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- comparison operators
- (Only some of the operators below are relevant to COMP9414 at University of New South Wales - see green colouring below.) Prolog has two main classes of comparison operators - arithmeticcomparison operators (and similar alphabetic comparison operators) and unification-style operators:
Comparison Definition Evaluates? X = Ysucceeds if X and Y unify (match) in the Prolog sense No X = Ysucceeds if X and Y do not unify; i.e. if not (X = Y)No T1 == T2succeeds if terms T1 and T2 are identical; e.g. names of variables have to be the same No T1 == T2succeeds if terms T1 and T2 are not identical No E1 =:= E2succeeds if values of expressions E1 and E2 are equal Yes E1 == E2succeeds if values of expressions E1 and E2 are not equal Yes E1 < E2succeeds if numeric value of expression E1 is < numeric value of E2 Yes E1 =< E2succeeds if numeric value of expression E1 is ≤ numeric value of E2 Yes E1 > E2succeeds if numeric value of expression E1 is > numeric value of E2 Yes E1 >= E2succeeds if numeric value of expression E1 is ≤ numeric value of E2 Yes T1 @< T2succeeds if T1 is alphabetically < T2 No T1 @=< T2succeeds if T1 is alphabetically ≤ T2 No T1 @> T2succeeds if T1 is alphabetically > T2 No T1 @>= T2succeeds if T1 is alphabetically ≥ T2 No See also
is.isis not a comparison operator, but is frequently confused with = by novice Prolog programmers. Briefly, you useX is Expto evaluate an arithmetic expression, likeY + 2, that contains an arithmetic operator, like+, and bind the resulting value to the variableXto the left of the the operatoris.As an example of
@<and its relatives,?- likes(mary, pizza) @< likes(mary, plums). true.
This succeeds becauselikesandmaryare the same in both terms, andpizzaalphabetically precedesplums.Comparison of fractional numbers: When comparing two fractional numbers, problems can arise from the approximate nature of representations of fractional numbers in computers. Sometimes it will work as expected, and sometimes not. For example, the query
1.21 =:= 1.1 * 1.1.fails with SWI Prolog on the computer where this article was written. You can and should work around this problem by, instead of testing fractional numbers for equality, doing something like the following:?- abs(1.21 - 1.1 * 1.1) < 0.000001. true.
abssignifies "absolute value":abs(X) = XifX >= 0andabs(X) = -XifX =< 0.
More generally, test forabs(X - Y) < Tiny, whereTinyis bound to small number (or is a small number, as in the example). How small to makeTinyabove depends on the particular case - you might need to use a smaller number ifXandYneed to be very close together to make your algorithm work correctly. consult- These pre-defined pseudo-predicates allow one to load Prolog code into a running Prolog interpreter. Example:
?- consult('myprogram.pl').This loads the contents of the Prolog program in the filemyprogram.plinto the running Prolog's database. Note that in SWI Prolog, at least, before loading the new facts and rules into the database, Prolog first removes all facts and rules that relate to procedures inmyprogram.pl. So ifmyprogram.plcontains, for example, the factlikes(jane, pizza)then all facts and rules aboutlikesthat have two arguments will be removed from the Prolog database before the new facts inmyprogram.plare loaded up. This is convenient when you are usingconsultto re-load a program after editing it (e.g. in another window, with the Prolog interpreter left running), but could be a little surprising if you were trying to load extra facts from a file.It is possible to consult more than one file at a time, by replacing the single file name with a list of files:
?- consult(['file1.pl', 'file2.pl', 'file3.pl']). % file1.pl compiled 0.00 sec, 524 bytes % file2.pl compiled 0.01 sec, 528 bytes % file3.pl compiled 0.00 sec, 524 bytes true.
It is also possible to abbreviate a consult call, simply typing the list of (one or more) files as a goal for Prolog:?- ['file1.pl', 'file2.pl', 'file3.pl'].
In some Prolog implementations, consultingusercauses the Prolog interpreter to read facts and rules from the user's terminal:?- [user]. |: likes(jane, pizza). |: bad_dog(Dog) :- |: bites(Dog, Person), |: is_human(Person), |: is_dog(Dog). |: <control-D> % user://1 compiled 0.00 sec, 1,156 bytes true. ?-
- current input stream
- At any given time, input to Prolog comes from the "current input stream". When Prolog starts up, the current input stream is the keyboard of the computer/workstation from which Prolog was started.
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