ideas
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solution.pl
103
solution.pl
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@ -1,3 +1,25 @@
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:- use_module(library(dcg/basics)).
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% DCG rule that checks if the remainder of the input
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% starts with Delim (a list), but does not consume
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% any input.
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lookahead(Delim , L, L) :-
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prefix(Delim , L).
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% Checks if the remainder of the input starts with one
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% of the items in the supplied list (as per lookahead /3).
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lookaheads([H|T]) -->
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lookahead(H), ! ; lookaheads(T).
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% Reads text until one of the delimiters in Delims is
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% encountered , then unifies R with the Prolog term parsed
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% from the text.
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% ! Will crash the parse if a delimiter is encountered
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% but the intervening text is not a Prolog term (you
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% don’t have this handle this case).
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prolog_term(R, Delims) -->
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string(S), lookaheads(Delims), !, {
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read_term_from_atom(S, R, []) }.
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:- dynamic tables/0.
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:- dynamic tables/0.
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/*
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/*
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Prints the names of all existing tables, one per line (use writeln/1).
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Prints the names of all existing tables, one per line (use writeln/1).
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@ -164,7 +186,86 @@ For example:
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findall(Values, selec(persons,[+id,+first],[],Values), Projections)
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findall(Values, selec(persons,[+id,+first],[],Values), Projections)
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returns: Projections = [[0, "Jeffrey"], [1, "Lorena"], [2, "Joseph"], ...
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returns: Projections = [[0, "Jeffrey"], [1, "Lorena"], [2, "Joseph"], ...
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*/
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*/
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/* selec(Table, Selectors, Conds, Projection). */
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selec(Table, Selectors, Conds, Projection) :-
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selec_help(Table, Selectors,[], [], ColumnNames, ColumnValue),
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%remove combinations that don't exist
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row(Table, Row),
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has_value(ColumnValue, Row),
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Projection = ColumnNames/ColumnValue.
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%base case
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selec_help(_, [], ColumnNames, ColumnValue, ColumnNames, ColumnValue).
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/**
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* Table: name of the table the selection is done one
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* [Selector|Selectors] name(s) of the column(s) for selection
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* AccCols: acumulator list for the column(s) name(s)
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* AccVals: acumulator list for the row(s) values
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* ColumnNames: List returning the column(s) name(s)
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* ColumnValue: List returning the row values for each column
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*/
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selec_help(Table, [Selector|Selectors], AccCols, AccVals, ColumnNames, ColumnValue) :-
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%check if table exist
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(tabl(Table, Cols) ->
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%check if column exist
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member(Selector, Cols) ->
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%find the value corresponding to the column in the row
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nth0(Index, Cols, Selector),
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row(Table, Row),
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nth0(Index, Row, Value),
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%go to next column tho evaluate
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selec_help(Table, Selectors, [Selector|AccCols], [Value|AccVals], ColumnNames, ColumnValue)
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;
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string_concat(Selector, " isn't a column of table ", X),
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string_concat(X, Table, Error),
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throw(Error)
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;
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string_concat(Table, " doesn't exist.", Error), throw(Error)
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).
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/*
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Check if a list has all the values in [Value|Rest]
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*/
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has_value([], _) :- !.
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has_value([Value|Rest], List) :-
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memberchk(Value, List),
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has_value(Rest, List).
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evaluate_condition_string(ConditionString) :-
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atom_codes(AtomCondition, ConditionString), % Convert string to atom
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read_term_from_codes(AtomCondition, Term, []), % Parse the term
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call(Term). % Evaluate the parsed term
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selec_column(Table, Column, Proj) :-
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tabl(Table, Cols),
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row(Table, Row),
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nth0(I, Cols, Column),
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nth0(I, Row, Value),
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Proj = Value.
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apply_cond(Table, Cond, Proj) :-
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tabl(Table, Cols),
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row(Table, Row),
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nth0(I, Cols, Column),
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nth0(I, Row, Value),
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split_string(Cond, " ", "", SubStrings),
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Decomp = SubStrings,
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Decomp = [Column, Op, Value],
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check_condition(Column, Op, Value),
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Proj = Row.
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check_condition(Left, =, Right) :-
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Left =:= Right.
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check_condition(Left, <, Right) :-
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Left < Right.
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check_condition(Left, >, Right) :-
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Left > Right.
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check_condition(Left, <=, Right) :-
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Left =< Right.
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check_condition(Left, >=, Right) :-
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Left >= Right.
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check_condition(Left, \=, Right) :-
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Left =\= Right.
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:- dynamic selec/3.
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:- dynamic selec/3.
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/*
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/*
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