PROLOG I/O I/O and the relational model. PROLOG I/O operators. –Not really predicates. I/O with characters. I/O with terms. I/O with lists. I/O with files. Turning lists of characters into atoms.

9.2 I//O And The Relational Model I/O does not fit into the relational model of computation. –I/O cannot be backtracked. –Theoretically could backtrack over input.  Very inefficient though. –Cannot backtrack output.  What’s said is said. PROLOG provides I/O operators as an “add on”. PROLOG I/O operators are not really predicates.

9.3 PROLOG I/O Operators PROLOG handles I/O via constructs which are essentially C++/Java style statements : get0(Ch) –Next input character = Ch. get(Ch) –Next valid input character = Ch. –A valid input character is any printable character except spaces, tabs and end of line characters. put(Ch) –Next output character = Ch. PROLOG treats characters as ordinal numbers (i.e. as ASCII/Unicode numbers). –The arguments of get0, get and put must be numbers, not characters.

9.4 PROLOG I/O Operators II nl –Next output character = end of line. read(X) –Next input term = X. write(X) –Next output term = X. Note that = means match. get0, get, put, nl, read and write cannot be backtracked in the usual way. –GNU PROLOG will backtrack over them but it cannot undo the I/O. –Some PROLOGs will not backtrack over them at all.

9.5 I/O With Characters Reading a single character : | ?- get0(Ch). a Ch = 97 yes | ?- Matching a single character : | ?- get0(97). a yes | ?- A common mistake ( ‘9’ has ASCII code 57 ) : | ?- get0(97). 97 no | ?-

9.6 I/O With Characters II Using ; (logical or) : | ?- get0(Ch) ; true. | ?- q yes | ?- get0(97) ; get0(97). | ?- qa yes | ?- First query works because of the true. Second query works because the second input character is ‘a’. get is identical to get0 except that it ignores all non-printable characters, spaces, tabs and end of line characters.

9.7 I/O With Characters III put takes a numeric argument and prints the corresponding character. | ?- put(97). a yes | ?- put(X). uncaught exception... | ?- X is , put(X). d | ?- put(97), nl, put(100). a d yes | ?- put requires its argument to be instantiated. –Same as the other arithmetic operators.

9.8 I/O With Terms PROLOG will input and output whole terms : | ?- read(X), read(Y). | ?- p(1,2). q(jim,joe). X = p(1,2) Y = q(jim,joe) yes | ?- write(q(a,b,c)), write(p(1,2,3)). q(a,b,c)p(1,2,3) yes | ?- X = 1, Y = 2, write(p(X,Y)). p(1,2) X = 1 Y = 2 yes | ?-

9.9 I/O With Lists Read characters up to a ‘.’ and compute a checksum : checkSum(Chs, N) :- get0(Ch), (Ch = 46, %. character Chs = [], N = 0 ; checkSum(NewChs, NewN), Chs = [Ch | NewChs], N is Ch + NewN ). | ?- checkSum(L, R). hello world. L = [104, ] R = 1116 | ?-

9.10 I/O With Lists II Using get : checkSum2(Chs, N) :- get(Ch), (Ch = 46, %. character Chs = [], N = 0 ; checkSum2(NewChs, NewN), Chs = [Ch | NewChs], N is Ch + NewN ). | ?- checkSum2(L, R). hello world. L = [104, ] R = 1084 | ?-

9.11 I/O With Lists III Printing a list of terms : printList([]). printList([T | Ts]) :- write(T), nl, printList(Ts). | ?- printList([hello, world]). hello world yes | ?-

9.12 I/O With Files PROLOG reads input from the standard input stream. –Normally connected to the keyboard. PROLOG writes output to the standard output stream. –Normally connected to the screen. The standard I/O streams can be redirected to files. see(fileName) : Connect input stream to file fileName. seen : Disconnect the input stream. tell(fileName) : Connect the output stream to file fileName. told : Disconnect the output stream. Similar mechanism to open and close in C++. –Voodoo in Java.

9.13 I/O With Files II see, seen, tell and told cannot be (properly) backtracked over. –GNU PROLOG backtracks over them but does not undo their effect on the file system. –Some PROLOGs will refuse to backtrack over them at all. Logical variables can be used as arguments to see and tell. –Arguments must be valid terms. The filename user means the keyboard or screen. – see(user) : connect the input stream to the keyboard. – tell(user) : connect the output stream to the screen.

9.14 Copying A File copyFile :- read(InFileName), read(OutFileName), see(InFileName), tell(OutFileName), echo, seen, told, see(user), tell(user). echo :- get0(Ch), (Ch = -1, ; put(Ch), echo ).

9.15 Copying A File II Most PROLOGs connect back to the standard streams after seen and told but it is safest not to rely on it. Not all PROLOGs return ASCII -1 when reading from an empty file. | ?- copyFile. temp. temp1. true ? (10 ms) yes | ?- The query succeeds and as a side effect copies the contents of the file temp to the file temp1.

9.16 Echoing The Keyboard To The Screen Could use echo on the standard I/O streams : | ?- echo. sljdkljhsdfljhsdf rwpwirnpfwpfhpwf true ? ; uncaught exception... | ?- Must use to send the end of file character. If we force a backtrack GNU PROLOG gives an error message. –Don’t try to backtrack I/O operations.

9.17 Turning Lists Of Characters Into Atoms name(A, Str) –Succeeds if the atom A matches with the contents of the string Str. To PROLOG a string is a list of ASCII numbers. –Goal succeeds if Str is the list of ASCII numbers of the characters which make up A. | ?- name(fred, Str). Str = [102,114,101,100] | ?- name(X,[102,114,101,100]) X = fred. | ?- name is most useful for converting text read from input into atoms. –Particularly useful with assert.

9.18 Turning Lists Of Characters Into Atoms II readAtom reads a string of characters from input and converts it into an atom. –The string is terminated by the ‘.’ character (ASCII 46 ). readAtom(A) :- readStr(Str), name(A,Str). readStr(Str) :- get0(Ch), (Ch = 46, Str = [] ; readStr(NewStr), Str = [Ch | NewStr] ).

9.19 Turning Lists Of Characters Into Atoms III | ?- readAtom(A). fred. A = fred ? yes | ?- readAtom(A), assertz(word(A)). fred. A = fred ? yes | ?- word(X). X = fred yes | ?- readAtom(A). a123->>>--<<<< aqa. A = ‘a123->>>--<<<< aqa’ yes | ?- read(A). a123->>>--<<<< aqa. uncaught exception.... | ?- readAtom is more powerful than PROLOG’s read.

9.20 Summary I/O is an “add on” to PROLOG. –Not relational. –PROLOG cannot undo I/O operations on backtracking. get0(Ch) : Next input character = Ch. get(Ch) : Next valid input character = Ch. put(Ch) : Next output character = Ch. PROLOG treats characters as ASCII/Unicode numbers. nl : Next output character = end of line. read(X) : Next input term = X. write(X) : Next output term = X.

9.21 Summary II PROLOG reads input from the standard input stream. PROLOG writes output to the standard output stream. see(fileName) : Connect input stream to file filename. seen : Disconnect input stream from file filename. tell(fileName) : Connect output stream to file filename. told : Disconnect output stream from file filename. Filename user means keyboard / screen. name(A, Str) : Succeeds if A matches with the string Str. –Can use name to convert strings into atoms. –Particularly useful with asserta and assertz.