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Input/Output and Debugging How to use IO Streams How to debug programs Help on coursework
IO Streams (concept same as unix) • In default, both input stream is computer keyboard and output stream is screen. • When we want to input from or output to files, we need to open IO streams, and close them at the end. These invoke the UNIX function fopen and fclose.
Built-ins for open/close IO Streams open(FileName, Mode, Stream) Close(Stream) FileName is an atom. Mode is one of: • read - open the file for input. • write - open the file for output. The file is created if it does not already exist, the file will otherwise be truncated. • append - open the file for output. The file is created if it does not already exist, the file will otherwise be appended to.
Two Levels of IO in Prolog There are two levels of IO built-ins in Prolog: 1. High level - dealing with Prolog terms; reading/writing a term at one time • write(X) • read(X) - X must be a term. 2. Low level - dealing with characters; reading/writing an ASCII code at one time get_code(X), put_code(X) – similar to getc/putc in C
- Standard IO - write(Term) read(Term) get_code(X) nl tab(N) - Files IO - write(Stream, Term) read(Stream, Term) get_code(Stream,X) nl(Stream) tab(Stream, N) Difference between standard IO and files IO Just add Stream Name!
An example – open a file and write something :- use_module(library(system)). % for datime mk_report:- open(‘report.txt’, write, S), datime(Year, Month, D, H, M, _), write(S, ‘Report’), nl(S), write(S, Year-Month-D), write(S, ‘ ’), write(S, H:M), nl(S), write(S, ‘I have talked to ’), userinfo(name, X), write(S, X), write(S,‘. He likes ’), userinfo(hobby, L), writelist(S, L), nl(S), close(S).
Debugging (1) How to fix syntax errors • Read error messages carefully • Common problems: missing brackets, semi-colons, full stops. • Watch out ‘here’ which tells you where went wrong
program([H|T):- do_something(H), program(T) --------------------------- ! Syntax error ! ] or operator expected ! in line 2 ! program ( [ H | T ! <<here>> ! ) :- do_something ( H ) , program ( T ) . ! Syntax error ! operator expected after expression ! in line 16 ! program ( [ H | T ] ) :- do_something ( H ) , program ( T ) ! <<here>> Examples of Error Messages
Debugging (2) Run time errors: • Existence error in user:progr/1This means progr/1 is not defined • In order to track down what is wrong, use ?- trace. To turn the debug mode on (notrace to switch it off)
The Procedure Box Control Flow Model *--------------------------* Call | | Exit ----> | d(X,Y) :- o(X,Y). | ----> | | | d(X,Z) :- | <---- | o(X,Y), d(Y,Z). | <---- Fail | | Redo *--------------------------*
The code is program([H|T]):- H=1, program(T). --------------------- | ?- program([_,_,_]). no | ?- trace. % The debugger will first creep – showing everything (trace) yes ?- program([_,_,_]). 1 1 Call: program([_435,_451,_467]) ? 2 2 Call: _435=1 ? 2 2 Exit: 1=1 ? 3 2 Call: program([_451,_467]) ? 4 3 Call: _451=1 ? 4 3 Exit: 1=1 ? 5 3 Call: program([_467]) ? 6 4 Call: _467=1 ? 6 4 Exit: 1=1 ? 7 4 Call: program() ? 7 4 Fail: program() ? When the trace mode is on, we can see everything step by step
Options when trace the program ?- program([_,_,_]). • 1 Call: program([_435,_451,_467]) ? You can type the following options: <return key> - creep (one step at the time) s - skip g - view ancestors a - abort n - switch off debug h - print out all options
If you don’t want to trace every step • leash(+Mode) • Leashing Mode determines the ports of invocation boxes at which you are to be prompted when you creep through your program • Mode is a list which can the following options: [call,exit,redo,fail,exception]).
try:- do1(X), do2(Y), do3(1). do1(a). do2(b). do3(c). | ?- try. no |?- leash([fail]). % Using leashing stopping at [fail] ports yes | ?- trace. % The debugger will first creep -- showing everything (trace) | ?- try. 1 1 Call: try 2 2 Call: do1(_550) 2 2 Exit: do1(a) 3 2 Call: do2(_545) 3 2 Exit: do2(b) 4 2 Call: do3(1) 4 2 Fail: do3(1) ? (debugger stops at the first failed point) Example of using “leash”
More help on course work (1) From ‘cango’ program’s output, you need to write out a detailed direction. It is very similar to write_list(L), if you just say to east, south, etc. % give_direction(DirList, DistanceList) give_direction(,):- nl. give_direction([H1|T1],[H2|T2]):- write(‘go ’), write(H1), write(‘ ’), write(H2), write(‘ meters, ’), give_direction(T1,T2). For a more human-like solution, you can say ‘turn left/right’, and indicate which side the room is. You need to use same trick used in ‘cango’: add pre-direction as an extra parameter
More help on course work (2) last term we gave you this program which handles conversation in a simple read/reply fashion, there is no continuation between dialogues. But, we do need some continued conversation. For example, chatbot may want to ask a few questions, it wants continually to ask until it get all answers. % last week’s code % fail-loop chatbot:- print_welcome, conversations. conversations:- repeat, print_prompt(you), readin(S), gen_reply(S,R), print_prompt(me), write_list(R), is_quit(S). % an example of forward-loop ask_location(X):- print_prompt(me), write(‘Please tell me where you are’), nl, print_prompt(you), readin(S), cont_ask_location(X, S). cont_ask_location(X, [H|_]):- is_in_db(H), !, X=H. % done cont_ask_location(X, _):- ask_location(X).