Is it possible in prolog to replace all white spaces of a string with some given character?
Example-
If I have a variable How are you today? and I want How_are_you_today?
For atoms
There are may ways in which this can be done. I find the following particularly simple, using atomic_list_concat/3:
?- atomic_list_concat(Words, ' ', 'How are you today?'), atomic_list_concat(Words, '_', Result).
Words = ['How', are, you, 'today?'],
Result = 'How_are_you_today?'.
For SWI strings
The above can also be done with SWI strings. Unfortunately, there is no string_list_concat/3 which would have made the conversion trivial. split_string/4 is very versatile, but it only does half of the job:
?- split_string("How are you today?", " ", "", Words).
Words = ["How", "are", "you", "today?"].
We can either define string_list_concat/3 ourselves (a first attempt at defining this is shown below) or we need a slightly different approach, e.g. repeated string_concat/3.
string_list_concat(Strings, Separator, String):-
var(String), !,
maplist(atom_string, [Separator0|Atoms], [Separator|Strings]),
atomic_list_concat(Atoms, Separator0, Atom),
atom_string(Atom, String).
string_list_concat(Strings, Separator, String):-
maplist(atom_string, [Separator0,Atom], [Separator,String]),
atomic_list_concat(Atoms, Separator0, Atom),
maplist(atom_string, Atoms, Strings).
And then:
?- string_list_concat(Words, " ", "How are you today?"), string_list_concat(Words, "_", Result).
Words = ["How", "are", "you", "today?"],
Result = "How_are_you_today?".
It all depends on what you mean by a string. SWI has several for them, some are generally available in any common Prolog and conforming to the ISO standard ; and some are specific to SWI and not conforming. So, let's start with those that are generally available:
###Strings as list of character codes — integers representing code points
This representation is often the default, prior to SWI 7 it was the default in SWI, too. The biggest downside is that a list of arbitrary integers can now be confused with text.
:- set_prolog_flag(double_quotes, codes).
codes_replaced(Xs, Ys) :-
maplist(space_repl, Xs, Ys).
space_repl(0' ,0'_).
space_repl(C, C) :- dif(C,0' ).
?- codes_replaced("Spaces !", R).
R = [83,112,97,99,101,115,95,95,33]
; false.
###Strings as list of characters — atoms of length 1
This representation is a bit cleaner since it does not confuse integers with characters, see this reply how to get more compact answers.
:- set_prolog_flag(double_quotes, chars).
chars_replaced(Xs, Ys) :-
maplist(space_replc, Xs, Ys).
space_replc(' ','_').
space_replc(C, C) :- dif(C,' ').
?- chars_replaced("Spaces !", R).
R = ['S',p,a,c,e,s,'_','_',!]
; false.
###Strings as atoms
#WouterBeek already showed you how this can be done with an SWI-specific built-in. I will reuse above:
atom_replaced(A, R) :-
atom_chars(A, Chs),
chars_replaced(Chs, Rs),
atom_chars(R, Rs).
?- atom_replaced('Spaces !',R).
R = 'Spaces__!'
; false.
So far everything applies to iso-prolog
###Strings as an SWI-specific, non-conforming data type
This version does not work in any other system. I mention it for completeness.
SWI-Prolog DCGs allows an easy definition, using 'push back' or lookahead argument:
?- phrase(rep_string(` `, `_`), `How are you`, R),atom_codes(A,R).
R = [72, 111, 119, 95, 97, 114, 101, 95, 121|...],
A = 'How_are_you'
The definition is
rep_string(Sought, Replace), Replace --> Sought, rep_string(Sought, Replace).
rep_string(Sought, Replace), [C] --> [C], rep_string(Sought, Replace).
rep_string(_, _) --> [].
edit To avoid multiple 'solutions', a possibility is
rep_string(Sought, Replace), Replace --> Sought, !, rep_string(Sought, Replace).
rep_string(Sought, Replace), [C] --> [C], !, rep_string(Sought, Replace).
rep_string(_, _) --> [].
Related
I can't get this Prolog DCG code working:
String1=" ",string_codes(String1,Codes),phrase(spaces(Output),Codes).
spaces(XXs) -->
[X], {X=32}, spaces(Xs),
{char_code(Ch,X), atom_concat(Ch,Xs,XXs)}, !. %%Space
spaces([]) --> [].
