Prolog executable writing String in codes

Question

I am trying to create an executable of following Prolog code:

item(one, 50, 40).
item(two, 80, 70).
item(three, 100, 55).
item(four, 50, 45).

maxMeanStudy:-  
    findall(M,item(_,M,_),L),
    max_member(Max,L),
    write("Maximum mean value:"),   % this line is not printed properly.
    writeln(Max),!.

main:-
    maxMeanStudy.

I am using following command to create executable as mentioned on this page: http://www.swi-prolog.org/pldoc/man?section=cmdlinecomp

$ swipl --goal=main --stand_alone=true -o myprog -c myques.pl

The executable created does not write the string "Maximum mean value:" in letters but writes it in codes:

$ ./myprog
[77,97,120,105,109,117,109,32,109,101,97,110,32,118,97,108,117,101,58]100

I am working on Linux (32bit). How can I solve this problem. Thanks for your help.

It looks like the double_quotes flag is reverted to codes. So your quick fix might be to either (a) include the directive, :- set_prolog_flag(double_quotes, string). at the beginning of your file, or (b) use single quotes for your strings so they are just atoms (then you don't have to worry about double quote interpretation). Of course, the best approach is (c) heed the answer @mat gave you. — lurker

mat mat · Accepted Answer · 2016-07-12T14:50:57

In almost all such cases, it turns out that:

You should not use side-effects in the first place. Instead, define relations that you can actually reason about. In your case, you are describing a relation between mean values and their maximum. Therefore, the name maximum_mean_value(Ms, M) suggests itself. And is_that_not_more_readable than mixingLowerAndUpperCaseLetters?

Let the toplevel do the printing for you, via pure queries like:

?- maximum_mean_value(Ms, M).
M = ... . % result is automatically shown by the toplevel!

If you really need to write something on the terminal, do it in a separate predicate. Avoid mixing pure and impure code.
Use format/2 for formatting output. For example:
```
maximum_mean_value(Ms, M),
format("Maximum mean value: ~d\n", [M]) 
```
Note how format/2 makes it easy to output text that involves other terms.

Prolog executable writing String in codes

1 Answers