I am trying to learn Erlang and I am working on the practice problems Erlang has on the site. One of them is:
Write the function time:swedish_date() which returns a string containing the date in swedish YYMMDD format:
time:swedish_date()
"080901"
My function:
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
[YYYY,MM,DD] = tuple_to_list(date()),
string:substr((integer_to_list(YYYY, 3,4)++pad_string(integer_to_list(MM))++pad_string(integer_to_list(DD)).
pad_string(String) ->
if
length(String) == 1 -> '0' ++ String;
true -> String
end.
I'm getting the following errors when compiled.
demo.erl:6: syntax error before: '.'
demo.erl:2: function swedish_date/0 undefined
demo.erl:9: Warning: function pad_string/1 is unused
error
How do I fix this?
After fixing your compilation errors, you're still facing runtime errors. Since you're trying to learn Erlang, it's instructive to look at your approach and see if it can be improved, and fix those runtime errors along the way.
First let's look at swedish_date/0:
swedish_date() ->
[YYYY,MM,DD] = tuple_to_list(date()),
Why convert the list to a tuple? Since you use the list elements individually and never use the list as a whole, the conversion serves no purpose. You can instead just pattern-match the returned tuple:
{YYYY,MM,DD} = date(),
Next, you're calling string:substr/1, which doesn't exist:
string:substr((integer_to_list(YYYY,3,4) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD))).
The string:substr/2,3 functions both take a starting position, and the 3-arity version also takes a length. You don't need either, and can avoid string:substr entirely and instead just return the assembled string:
integer_to_list(YYYY,3,4) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD)).
Whoops, this is still not right: there is no such function integer_to_list/3, so just replace that first call with integer_to_list/1:
integer_to_list(YYYY) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD)).
Next, let's look at pad_string/1:
pad_string(String) ->
if
length(String) == 1 -> '0' ++ String;
true -> String
end.
There's a runtime error here because '0' is an atom and you're attempting to append String, which is a list, to it. The error looks like this:
** exception error: bad argument
in operator ++/2
called as '0' ++ "8"
Instead of just fixing that directly, let's consider what pad_string/1 does: it adds a leading 0 character if the string is a single digit. Instead of using if to check for this condition — if isn't used that often in Erlang code — use pattern matching:
pad_string([D]) ->
[$0,D];
pad_string(S) ->
S.
The first clause matches a single-element list, and returns a new list with the element D preceded with $0, which is the character constant for the character 0. The second clause matches all other arguments and just returns whatever is passed in.
Here's the full version with all changes:
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
{YYYY,MM,DD} = date(),
integer_to_list(YYYY) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD)).
pad_string([D]) ->
[$0,D];
pad_string(S) ->
S.
But a simpler approach would be to use the io_lib:format/2 function to just format the desired string directly:
swedish_date() ->
io_lib:format("~w~2..0w~2..0w", tuple_to_list(date())).
First, note that we're back to calling tuple_to_list(date()). This is because the second argument for io_lib:format/2 must be a list. Its first argument is a format string, which in our case says to expect three arguments, formatting each as an Erlang term, and formatting the 2nd and 3rd arguments with a width of 2 and 0-padded.
But there's still one more step to address, because if we run the io_lib:format/2 version we get:
1> demo:swedish_date().
["2015",["0",56],"29"]
Whoa, what's that? It's simply a deep list, where each element of the list is itself a list. To get the format we want, we can flatten that list:
swedish_date() ->
lists:flatten(io_lib:format("~w~2..0w~2..0w", tuple_to_list(date()))).
Executing this version gives us what we want:
2> demo:swedish_date().
"20150829"
Find the final full version of the code below.
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
lists:flatten(io_lib:format("~w~2..0w~2..0w", tuple_to_list(date()))).
UPDATE: #Pascal comments that the year should be printed as 2 digits rather than 4. We can achieve this by passing the date list through a list comprehension:
swedish_date() ->
DateVals = [D rem 100 || D <- tuple_to_list(date())],
lists:flatten(io_lib:format("~w~2..0w~2..0w", DateVals)).
This applies the rem remainder operator to each of the list elements returned by tuple_to_list(date()). The operation is needless for month and day but I think it's cleaner than extracting the year and processing it individually. The result:
3> demo:swedish_date().
"150829"
There are a few issues here:
You are missing a parenthesis at the end of line 6.
You are trying to call integer_to_list/3 when Erlang only defines integer_to_list/1,2.
This will work:
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
[YYYY,MM,DD] = tuple_to_list(date()),
string:substr(
integer_to_list(YYYY) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD))
).
pad_string(String) ->
if
length(String) == 1 -> '0' ++ String;
true -> String
end.
In addition to the parenthesis error on line 6, you also have an error on line 10 where yo use the form '0' instead of "0", so you define an atom rather than a string.