I feel like an improved solution would probably be something like this:
spaces(Spaces) --> " ", spaces(S0), { atom_concat(' ', S0, Spaces) }.
spaces('') --> [].
There's no real need to ask what the char is for code 32, you know it's a space. Also, [X], {X=32} from your answer is better as [32], which is still better as " ".
I solved this by changing [] in the base case to ''.
spaces(XXs) -->
[X], {X=32}, spaces(Xs),
{char_code(Ch,X), atom_concat(Ch,Xs,XXs)}, !. %% Space
spaces('') --> [].
String1 = " ",
Codes = [32, 32, 32],
Output = ' '.
If you are doing DCG and using SWI-Prolog there is a library of often used predicates and DCG clauses in dcgbasics. This can be used in code with
:- use_module(library(dcg/basics)).
To list the code for the predicates use listing/1, e.g.
?- listing(dcg_basics:_).
The library has a DCG clause blanks//0 that does what you want, e.g.
?- listing(dcg_basics:blanks).
blanks(A, B) :-
blank(A, C),
!,
D=C,
blanks(D, B).
blanks(A, A).
true.
?- listing(dcg_basics:blank).
blank([C|A], B) :-
nonvar(C),
code_type(C, space),
B=A.
true.
which as DCG is
blank -->
[C],
{
nonvar(C),
code_type(C,space)
}.
blanks -->
blank, !, blanks.
blanks --> [].
NB
The library version uses character codes and not characters.
?- string_codes("",Codes),phrase(blanks,Codes,Rest).
Codes = Rest, Rest = [].
?- string_codes(" ",Codes),phrase(blanks,Codes,Rest).
Codes = [32],
Rest = [].
?- string_codes(" ",Codes),phrase(blanks,Codes,Rest).
Codes = [32, 32],
Rest = [].
?- string_codes(" ",Codes),phrase(blanks,Codes,Rest).
Codes = [32, 32, 32],
Rest = [].
I have to solve a homework but I have a very limited knowledge of Prolog. The task is the following:
Write a Prolog program which can list all of those substrings of a string, whose length is at least two character and the first and last character is the same.
For example:
?- sameend("teletubbies", R).
R = "telet";
R = "ele";
R = "eletubbie";
R = "etubbie";
R = "bb";
false.
My approach of this problem is that I should iterate over the string with head/tail and find the index of the next letter which is the same as the current (it satisfies the minimum 2-length requirement) and cut the substring with sub_string predicate.
This depends a bit on what you exactly mean by a string. Traditionally in Prolog, a string is a list of characters. To ensure that you really get those, use the directive below. See this answer for more.
:- set_prolog_flag(double_quotes, chars).
sameend(Xs, Ys) :-
phrase( ( ..., [C], seq(Zs), [C], ... ), Xs),
phrase( ( [C], seq(Zs), [C] ), Ys).
... --> [] | [_], ... .
seq([]) -->
[].
seq([E|Es]) -->
[E],
seq(Es).
if your Prolog has append/2 and last/2 in library(lists), it's easy as
sameend(S,[F|T]) :-
append([_,[F|T],_],S),last(T,F).
One of my assignments ask us to build a prolog tokenizer. Right now I wrote a predicate that can change space and tab it new line. But I don't know how to implement that into the main program.
The replace part looks like this:
replace(_, _, [], []).
replace(O, R, [O|T], [R|T2]):- replace(O, R, T, T2).
replace(O, R, [H|T], [H|T2]) :- H \= O, replace(O, R, T, T2).
And the Main part has a predicate called removewhite(list1 list2)
So how can I let removewhite execute replace?
You are a bit 'off trail' toward a tokenizer: removewhite/2 isn't going to buy you any useful functionality. Instead, consider a DCG (of course if your Prolog offers this functionality):
tokenize(String, Tokens) :- phrase(tokenize(Tokens), String).
tokenize([]) --> [].
tokenize(Tokens) --> skip_spaces, tokenize(Tokens).
tokenize([Number|Tokens]) --> number(Number), tokenize(Tokens).
skip_spaces --> code_types(white, [_|_]).
number(N) --> code_types(digit, [C|Cs]), {number_codes(N,[C|Cs])}.
code_types(Type, [C|Cs]) --> [C], {code_type(C,Type)}, !, code_types(Type, Cs).
code_types(_, []) --> [].
despite the simplicity, this is a fairly efficient scanner, easily extensible.
