String function clause matching - erlang

I'm running into a problem when writing some simple erlang code for an old Advent of Code task.
The following program is supposed to read lines, group characters in a string by occurrence and then count the number of lines that have a repeat of three characters.
count_occurrences([], Map) -> Map;
count_occurrences([H | T], Map) ->
count_occurrences(T, maps:put(H, maps:get(H, Map, 0) + 1, Map)).
count(Line, Count) ->
Map = count_occurrences(Line, #{}),
case lists:member(3, maps:values(Map)) of
true -> Count + 1;
false -> Count
end.
run() ->
{ok, Binary} = file:read_file("data.txt"),
Lines = binary:split(Binary, <<"\n">>, [global]),
Result = lists:foldl(fun count/2, 0, Lines),
Result.
However, I get this error message:
10> c(day2).
{ok,day2}
11> day2:run().
** exception error: no function clause matching day2:count_occurrences(<<"bpacnmelhhzpygfsjoxtvkwuor">>,#{}) (day2.erl, line 5)
in function day2:count/2 (day2.erl, line 10)
in call from lists:foldl/3 (lists.erl, line 1263)
I don't understand why <<"bpacnmelhhzpygfsjoxtvkwuor">>,#{} doesn't match the second "count_occurrences" function clause - a string is the same as a list, right? Why doesn't it match [H | T]?

Check out this example:
-module(a).
-compile(export_all).
go([_H|_T], _X) ->
"First arg was a list";
go("a", _X) ->
"First arg was a string";
go(<<"a">>, _X) ->
"First arg was a binary".
In the shell:
5> a:go(<<"a">>, #{a=>1, b=>2}).
"First arg was a binary"
and:
6> a:go("a", #{a=>1, b=>2}).
"First arg was a list"
a string is the same as a list, right?
Yes, a double quoted string is a shortcut for creating a list of integers where the integers in the list are the ascii codes of the characters. Hence, the second function clause above will never match:
a.erl:6: Warning: this clause cannot match because a previous clause
at line 4 always matches
But....a binary, such as <<"abc">> is NOT a string, and therefore a binary is not a shortcut for creating a list of integers.
8> "a" =:= [97].
true
Okay, you knew that. But, now:
9> "a" =:= <<"a">>.
false
10> <<"a">> =:= <<97>>.
true
11> "a" =:= <<97>>.
false
And, finally:
13> <<"abc">> =:= <<97, 98, 99>>.
true
The last example shows that specifying a double quoted string inside a binary is just a shortcut for specifying a comma separated list of integers inside a binary--however specifying a double quoted string inside a binary does not somehow convert the binary to a list.

Note that you can also iterate through a binary with only slightly different syntax:
count_occurrences(<<>>, Map) -> Map;
count_occurrences(<<H, T/binary>>, Map) ->
count_occurrences(T, maps:put(H, maps:get(H, Map, 0) + 1, Map)).
By default, H is assumed to be a byte, but you can add modifiers to specify how many bits you want to select, and more. See the documentation for the Bit Syntax.

You get this error cuz function count_occurrences/2 expect first argument list - [<<"bpacnmelhhzpygfsjoxtvkwuor">>] or "bpacnmelhhzpygfsjoxtvkwuor" but was put binary - <<"bpacnmelhhzpygfsjoxtvkwuor">>. Double check input data Line in function count/2 of module day2.erl at line 10:
1> is_list([]).
true
2> is_list("").
true
3> is_list(<<"">>).
false
4> is_list(binary_to_list(<<"">>)).
true

Related

Erlang: Get first n characters of a string

I have credit card number, let's say 5940043543536. And for security purposes I only want to display the first four digits.
How would one do that in erlang?
A string in Erlang is just a list of integers, so you can use lists:sublist/3:
1> String = "5940043543536".
"5940043543536"
2> lists:sublist(String, 1, 4).
"5940"
Note that the position argument starts from 1 and not 0.
In case you are receiving binary (instead of string)
binary:part(<<"123455678901234">>, 1, 4).
<<"2345">>
or if you need get last four digits
binary:part(<<"123455678901234">>, {byte_size(<<"123455678901234">>), -4}).
<<"1234">>
newer versions of Erlang have built in string functions. For your case
1> string:slice("123455678901234", 1, 4).
"1234"
there is a string:substring function too, which works the same way, but it has been depreciated for slice.
You can try use pattern matching:
1> String = "5940043543536".
"5940043543536"
2> [A,B,C,D|_] = String.
"5940043543536"
3> [A,B,C,D].
"5940"
Or you can create your own function, eg:
1> String = "5940043543536".
"5940043543536"
2> GetDigits = fun F(_, Acc, 0) -> lists:reverse(Acc);
F([H|T], Acc, N) -> F(T, [H|Acc], N - 1) end.
#Fun<erl_eval.43.91303403>
3> GetDigits(String, [], 4).
"5940"

Find substring in a string using start and end points from List

I have a list like List = [{0,12},{0,12},{-1,0},{0,12},{0,4},{1,2}] and a string Str = "https://www.youtube.com/watch?v=WQfdwsPao9E", now I've to find all the substrings using start and end point from list.
I want substrings to be returned in a List like ["https://www","https://www",..]
I tried using this:
C=lists:map(fun({X,Y}) -> string:sub_string(Str,X,Y) end,List)
1> List = [{0,12},{0,12},{-1,0},{0,12},{0,4},{1,2}].
[{0,12},{0,12},{-1,0},{0,12},{0,4},{1,2}]
2> Str = "https://www.youtube.com/watch?v=WQfdwsPao9E".
"https://www.youtube.com/watch?v=WQfdwsPao9E"
3> Len = length(Str).
43
4> [string:sub_string(Str,max(1,X),min(Len,Y)) || {X,Y} <- List].
["https://www.","https://www.",[],"https://www.","http",
"ht"]
5>
you may have to adjust the indexes in the string to fit exactly to your need.
[edit] It looks like I didn't interpret correctly what is the meaning of the tuple. I think it is {Fist_Char_Index, Char_Number}, or {-1,0} if no match is found. So you should use:
[string:sub_string(Str,X+1,X+Y) || {X,Y} <- List, {X,Y} =/= {-1,0}].

