1. FATIH UNIVERSITY Department of Computer Engineering Input and Output Notes for Ch.6 of Bratko For CENG 421 Fall03

2. FATIH UNIVERSITY Department of Computer Engineering Communication with Files In principle, –program can read data from several input files (input streams). –Program can write data to several output files (output streams). –Default i/o stream called user –Any other name can be given to the other streams But at any time only one input and output stream is active.( namely current input and output stream)

3. FATIH UNIVERSITY Department of Computer Engineering Opening and Closing Files To change the current input stream: –see( Filename). –see( user). –seen. To change the current output stream: –tell( Filename). –tell( user). –told.

4. FATIH UNIVERSITY Department of Computer Engineering Opening and Closing Files –see( Filename). Succeeds if there is nothing wrong with the file. Causes a side effect: The current input stream is changed to Filename. –Typical operations for reading from a file: see( Filename). read_from_file(Info). see(user).

5. FATIH UNIVERSITY Department of Computer Engineering Opening and Closing Files –tell( Filename). Succeeds if there is nothing wrong with the file. Causes a side effect: The current output stream is changed to Filename. –Typical operations for writing to a file: tell( Filename). write_to_file(Info). tell(user).

6. FATIH UNIVERSITY Department of Computer Engineering Opening and Closing Files seen –Closes the current input file told –Closes the current output file Sequential access is the most frequently used one Some implementations use Random access.

7. FATIH UNIVERSITY Department of Computer Engineering Sequential Access Sequential files behaves like terminals Request for reading  –read from the current position in the current input stream –Current position will be moved to the next unread item. –Request for reading at the end of the file returns the atom end_of_file

8. FATIH UNIVERSITY Department of Computer Engineering Sequential Access Request for writing  –Append to the end of the current output stream –Not possible to go backward and to overwrite part of a file.

9. FATIH UNIVERSITY Department of Computer Engineering Files in Prolog Two ways in which the files can be viewed –Consider characters as the basic elements Read/write one character at a time Use get, get0, put built-in predicates. –Consider bigger units (or terms in Prolog) of information as the building blocks of the file. Read/write one term at a time Use read, write built-in predicates.

10. FATIH UNIVERSITY Department of Computer Engineering Reading and Writing Terms read( X). –reads a term from the current input stream –deterministic: no backtracking on failure –X = end_of_file when at eof –Each term in the input file must be followed by a full stop and a space or a carriage return. write( X). –displays term X –Knows how to display all terms Additional built-in predicates for formatting: –tab(N):put N spaces –nl: new line –Examples of the book on pages 147-154.

11. FATIH UNIVERSITY Department of Computer Engineering Manipulating Characters put( C): A Character is written. –C is an ASCII code get0( C): A single char is read. –C is instantiated to an ASCII code get( C): A single char is read. –Like get0( C), but skips blanks(non-printable chars). Example of the book on page 155

12. FATIH UNIVERSITY Department of Computer Engineering Constructing and Decomposing Atoms name/2 –Name( Atom, L) if L is the list of ASCII codes of the characters that make up Atom –name is bi-directional –name(zx232,[122,120,50,51,50] –Example: Prefix test: Atoms=order1, order2, driver1,driver2, taxia1, taxilux taxi(X) tests whether X represents a taxi taxi(X) :-name(X,XL), name(taxi,TL), conc(TL,_,XL).

13. FATIH UNIVERSITY Department of Computer Engineering Example % Figure 6.2 A procedure to transform a sentence into a list of atoms. /*Procedure getsentence reads in a sentence and combines the words into a list of atoms. For example getsentence( Wordlist) produces Wordlist = [ 'Mary', was, pleased, to, see, the, robot, fail] if the input sentence is: Mary was pleased to see the robot fail. */

14. FATIH UNIVERSITY Department of Computer Engineering getsentence( Wordlist) :- get0( Char), getrest( Char, Wordlist). getrest( 46, [] ) :- !. % End of sentence: 46 = ASCII for '.' getrest( 32, Wordlist) :- !, % 32 = ASCII for blank getsentence( Wordlist). % Skip the blank getrest( Letter, [Word | Wordlist] ) :- getletters( Letter, Letters, Nextchar), % Read letters of current word name( Word, Letters), getrest( Nextchar, Wordlist). getletters( 46, [], 46) :- !. % End of word: 46 = full stop getletters( 32, [], 32) :- !. % End of word: 32 = blank getletters( Let, [Let | Letters], Nextchar) :- get0( Char), getletters( Char, Letters, Nextchar).