In SWI-Prolog, that has (non ISO compliant) extensions for efficient handling of strings, this can be called from top level like:
?- tokenize(`123 4 567 `, L).
L = [123, 4, 567]
or
?- atom_codes('123 4 567 ',Cs), tokenize(Cs, L).
Cs = [49, 50, 51, 32, 32, 52, 32, 53, 54|...],
L = [123, 4, 567]
Btw, in SWI-Prolog, number//1 is predefined (with much more functionality, of course) in library(dcg/basics).
Anyway, about your question
how can I let removewhite execute replace?
I feel you're really 'barking the wrong tree': removing a space - that actually is a separator - will mess up your input...
You can write a more "powerfull" predicate
replace_all(_, _, [], []).
replace_all(L, R, [X|T], [R|T2]):-
member(X, L),
replace_all(L, R, T, T2).
replace_all(L, R, [X|T], [X|T2]) :-
\+ member(X, L),
replace_all(L, R, T, T2).
Then, you will have
removewhite(List1, List2) :-
remove_all([' ', '\t'], '\n', List1, List2).
Write a program that deletes vowels (String, NoVowelsString) that deletes all vowels from a given string.
So far I've got the condition vowel(X):- member(X,[a,e,i,o,u]). Then I thought of the one that deletes all the elements from the other list:
delete2([],L1,L1).
delete2([H|T],L1,L3) :-
delete2(H,L1,R2),
delete2(T,R2,L3).
So having these two I thought that I could put a condition to those elements being deleted that they have to be a member of [a,e,i,o,u]. Though I still haven't got anywhere.
The following is based on the reification of term equality/inequality.
First, we first define list_memberd_t/3, which behaves just like the memberd_truth/3 but has a different argument order:
list_memberd_t([] ,_,false).
list_memberd_t([Y|Ys],X,Truth) :-
if_(X=Y, Truth=true, list_memberd_t(Ys,X,Truth)).
list_memberd_truth(Xs,X,Truth) :- list_memberd_t(Xs,X,Truth).
For the sake of brevity, let's define memberd_t/3 based on list_memberd_t/3:
memberd_t(X,Xs,Truth) :- list_memberd_t(Xs,X,Truth).
As a parallel to library(apply), let's define tinclude/3:
:- meta_predicate tinclude(2,?,?).
tinclude(P_2,Xs,Zs) :-
list_tinclude_list(Xs,P_2,Zs).
list_tinclude_list([], _P_2,[]).
list_tinclude_list([E|Es],P_2,Fs0) :-
if_(call(P_2,E), Fs0 = [E|Fs], Fs0 = Fs),
list_tinclude_list(Es,P_2,Fs).
tfilter/3 is another name for tinclude/3:
tfilter(P_2,As,Bs) :-
tinclude(P_2,As,Bs).
Next, we define the meta-predicate texclude/3, the opposite of tinclude/3:
:- meta_predicate texclude(2,?,?).
texclude(P_2,Xs,Zs) :-
list_texclude_list(Xs,P_2,Zs).
list_texclude_list([],_,[]).
list_texclude_list([E|Es],P_2,Fs0) :-
if_(call(P_2,E), Fs0 = Fs, Fs0 = [E|Fs]),
list_texclude_list(Es,P_2,Fs).
Now let's use them together!
?- texclude(list_memberd_truth([a,e,i,o,u]),
[d,e,l,e,t,e,' ',v,o,w,e,l,s,' ',i,n,' ',a,' ',l,i,s,t], Filtered).
Filtered = [d, l, t, ' ',v, w, l,s,' ', n,' ', ' ',l, s,t].
Edit
As an alternative to using above texclude/3, let's use tinclude/3 with an auxiliary predicate not/3 to flip the truth value:
:- meta_predicate not(2,?,?).
not(P_2,X,Truth) :-
call(P_2,X,Truth0),
truth_flipped(Truth0,Truth).
truth_flipped(true,false).
truth_flipped(false,true).