Erlang: syntax error before: ","word"

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.

Erlang and io:read()

I have a problem, I don't know why the program doesn't work correctly.
When I run the program and I insert atom c, then the program calls function io:read() forever.
Thank you for your help and I'm sorry for my English.
-module(temperature).
-export([run/0, convert/2]).
run() ->
run(true).
run(true) ->
{ok, Choice} = io:read("Convert to degrees Celsius or convert to degrees Fahrenheit? c/f :"),
{ok, Temp} = io:read("Insert temperature: "),
{UnitTemp, Convert} = convert(Choice, Temp),
io:format("The converted temperature: ~f ~s\n", [Convert, UnitTemp]),
{ok, Continue} = io:read("New temperature? true/false :"),
run(Continue);
run(false) ->
ok.
convert(c, Fahrenheit) -> {'Celsius', 5 * (Fahrenheit - 32) / 9};
convert(f, Celsius) -> {'Fahrenheit', 9 * Celsius / 5 + 32}.
io:read reads a term, so it does not stop, until you finish your term with ..
1> io:read("Enter term: ").
Enter term: {foo, bar}.
{ok,{foo,bar}}
2> io:read("This will give error: ").
This will give error: }foo
This will give error: .
{error,{1,erl_parse,["syntax error before: ","'}'"]}}
So you can simply type c..
Alternatively, you can use io:get_chars/2 if you don't want to type the dot. The first argument is prompt and second one is number of chars to read, so in your case, it will be:
io:get_chars("prompt ", 1).
prompt c
"c"
Remember, that after typing c, you still have to hit enter and now, you should pattern match on string "c" instead of atom c.

Matching tuples with don't-care variables in Erlang

I am looking for a way to find tuples in a list in Erlang using a partial tuple, similarly to functors matching in Prolog. For example, I would like to following code to return true:
member({pos, _, _}, [..., {pos, 1, 2}, ...])
This code does not work right away because of the following error:
variable '_' is unbound
Is there a brief way to achieve the same effect?
For simple cases it's better to use already mentioned lists:keymember/3. But if you really need member function you can implement it yourself like this:
member(_, []) ->
false;
member(Pred, [E | List]) ->
case Pred(E) of
true ->
true;
false ->
member(Pred, List)
end.
Example:
>>> member(fun ({pos, _, 2}) -> true; (_) -> false end, [..., {pos, 1, 2}, ...]).
Use lists:keymember/3 instead.
You can do it with a macro using a list comprehension:
-define(member(A,B), length([0 || A <- B])>0).
?member({pos, _, _}, [{width, 17, 42}, {pos, 1, 2}, totally_irrelevant]).
It is not very efficient (it runs through the whole list) but it is the closest I can think to the original syntax.
If you want to actually extract the elements that match you just remove 'length' and add a variable:
-define(filter(A,B), [_E || A =_E <- B]).
You could do it using list comprehension:
Matches = [ Match || {Prefix, _, _} = Match <- ZeList, Prefix == pos].
Another possibility would be to do what match specs do and use the atom '_' instead of a raw _. Then, you could write a function similar to the following:
member(X, List) when is_tuple(X), is_list(List) ->
member2(X, List).
% non-exported helper functions:
member2(_, []) ->
false;
member2(X, [H|T]) when not is_tuple(H); size(X) =/= size(H) ->
member2(X, T);
member2(X, [H|T]) ->
case is_match(tuple_to_list(X), tuple_to_list(H)) of
true -> true;
false -> member2(X, T)
end.
is_match([], []) ->
true;
is_match(['_'|T1], [_|T2]) ->
is_match(T1, T2);
is_match([H|T1], [H|T2]) ->
is_match(T1, T2);
is_match(_, _) ->
false.
Then, your call would now be:
member({pos, '_', '_'}, [..., {pos, 1, 2}, ...])
This wouldn't let you match patterns like {A, A, '_'} (checking where the first two elements are identical), but if you don't need variables this should work.
You could also extend it to use variables using a similar syntax to match specs ('$1', '$2', etc) with a bit more work -- add a third parameter to is_match with the variable bindings you've seen so far, then write function clauses for them similar to the clause for '_'.
Granted, this won't be the fastest method. With the caveat that I haven't actually measured, I expect using the pattern matching in the language using a fun will give much better performance, although it does make the call site a bit more verbose. It's a trade-off you'll have to consider.
May use ets:match:
6> ets:match(T, '$1'). % Matches every object in the table
[[{rufsen,dog,7}],[{brunte,horse,5}],[{ludde,dog,5}]]
7> ets:match(T, {'_',dog,'$1'}).
[[7],[5]]
8> ets:match(T, {'_',cow,'$1'}).
[]

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