CSIS 123A Lecture 8

1. CSIS 123A Lecture 8 Streams & File IO Glenn Stevenson CSIS 113A MSJC

2. Introduction • Streams • Special objects • Deliver program input and output • File I/O • Uses inheritance • Covered Later • File I/O very useful, so covered here Glenn Stevenson CSIS 113A MSJC

3. Streams • A flow of characters • 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 Glenn Stevenson CSIS 113A MSJC

4. 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 Glenn Stevenson CSIS 113A MSJC

5. Streams Usage Like cin, cout • Consider: • Given program defines stream inStreamthat comes from some file:int theNumber;inStream >> theNumber; • Reads value from stream, assigned to theNumber • Program defines stream outStream that goesto some fileoutStream << “theNumber is “ << theNumber; • Writes value to stream, which goes to file Glenn Stevenson CSIS 113A MSJC

6. Files • We’ll use text files • Reading from file • When program takes input • Writing to file • When program sends output • Start at beginning of file to end • Other methods available • We’ll discuss this simple text file access here Glenn Stevenson CSIS 113A MSJC

7. File Connection • Must first connect file to stream object • For input: • File  ifstream object • For output: • File  ofstream object • Classes ifstream and ofstream • Defined in library <fstream> • Named in std namespace Glenn Stevenson CSIS 113A MSJC

8. File I/O Libraries • To allow both file input and output in yourprogram:#include <fstream>using namespace std; OR#include <fstream>using std::ifstream;using std::ofstream; Glenn Stevenson CSIS 113A MSJC

9. Declaring Streams • Stream must be declared like any otherclass variable:ifstream inStream; ofstream outStream; • Must then ‘connect’ to file:inStream.open(“infile.txt”); • Called ‘opening the file’ • Uses member function open • Can specify complete pathname Glenn Stevenson CSIS 113A MSJC

10. Streams Usage • Once declared  use normally!int oneNumber, anotherNumber; inStream >> oneNumber >> anotherNumber; • Output stream similar:ofstream outStream; outStream.open(“outfile.txt”); outStream << “oneNumber = “ << oneNumber << “ anotherNumber = “ << anotherNumber; • Sends items to output file Glenn Stevenson CSIS 113A MSJC

11. File Names • Programs and files • Files have two names to our programs • External file name • Also called ‘physical file name’ • Like ‘infile.txt’ • Sometimes considered ‘real file name’ • Used only once in program (to open) • Stream name • Also called ‘logical file name’ • Program uses this name for all file activity Glenn Stevenson CSIS 113A MSJC

12. Closing Files • Files should be closed • When program completed getting input orsending output • Disconnects stream from file • In action:inStream.close(); outStream.close(); • Note no arguments • Files automatically close when programends Glenn Stevenson CSIS 113A MSJC

13. File Flush • Output often ‘buffered’ • Temporarily stored before written to file • Written in ‘groups’ • Occasionally might need to force writing:outStream.flush(); • Member function flush, for all output streams • All buffered output is physically written • Closing file automatically calls flush() Glenn Stevenson CSIS 113A MSJC

14. File Example Glenn Stevenson CSIS 113A MSJC

15. Appending to a File • Standard open operation begins withempty 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 • 2nd argument is class ios defined constant • In <iostream> library, std namespace Glenn Stevenson CSIS 113A MSJC

16. Alternative Syntax for File Opens • Can specify filename at declaration • Passed as argument to constructor • ifstream inStream;inStream.open(“infile.txt”); EQUIVALENT TO:ifstream inStream(“infile.txt”); Glenn Stevenson CSIS 113A MSJC

17. Checking File Open Success • File opens could fail • If input file doesn’t exist • No write permissions to output file • Unexpected results • Member function fail() • Place call to fail() to check stream operationsuccessinStream.open(“stuff.txt”);if (inStream.fail()){ cout << “File open failed.\n”; exit(1);} Glenn Stevenson CSIS 113A MSJC

18. Character I/O with Files • All cin and cout character I/O same forfiles! • Member functions work same: • get, getline • put, putback, • peek, ignore Glenn Stevenson CSIS 113A MSJC