Sample query:
?- tinclude(not(list_memberd_truth([a,e,i,o,u])),
[d,e,l,e,t,e,' ',v,o,w,e,l,s,' ',i,n,' ',a,' ',l,i,s,t], Filtered).
Filtered = [d, l, t, ' ',v, w, l,s,' ', n,' ', ' ',l, s,t].
here a solution using DCG. Note how the 'output' is obtained (no arguments passing, only difference lists)
novowels --> ("a";"e";"i";"o";"u"), !, novowels.
% or ..
% novowels --> [C], {memberchk(C, "aeiou")}, !, novowels.
novowels, [C] --> [C], !, novowels.
novowels --> [].
I must confess the second cut doesn't like me, but seems required.
test:
?- phrase(novowels, "abcdefghilmnopq", L),format('~s',[L]).
bcdfghlmnpq
L = [98, 99, 100, 102, 103, 104, 108, 109, 110|...].
edit About the second cut, it seems required by 'left hand' notation: if I code with argument, without cut, I get a correct parsing:
novowels(Cs) --> ("a";"e";"i";"o";"u"), !, novowels(Cs).
% novowels(Cs) --> [C], {memberchk(C, "aeiou")}, !, novowels(Cs).
novowels([C|Cs]) --> [C], novowels(Cs).
novowels([]) --> [].
test:
?- phrase(novowels(L), "abcdefghilmnopq"),format('~s',[L]).
bcdfghlmnpq
L = [98, 99, 100, 102, 103, 104, 108, 109, 110|...] ;
false.
I wonder if this is a bug of the DCG translator, or (more probably) my fault...
Here is the code
deleteV([H|T],R):-member(H,[a,e,i,o,u]),deleteV(T,R),!.
deleteV([H|T],[H|R]):-deleteV(T,R),!.
deleteV([],[]).
What it does?
First it question itself?It's the head a vowel
Yes->We ignore it.
No->We need it.
If it finds an empty list, it constructs the result list, and when returning from backtracking it appends the consonats in front.
This code was tested in SWIProlog.
I came up w/ the following code to replace all occurences of Find w/ Replace in Request & put the answer in Result. This is using a DCG, so they are all lists of character codes. The predicate that client code would use is substitute.
findReplace(_, _, [], []) -->
[]. % The end.
findReplace(Find, Replace, Result, ResultRest) -->
Find, % Found Find.
{ append(Replace, Intermediate, Result) }, % Put in Replace in Find's place.
!, % Make sure we don't backtrack & interpret Find as the next case.
findReplace(Find, Replace, Intermediate, ResultRest).
findReplace(Find, Replace, [ C | Intermediate ], ResultRest) -->
[ C ], % Any other character.
findReplace(Find, Replace, Intermediate, ResultRest).
substitute(Find, Replace, Request, Result):-
phrase(findReplace(Find, Replace, Result, []), Request).
This works in SWI-Prolog. Does anyone have any comments on how I could improve it? I'm learning how to use DCG's & difference lists. E.g., I put in the cut so that, after finding Find, prolog doesn't ever backtrack & interpret that as an ordinary character in the [ C ] case. Is this needed, or is there a more declarative way of doing so?
Another question - is there a predicate already available to do the same thing that substitute does, maybe on atoms?
Thanks in advance.
Consider using semicontext notation to replace subsequences in DCGs:
eos([], []).
replace(_, _) --> call(eos), !.
replace(Find, Replace), Replace -->
Find,
!,
replace(Find, Replace).
replace(Find, Replace), [C] -->
[C],
replace(Find, Replace).
substitute(Find, Replace, Request, Result):-
phrase(replace(Find, Replace), Request, Result).
Example:
?- substitute("a", "b", "atesta", R), atom_codes(A, R).
R = [98, 116, 101, 115, 116, 98],
A = btestb.
Also, underscores_are_much_more_readable thanMixedCaseNamesAsYouSee.
About the second question, i.e. working with atoms, I wrote this utility perusing atomic_list_concat
%% replace_word(+Old, +New, +Orig, -Replaced)
%% is det.
%
% string replacement
% doesn't fail if not found
%
replace_word(Old, New, Orig, Replaced) :-
atomic_list_concat(Split, Old, Orig),
atomic_list_concat(Split, New, Replaced).
Example:
?- replace_word(a, b, atesta, X).
X = btestb.