ideas

solution.pl

@@ -1,3 +1,25 @@
+:- use_module(library(dcg/basics)).
+% DCG rule that checks if the remainder of the input
+% starts with Delim (a list), but does not consume
+% any input.
+lookahead(Delim , L, L) :-
+	prefix(Delim , L).
+% Checks if the remainder of the input starts with one
+% of the items in the supplied list (as per lookahead /3).
+lookaheads([H|T]) -->
+	lookahead(H), ! ; lookaheads(T).
+% Reads text until one of the delimiters in Delims is
+% encountered , then unifies R with the Prolog term parsed
+% from the text.
+% ! Will crash the parse if a delimiter is encountered
+% but the intervening text is not a Prolog term (you
+% don’t have this handle this case).
+prolog_term(R, Delims) -->
+	string(S), lookaheads(Delims), !, {
+	read_term_from_atom(S, R, []) }.
+
+
+
 :- dynamic tables/0.
 /*
 Prints the names of all existing tables, one per line (use writeln/1).
@@ -164,7 +186,86 @@ For example:
 findall(Values, selec(persons,[+id,+first],[],Values), Projections)
 returns: Projections = [[0, "Jeffrey"], [1, "Lorena"], [2, "Joseph"], ...
 */
-/* selec(Table, Selectors, Conds, Projection). */
+selec(Table, Selectors, Conds, Projection) :-
+    selec_help(Table, Selectors,[], [], ColumnNames, ColumnValue),
+	%remove combinations that don't exist
+	row(Table, Row),
+	has_value(ColumnValue, Row),
+    Projection = ColumnNames/ColumnValue.
+
+%base case
+selec_help(_, [], ColumnNames, ColumnValue, ColumnNames, ColumnValue).
+/**
+ * Table: name of the table the selection is done one
+ * [Selector|Selectors] name(s) of the column(s) for selection
+ * AccCols: acumulator list for the column(s) name(s)
+ * AccVals: acumulator list for the row(s) values
+ * ColumnNames: List returning the column(s) name(s)
+ * ColumnValue: List returning the row values for each column 
+*/
+selec_help(Table, [Selector|Selectors], AccCols, AccVals, ColumnNames, ColumnValue) :-
+	%check if table exist
+    (tabl(Table, Cols) ->
+		%check if column exist
+    	member(Selector, Cols) ->
+			%find the value corresponding to the column in the row
+			nth0(Index, Cols, Selector),
+        	row(Table, Row),
+        	nth0(Index, Row, Value),
+			%go to next column tho evaluate
+        	selec_help(Table, Selectors, [Selector|AccCols], [Value|AccVals], ColumnNames, ColumnValue)
+    	;   
+			string_concat(Selector, " isn't a column of table ", X),
+			string_concat(X, Table, Error),
+			throw(Error)
+	;
+		string_concat(Table, " doesn't exist.", Error), throw(Error)
+    ).
+/*
+Check if a list has all the values in [Value|Rest]
+*/
+has_value([], _) :- !.
+has_value([Value|Rest], List) :-
+    memberchk(Value, List),
+	has_value(Rest, List).
+
+evaluate_condition_string(ConditionString) :-
+    atom_codes(AtomCondition, ConditionString), % Convert string to atom
+    read_term_from_codes(AtomCondition, Term, []), % Parse the term
+    call(Term). % Evaluate the parsed term
+
+selec_column(Table, Column, Proj) :-
+	tabl(Table, Cols),
+	row(Table, Row),
+	nth0(I, Cols, Column),
+	nth0(I, Row, Value),
+	Proj = Value.
+
+apply_cond(Table, Cond, Proj) :-
+	tabl(Table, Cols),
+	row(Table, Row),
+	nth0(I, Cols, Column),
+	nth0(I, Row, Value),
+	split_string(Cond, " ", "", SubStrings),
+	Decomp = SubStrings,
+	Decomp = [Column, Op, Value],
+	check_condition(Column, Op, Value),
+	Proj = Row.
+
+check_condition(Left, =, Right) :-
+    Left =:= Right.
+check_condition(Left, <, Right) :-
+    Left < Right.
+check_condition(Left, >, Right) :-
+    Left > Right.
+check_condition(Left, <=, Right) :-
+    Left =< Right.
+check_condition(Left, >=, Right) :-
+    Left >= Right.
+check_condition(Left, \=, Right) :-
+    Left =\= Right.
+	
+	
 
 :- dynamic selec/3.
 /*