19. Checking End of File • Use loop to process file until end • Typical approach • Two ways to test for end of file • Member function eof()inStream.get(next);while (!inStream.eof()){ cout << next; inStream.get(next);} • Reads each character until file ends • eof() member function returns bool Glenn Stevenson CSIS 113A MSJC

20. End of File Check with Read • Second method • read operation returns bool value!(inStream >> next) • Expression returns true if read successful • Returns false if attempt to read beyond end of file • In action:double next, sum = 0;while (inStream >> next) sum = sum + next;cout << “the sum is “ << sum << endl; Glenn Stevenson CSIS 113A MSJC

21. Random Access to Files • Sequential Access • Most commonly used • Random Access • Rapid access to records • Perhaps very large database • Access ‘randomly’ to any part of file • Use fstream objects • input and output Glenn Stevenson CSIS 113A MSJC

22. Random Access Tools • Opens same as istream or ostream • Adds second argument • fstream rwStream;rwStream.open(“stuff”, ios::in | ios:: out); • Opens with read and write capability • Move about in file • rwStream.seekp(1000); • Positions put-pointer at 1000th byte • rwStream.seekg(1000); • Positions get-pointer at 1000th byte Glenn Stevenson CSIS 113A MSJC

23. Random Access Sizes • To move about  must know sizes • sizeof() operator determines number of bytesrequired for an object:sizeof(s) //Where s is string s = “Hello”sizeof(10)sizeof(double)sizeof(myObject) • Position put-pointer at 100th record of objects:rwStream.seekp(100*sizeof(myObject) – 1); Glenn Stevenson CSIS 113A MSJC

24. More seekp, seekg • ostream&  seekp ( streampos pos ); • move the file pointer to a specific offset from where it currently is • ostream&  seekp ( streamoff off , ios_base::seekdir dir ); • allows you to specify a base point to offset from one of the following • ios_base::beg seek relative to beginning of file • ios_base::end seek relative to end of file • ios_base::cur seek relative to current position Glenn Stevenson CSIS 113A MSJC

25. tellp tellg • Similar to seekp and seekg • Except they return the current position in the file. • tellp is part of ostream • tellg is part of istream Glenn Stevenson CSIS 113A MSJC

26. Bitwise operators • & - bitwise AND • | - bitwise OR • ^ - bitwise XOR • ~ - ones compliment • << - left shift • >> - right shift

27. Bitwise AND & • In order for result to be one both bits must be 1: 1 & 1 = 1 1 & 0 = 0 0 & 1 = 0 0 & 0 = 0 • An Example • int x = 7 , y = 3;int z = x & y; // z = 3 01110011---------0011

28. Bitwise OR | • In order for result to be one either bit must be 1: 1 | 1 = 1 1 | 0 = 1 0 | 1 = 1 0 | 0 = 0 • Can be used for adding. • Take previous example x | y // 7 | 3 01110011--------- // result is 7! 0111

29. Bitwise XOR ^ operator • Returns 1 if one of the values is 1 but not both: 1 ^ 1 = 0 1 ^ 0 = 1 0 ^ 1 = 1 0 ^ 0 = 0 01110011--------- // result is 4!0100

30. More XOR • Used perfectly to toggle a bit • Suppose you want a function that will toggle bit 3 on or off. • Create a mask : • 0001000 • XOR your bits with the mask: int mask = 8;int bitPattern = 170; //10101010 bitPattern^=mask;

31. XOR Toggle result • First time toggle results 1010101000001000--------------10100010 bitPattern now holds 162 • XOR again 1010001000001000--------------10101010 bitPattern now holds 170

32. ~ Ones Compliment • Simply flips all bits • Ones to zeros, zeros to ones int x = 3; // 011x = ~x;; // 11111111100 • Combine it with the & operator to ensure a bit is off! int b = 50; int c = b & ~0x10; 00110010 - b & 11101111 - ~0x10 ---------- 00100010 - result

33. Left Shift << operator • Shifts bits to the left • Essentially multiplying the number by 2 int x = 4; x = x << 1; 100 // binary x = 4 After shift x = 1000; // binary x or 8

34. Right Shift >> Operator • Works same as left except moves bits to the right. Essentially divides number by 2 int x = 4; x = x >> 1; // x now becomes 2