I understand you are doing this for educational purpose, but I encourage you to dig into erlang libraries, it is something you will have to do. For a common problem like this, it already exists function that help you:
swedish_date() ->
{YYYY,MM,DD} = date(), % not useful to transform into list
lists:flatten(io_lib:format("~2.10.0B~2.10.0B~2.10.0B",[YYYY rem 100,MM,DD])).
% ~X.Y.ZB means: uses format integer in base Y, print X characters, uses Z for padding
I have been busy with a sudoku solver in Erlang yesterday and today. The working functionality I have now is that I can check if a sudoku in the form of a list, e.g.,
[6,7,1,8,2,3,4,9,5,5,4,9,1,7,6,3,2,8,3,2,8,5,4,9,1,6,7,1,3,2,6,5,7,8,4,9,9,8,6,4,1,2,5,7,3,4,5,7,3,9,8,6,1,2,8,9,3,2,6,4,7,5,1,7,1,4,9,3,5,2,8,6,2,6,5,7,8,1,9,3,4].
is valid or not by looking at the constraints (no duplicates in squares, rows, and columns).
This function is called valid(S) which takes a sudoku S and returns true if it is a valid sudoku and false if it is not. The function ignores 0's, which are used to represent empty values. This is an example of the same sudoku with some random empty values:
[0,7,1,8,2,3,4,0,5,5,4,9,0,7,6,3,2,8,3,0,8,5,0,9,1,6,7,1,3,2,6,5,7,8,4,9,0,8,6,4,1,2,5,7,0,4,5,7,3,9,8,6,1,0,8,9,3,2,6,4,7,5,1,7,1,4,9,3,0,2,8,6,2,6,5,7,8,1,9,3,4].
The next step is to find the first 0 in the list, and try a value from 1 to 9 and check if it produces a valid sudoku. If it does we can continue to the next 0 and try values there and see if it is valid or not. Once we cannot go further we go back to the previous 0 and try the next values et cetera until we end up with a solved sudoku.
The code I have so far looks like this (based on someone who got it almost working):
solve(First,Nom,[_|Last]) -> try_values({First,Nom,Last},pos()).
try_values(_,[]) -> {error, "No solution found"};
try_values({First,Nom,Last},[N|Pos]) ->
case valid(First++[N]++Last) of
true ->
case solve({First++[N]},Nom,Last) of
{ok,_} -> {ok, "Result"};
{error,_} -> try_values({First,N,Last},Pos)
end;
false -> try_values({First,N,Last},Pos)
end.
pos() is a list consisting of the values from 1 to 9. The idea is that we enter an empty list for First and a Sudoku list for [_|Last] in which we look for a 0 (Nom?). Then we try a value and if the list that results is valid according to our function we continue till we fail the position or have a result. When we fail we return a new try_values with remaining (Pos) values of our possibitilies.
Naturally, this does not work and returns:
5> sudoku:solve([],0,S).
** exception error: bad argument
in operator ++/2
called as {[6]}
++
[1,1,8,2,3,4,0,5,5,4,9,0,7,6,3,2,8,3,2,8,5,4,9,1,6,7,1,3,2|...]
in call from sudoku:try_values/2 (sudoku.erl, line 140)
in call from sudoku:try_values/2 (sudoku.erl, line 142)
With my inexperience I cannot grasp what I need to do to make the code logical and working. I would really appreciate it if someone with more experience could give me some pointers.
try_values([], []) -> error("No solution found");
try_values([Solution], []) -> Solution;
try_values(_, []) -> error("Bad sudoku: multiple solutions");
try_values(Heads, [0|Tail]) ->
NewHeads = case Heads of
[] -> [[P] || P <- pos()];
_ -> [Head++[P] || P <- pos(), Head <- Heads]
end,
ValidHeads = [Head || Head <- NewHeads, valid(Head++Tail)],
try_values(ValidHeads, Tail);
try_values([], [H|Tail]) -> try_values([[H]], Tail);
try_values(Heads, [H|Tail]) -> try_values([Head++[H] || Head <- Heads], Tail).
solve(Board) ->
case valid(Board) of
true -> try_values([], Board);
false -> error("No solution found")
end.
try_values does what you described. It builds solution by going through Board, trying all possible solutions (from pos()) when it finds 0 and collecting valid solutions in ValidHeads to pass them further to continue. Thus, it goes all possible ways, if at some point there are multiple valid sudoku they all will be added to Heads and will be tested on validity on following steps. solve is just a wrapper to call try_values([], Board).
Basically, the way to iterate recursively over 0's is to skip all non-zeros (2 last try_values expression) and do the job on zeros (fourth try_values expression).
First three try_values expressions check if solution is exist and single and return it in that case.