Prolog - Print the Value of a Variable
I Just Cannot Figure out How to Print the Value of X. Here Is What I Tried in the Toplevel: 59? - Read(X). |: 2. X = 2. 60? - Write(X). _G253 True. What Is...
I just cannot figure out how to print the value of X. Here is what I tried in the toplevel:
59 ?- read(X).
|: 2.
X = 2.
60 ?- write(X).
_G253
true.
What is _G253? I dont want the index number, I want the value X is bound to.
What should I do in order to print the value of X?
2 Answers
When you type write(X). at the interactive prompt, and nothing else, X is not bound to anything in particular. If you want to read X from the user and then write it, try typing read(X), write(X). at the prompt.
?- read(X), write(X).
|: 28.
28
X = 28.
SWI Prolog does keep a history of top-level bindings; type help. to go into the manual, then search for bindings or just navigate to section 2.8 of the manual, 'Reuse of top-level bindings'. There, you can learn that the most recent value of any variable bound in a successful top-level goal is retained, and can be referred to using the name of the variable, prefixed with a dollar sign. So interactions like the following are possible:
?- read(X).
|: 42.
X = 42.
?- write($X).
42
true.
But a top-level goal that just happens to use the variable name X will be interpreted as using a fresh variable; to do otherwise would violate the normal semantics of Prolog.
prolog - take as input and print the value of a variable.
go:- write('Enter a name'),nl,
read(Name),nl,
print(Name).
print(Name):- write(Name),write(', Hello !!!').
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