N;
input_file_stream
>> N;
 Output stream similar:
ofstream output_file_stream;
output_file_stream.open("outfile.txt");
output_file_stream << "Number = " << N;
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Closing Files
 Files should be closed
 When program completed getting input or
sending output
 Disconnects stream from file
 In action:
input_file_stream.close();
output_file_stream.close();
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File Example:
Display 12.1 Simple File
Input/Output (1 of 2)
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File Example:
Display 12.1 Simple File
Input/Output (1 of 2)
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Appending to a File
 Standard of stream open operation begins
with empty file
 Even if file exists  contents lost
 Open for append:
ofstream outStream;
outStream.open("important.txt", ios::app);
 If file doesn’t exist  creates it
 If file exists  appends to end
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Compare Console I/O and File
I/O
 All cin functions are same for files!
 Read in (send out) integers (int)
 Read in (send out) characters (char)
 Read in (send out) floating-point s (float)
 ….
 For String operating:
 >> read one word (separated by spaces,
new lines)
 getline() read one line of words
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Compare Console I/O and File
I/O (continued)
 For console String (C-string)
operations

char S[100];
cin >> S;
 read one word

char S[100];
cin.getline(S, 20) ;
 read one line of words
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Compare Console I/O and File
I/O (continued)
 For file String (C-string) operations
ifstream ifm;
ifm.open("infile.txt");

char S[100];
ifm >> S;
 read one word

char S[100];
ifm.getline(S, 20) ;
 read one line of words
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Compare Console I/O and File
I/O (continued)
 cout is same for file operations (say
fout)
ofstream fout;
fout.open(“output.txt", ios::app);
cout << “whatever\n”
fout << “whatever\n”
 Whatever displayed on screen will be
same as those saved in file
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More I/O member functions
 See Appendix 4 of the textbook
Stream_var.open(“File name”);
Stream_var.close();
Strean_var.eof();
Stream_var.get(char_var);
Stream_var.getline(cstring_var, length);
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Checking End of File
 Use loop to process file until end
 An extraction operator actually returns a boolean
value
inStrem >> next
Return true if the read was successful (space and
new line will not be read) and returns false when
code attempts to read beyond the end of the file
Int next, sum =0;
while (inStream >> next)
{
sum = sum + next;
}
cout << “ the sum is “ << sum << endl;
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Checking End of File
 Use loop to process file until end
inStrem.get(next)
Return true if the read was successful and returns
false when code attempts to read beyond the end
of the file
char next, sum =0;
while (inStream.get(next))
{
sum = sum + 1;
}
cout << “ the total number of characters is “ << sum << endl;
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Checking End of File
 Use loop to process file until end
inStrem.getline(next, length)
Return true if the read was successful and returns
false when code attempts to read beyond the end
of the file
Char[] next, sum =0;
while (inStream.getline(next, 100))
{
sum = sum + 1;
}
cout << “ the total number of lines is “ << sum << endl;
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Which Function to Write
 The easiest one and works for any text
data
 <<
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Which Function to Read
 Numbers (Integer / floating-point) file:
numbers are separated by blank space(s)
 >>
 Text file: words are separated by blank
space(s): each word is going to be processed
separately
 >>
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Which Function to Read
(continued)
 Character files
 each character (including space) needs to be
processed individually
 get(char s)
 Ignore spaces, new lines, and tab
 >>
 Character files: one line is a record
 getline(char s[], int size)
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Tools: File Names as Input
 Stream open operation
 Argument to open() is string type
 Can be literal (used so far) or variable
char fileName[16];
ifstream inStream;
cout << "Enter file name: ";
cin >> fileName;
inStream.open(fileName);
 Provides more flexibility
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File Access Applications
 Copy files
 Copy char by char
 Copy line by line
 Process files:
 Determine how many specific characters
 Count words
 Count lines
 Work on numbers
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Read and Write can be
Performed at the Same Time
 Opens same as istream or ostream
 Adds second argument
 fstream rwStream;
rwStream.open("stuff", ios::in | ios:: out);
 Opens with read and write capability
 It is confusing where to read and where to
write: not recommend to use.
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Random Access to Files
 Sequential Access
 Most commonly used
 So far we have talked about
 Random Access
 Rapid access to records
 Access "randomly" to any part of file
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Random Access Tools
 Move about in file
 rwStream.seekp(1000);
Positions put-pointer at 1000th byte
 rwStream.seekg(1000);
Positions get-pointer at 1000th byte
 Not a requirement for this course
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Summary of File Operations
 Read from a file
 Write to a new or existing file
 Delete a file
 Copy a file
 Move a file
 Modify a file
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