Question

I am trying to read an integer value from a simple text file.
Row in my input file looks like this:

1 4 15 43
2 4 12 33
... (many rows of 4 integers)

I have opened the file in the following way:

{ok, IoDev} = file:open("blah.txt",[read])

But only thing that I manage to read are bytes with all the functions available.

What I finally want to get from file are tuples of integers.

{1 4 15 43}
{2 4 12 33}
...

Answer 1

You must first use file:read_line/1 to read a line of text, and use re:split/2 to get a list of strings containing numbers. Then use the list_to_integer BIF to get integers.

Here’s an example (surely there is a better solution):

#!/usr/bin/env escript
%% -*- erlang -*-

main([Filename]) ->
    {ok, Device} = file:open(Filename, [read]),
    read_integers(Device).

read_integers(Device) ->
    case file:read_line(Device) of
    eof ->
        ok;
    {ok, Line} ->
        % Strip the trailing \n (ASCII 10)
        StrNumbers = re:split(string:strip(Line, right, 10), "\s+", [notempty]),
        [N1, N2, N3, N4] = lists:map(fun erlang:list_to_integer/1, 
                     lists:map(fun erlang:binary_to_list/1, 
                           StrNumbers)), 
        io:format("~w~n", [{N1, N2, N3, N4}]),
        read_integers(Device)
    end.

(EDIT)

I found a somewhat simpler solution that uses io:fread to read formatted input. It works well in your case but fails badly when the file is badly constructed.

#!/usr/bin/env escript
%% -*- erlang -*-

main([Filename]) ->
    {ok, Device} = file:open(Filename, [read]),
    io:format("~w~n", [read_integers(Device)]).

read_integers(Device) ->
    read_integers(Device, []).

read_integers(Device, Acc) ->
    case io:fread(Device, [], "~d~d~d~d") of
    eof ->
        lists:reverse(Acc);
    {ok, [D1, D2, D3, D4]} ->
        read_integers(Device, [{D1, D2, D3, D4} | Acc]);
    {error, What} ->
        io:format("io:fread error: ~w~n", [What]),
        read_integers(Device, Acc)
    end.