Issues with reading file in erlang - functional-programming

Question

So, I am trying to read and write into a file.
While writing into the file, I need to check if a particular index exist in file then I don't write and throw error.
The data in file will look like this:
{1,{data,dcA,1}}.
{2, {data, dcA, 2}}.
{3,{data,dcA,3}}.
I added the dot at the end of each line because file:consult() needs the file like this.
Which is in this format.
{Index, {Data, Node, Index}}
When I have to add a new file, I check with this Index.
Here's what I have tried so far - https://pastebin.com/apnWLk45
And I run it like this:
193> {ok, P9} = poc:start(test1, self()).
{ok,<0.2863.0>}
194> poc:add(P9, Node, {6, data}).
In poc:add/3, P9 is the process id from the file:open.
I defined before in shell as dcA
And the third is the data - which is in this format - {Index, data}
Since I am using file:consult/1, it takes the filename as parameter. At that point, I only have process id. So I take the name from
file:pid2name(_Server).
This runs perfectly when I run it for the first time.
When I run this again - poc:add(P9, Node, {6, data2}), I get an error in this line file:pid2name(_Server).
exception error: no match of right hand side value undefined
How can I solve this issue?
I am new to Erlang. Just been a week that I started learning.

Answer 1

I am trying to read and write into a file. While writing into the
file, I need to check if a particular index exist in file then I don't
write and throw error.
A DETS table can easily do what you want:
-module(my).
-compile(export_all).
open_table() ->
dets:open_file(my_data, [{type, set}, {file, "./my_data.dets"}]).
close_table() ->
dets:close(my_data).
clear_table() ->
dets:delete_all_objects(my_data).
insert({Key, _Rest}=Data) ->
case dets:member(my_data, Key) of
true -> throw(index_already_exists);
false -> dets:insert(my_data, Data)
end.
all_items() ->
dets:match(my_data, '$1').
In the shell:
~/erlang_programs$ erl
Erlang/OTP 20 [erts-9.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V9.2 (abort with ^G)
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:open_table().
{ok,my_data}
3> my:clear_table().
ok
4> my:all_items().
[]
5> my:insert({1, {data, a, b}}).
ok
6> my:insert({2, {data, c, d}}).
ok
7> my:insert({3, {data, e, f}}).
ok
8> my:all_items().
[[{1,{data,a,b}}],[{2,{data,c,d}}],[{3,{data,e,f}}]]
9> my:insert({1, {data, e, f}}).
** exception throw: index_already_exists
in function my:insert/1 (my.erl, line 15)
When I run this again - poc:add(P9, Node, {6, data2}), I get an error
in this line file:pid2name(_Server):
exception error: no match of right hand side value undefined
When a process opens a file, it becomes linked to a process that handles the file I/O, which means that if the process that opens the file terminates abnormally, the I/O process will also terminate. Here is an example:
-module(my).
-compile(export_all).
start() ->
{ok, Pid} = file:open('data.txt', [read, write]),
spawn(my, add, [Pid, x, y]),
exit("bye").
add(Pid, _X, _Y) ->
timer:sleep(1000), %Let start() process terminate.
{ok, Fname} = file:pid2name(Pid),
io:format("~s~n", [Fname]).
In the shell:
1> c(my).
my.erl:2: Warning: export_all flag enabled - all functions will be exported
{ok,my}
2> my:start().
** exception exit: "bye"
in function my:start/0 (my.erl, line 7)
3>
=ERROR REPORT==== 25-Jun-2018::13:28:48 ===
Error in process <0.72.0> with exit value:
{{badmatch,undefined},[{my,add,3,[{file,"my.erl"},{line,12}]}]}
According to the pid2name() docs:
pid2name(Pid) -> {ok, Filename} | undefined
the function can return undefined, which is what the error message is saying happened.