I have no idea what is the problem,
this is the code-
solve_bdd(BddTree, ListVars) ->
findRes(BddTree, maps:from_list(ListVars++[{one, 1}, {zero, 0}])).
findRes(BddTree, Map) when is_record(BddTree, node)-> Val = maps:get(getName(BddTree)), Name = getName(BddTree),
if Name=='one' or Name=='zero' -> maps:get(getName(BddTree));
(Val==1 or Val=='true') -> findRes(getRight(BddTree), Map);
(Val==0 or Val=='false') -> findRes(getLeft(BddTree), Map);
true -> error
end;
findRes(_, _) -> error.
And the shell errors-
exf.erl:183: syntax error before: '=='
exf.erl:180: function findRes/2 undefined
exf.erl:21: Warning: function getRight/1 is unused
exf.erl:22: Warning: function getLeft/1 is unused
error
When there is multiple conditions, You should group operands of or operator in parentheses:
1> false or false.
false
2> false == true or false == true.
* 1: syntax error before: '=='
2> (false == true) or (false == true).
false
Also maps:get/1 (function get in module maps which accepts 1 parameter) that you used:
maps:get( getName(BddTree) )
does not exists! But you can use maps:get/2 or maps:get/3.
Most of the time you can use case expression instead of if expression.
Also sometimes it's better to use orlese operator instead of or.
It's better to not handle anything! instead of handling both 0 and 1 and boolean types, you can use one of them and remove unnecessary checks.
By convention in Erlang it's better to write function and record names in Snake_case.
BTW, your findRes/2 function would be like:
%%% I don't know what work you expect from this function so if it's not working
%%% just like your own, try to fix it!
% findRes -> find_res
find_res(BddTree, Map) when is_record(BddTree, node) ->
% Sounds like BddTree is a record. If by `get_name/1` you just want to
% access one of it's elements, you can simply write BddTree#node.<ELEMENT_NAME>
% getName -> get_name
case get_name(BddTree) of
% you don't have to use ' character for atoms:
Name when Name == one orelse Name == zero ->
% I thinkd you've missed `Map`:
maps:get(get_name(BddTree), Map);
_ ->
% I do not use 0 and 1 and just use boolean type:
find_res(
% I thinkd you've missed `Map`:
case maps:get(get_name(BddTree), Map) of
Val when Val -> % when Val == true
% getRight -> get_right
get_right(BddTree);
_ -> % Assume false
get_left(BddTree)
end,
Map
)
end;
find_res(_, _) -> error.
And let's look at above code without comments:
find_res(BddTree, Map) when is_record(BddTree, node) ->
case get_name(BddTree) of
Name when Name == one orelse Name == zero ->
maps:get(get_name(BddTree), Map);
_ ->
find_res(
case maps:get(get_name(BddTree), Map) of
Val when Val -> % when Val == true
get_right(BddTree);
_ ->
get_left(BddTree)
end,
Map
)
end;
find_res(_, _) -> error.
Related
I have the following functions:
search(DirName, Word) ->
NumberedFiles = list_numbered_files(DirName),
Words = make_filter_mapper(Word),
Index = mapreduce(NumberedFiles, Words, fun remove_duplicates/3),
dict:find(Word, Index).
list_numbered_files(DirName) ->
{ok, Files} = file:list_dir(DirName),
FullFiles = [ filename:join(DirName, File) || File <- Files ],
Indices = lists:seq(1, length(Files)),
lists:zip(Indices, FullFiles). % {Index, FileName} tuples
make_filter_mapper(MatchWord) ->
fun (_Index, FileName, Emit) ->
{ok, [Words]} = file:consult(FileName), %% <---- Line 20
lists:foreach(fun (Word) ->
case MatchWord == Word of
true -> Emit(Word, FileName);
false -> false
end
end, Words)
end.
remove_duplicates(Word, FileNames, Emit) ->
UniqueFiles = sets:to_list(sets:from_list(FileNames)),
lists:foreach(fun (FileName) -> Emit(Word, FileName) end, UniqueFiles).
However, when i call search(Path_to_Dir, Word) I get:
Error in process <0.185.0> with exit value:
{{badmatch,{error,{1,erl_parse,["syntax error before: ","wordinfile"]}}},
[{test,'-make_filter_mapper/1-fun-1-',4,[{file,"test.erl"},{line,20}]}]}
And I do not understand why. Any ideas?
The Words variable will match to content of the list, which might not be only one tuple, but many of them. Try to match {ok, Words} instead of {ok, [Words]}.
Beside the fact that the function file:consult/1 may return a list of several elements so you should replace {ok,[Words]} (expecting a list of one element = Words) by {ok,Words}, it actually returns a syntax error meaning that in the file you are reading, there is a syntax error.
Remember that the file should contain only valid erlang terms, each of them terminated by a dot. The most common error is to forget a dot or replace it by a comma.
