File Handling
Lab #10
LAB #10
FILE HANDLING
OBJECTIVE
Write programs to implement file handling.
THEORY

Prolog uses a current_read_device (normally the keyboard) for reading input, and
a current_write_device (normally the screen) for sending output. You can specify
other devices, and can reassign the current input and output devices at run time.
(This reassigning is known as redirecting I/O.)

Prolog's basic file-access predicates are:



openread (open a file for reading)

openwrite (open a file for writing)

openappend (open a file for appending)

openmodify (open a file for modification)

openfile (general purpose file opener)

filemode (set a file to text mode or binary mode)

closefile (close a file)

readdevice (reassign the current_read_device or get its name)

writedevice (reassign the current_write_device or get its name)
To access files you use the FILE domain, which has five predefined alternatives:

keyboard (for reading from the keyboard)

screen (for writing to the screen)

stdin (for reading from standard input)

stdout (for writing to standard output)

stderr (for writing to standard error)
To work within and manipulate files, you use the following standard predicates:

filepos (controls the position where reading or writing takes place)

eof (checks whether the file position during a read operation is at the end of
the file)

flush (forces the contents of the internal buffer to be written to a file)

existfile (verifies that a file exists)

searchfile (locates a file among several directories)
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File Handling





deletefile (deletes a file)

renamefile (renames a file)

copyfile (copies a file)

fileattrib (gets or sets file attributes)

disk (changes the current disk and directory/subdirectory)
Lab #10
Binary blocks may be transferred using:

readblock (reads a binary block from a file)

writeblock (writes a binary block to a file)

file_bin transfers between whole files and binary blocks
To search directories, the following standard predicates are available:

diropen (opens a directory for searching)

dirmatch (finds matching files in a directory)

dirclose (closes a directory)

dirfiles (non-deterministically matches files in a directory)
To manipulate file names, the following are available:

filenamepath (joins or splits qualified file names)

filenameext (joins or splits file names and extensions)
The standard predicate readterm allows your program to access facts in a file at
run time. readterm can read any object written by write, plus facts describing
database predicates.
EXAMPLE PROGRAMS
Program 1
DOMAINS
file = input; output
PREDICATES
convert_file
nondeterm repfile(FILE)
run
CLAUSES
convert_file :-
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File Handling
Lab #10
repfile(input),
readln(Ln),
upper_lower(LnInUpper,Ln),
/* converts the string
to uppercase */
write(LnInUpper),nl,
fail.
convert_file.
repfile(_).
repfile(F):not(eof(F)),
repfile(F).
run:write("Which file do you want convert ?"),
readln(InputFileName),nl,
write("What is the name of the output file ?"),
readln(OutputFileName),nl,
openread(input, InputFileName),
readdevice(input),
openwrite(output, OutputFileName),
writedevice(output),
convert_file,
closefile(input),
closefile(output).
GOAL
run.
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File Handling
Lab #10
Program 2
DOMAINS
file = input
PREDICATES
inspect_positions(file)
CLAUSES
inspect_positions(UserInput):readdevice(UserInput),
nl,write("Position No? "),
readln(X),
term_str(ulong,Posn,X),
readdevice(input),
filepos(input,Posn,0),
readchar(Y),nl,
write("Char is: ",Y),
inspect_positions(UserInput).
GOAL
write("Which file do you want to work with ?"),nl,
readln(FileName),
openread(input, FileName),
readdevice(UserInput),
inspect_positions(UserInput).
EXERCISES
1. Type the above examples in Visual Prolog environment and execute them
individually.
2. Explain the working of the above examples.
3. Write a Prolog program, which utilizes most of the file-related predicates, listed
above.
4. Modify example 2 for handling individual lines of a text file.
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