I have the following code
let rec function1 element1 element2 = function
| [] -> []
| [a;b;c;d;e;f]::t -> if true then if true then [a]::(function1 element1 element2 t) else (function1 element1 element2 t)
| h :: t -> (function1 element1 element2 t);
,but it wont let me check if statement 1 and statement 2 is true
I keep getting
| [a;b;c;d;e;f]::t -> if true then if true then [a]::(function1 element1 element2 t) else (function1 element1 element2 t)
------------------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
stdin(3,31): error FS0001: This expression was expected to have type
'a list
but here has type
unit
I tried many different things and it doesnt seem like i am trying something that shouldnt work. Help please
You statement is read as:
if true
then
if true
then something
else somethingElse
// missing else
If statement might omit else branch if type of overall expression should be unit. So when you omit else you can think of just adding else () by the compiler.
On the other hand you are defining that match statement should return a list.
As you have 2 if's but only 1 else branch, the compiler has inferred the return type to be unit as only 1 of the if expressions returns a non-unit.
You could fix this by either
1: remove the nesting - it's not necessary when you could && the conditions
2: add the extra else branch (which in you're case would return (function1 element1 element2 t))
I have the following code to return a record from a list of records that has a field with value equal to Accountnumber.
lookup(AccountNumber, [#account{no=AccountNumber} = Rec | _]) ->
Rec;
lookup(AccountNumber, [_| T]) ->
lookup(AccountNumber, T);
lookup(AccountNumber, []) ->
not_found.
The above code works fine, but when I try to convert it to filter using the following code:
lookup(AccountNumber, DBRef) ->
lists:filter(fun(#account{no=AccountNumber} = Rec) -> Rec end, DBRef).
I got the following error:
** exception error: no case clause matching #account{no = 2,balance = 0,pin = undefined,name = "Ali",
transactions = []}
in function lists:'-filter/2-lc$^0/1-0-'/2 (lists.erl, line 1271)
What is the reason for the error?
There are multiple problems in the code
1.The filter should always return atom true or false for all list elements. This is causing you the error.
2.When the variable outside the fun block is used in fun header, they are not patterned matched, the variable outside is masked. Hence the pattern match fails.
You can see the modified code below.
lookup(AccountNumber, DBRef) ->
lists:filter(
fun(#account{no=AccNo}) when AccNo =:= AccountNumber -> true;
(_) -> false
end, DBRef).
I'd like to know if there is a function in Erlang can help me know whether an element is in a tuple or not. Like sets:is_element/2.
Tuple = {aaa,bbb,ccc}.
is_element_of_tuple(Tuple, aaa) % => true
is_element_of_tuple(Tuple, ddd) % => false
You can always transform the tuple to a list using tuple_to_list/1:
is_element_of_tuple(Tuple, Element) ->
lists:member(Element, tuple_to_list(Tuple)).
The simple answer is: no there is no function to do this. You have to write your own loop which traverses all the elements of a tuple until it either finds or does not find it. You an either convert the tuple to a list as above or write your own loop, something like:
is_element_of_tuple(E, Tuple) ->
is_element_of_tuple(E, Tuple, 1, tuple_size(Tuple)).
is_element_of_tuple(E, T, I, S) when I =< S ->
case element(I, T) of
E -> true;
_ -> is_element_of_tuple(E, T, I+1, S)
end;
is_element_of_tuple(_, _, _, _) -> false. %Done all the elements
Using a case and matching in this way means we check for exact equality, and it is probably a little faster than using =:= and checking if that returns true or false.
Working with Erlang's case, I'm facing a problem. The problem is the following:
other languages:
switch(A)
{
case "A" : case "B" :
//do something
break;
}
So, how to achieve the same thing using Erlang? Because sometimes it is very important to put conditions like these, to avoid overhead.
May be guards are what you want.
the_answer_is(N) when A == "A"; A == "B";
; - is OR
, - is AND
You can use case expressions in Erlang. The syntax is:
case Expression of
Pattern1 [when Guard1] -> Expr_seq1;
Pattern2 [when Guard2] -> Expr_seq2;
...
end
To quote Pragmatic Erlang:
case is evaluated as follows. First,
Expression is evaluated; assume this
evaluates to Value. Thereafter, Value
is matched in turn against Pattern1
(with the optional guard Guard1),
Pattern2, and so on, until a match is
found. As soon as a match is found,
then the corresponding expression
sequence is evaluated—the result of
evaluating the expression sequence is
the value of the case expression. If
none of the patterns match, then an
exception is raised.
An example:
filter(P, [H|T]) ->
case P(H) of
true -> [H|filter(P, T)];
false -> filter(P, T)
end;
filter(P, []) ->
[].
filter(P , L); returns a list of all those elements X in L for which P(X) is true. This can be written using pattern matching, but the case construct makes the code cleaner. Note that choosing between pattern matching and case expressions is a matter of taste, style and experience.
Not my favorite style, but you can do something like:
case A of
_ when A == "A";
A == "B" -> do_ab();
_ when A == "C";
_ when A == "D" -> do_cd();
_ -> do_default()
end.