I am going through the tutorials of riak and Erlang, I have stored data in riak by the riak-erlang-client and i did the following:
1> {ok,Pid} = riakc_pb_socket:start_link("127.0.0.1", 8087).
{ok,<0.34.0>}
2> Val1 = [1,2,3].
[1,2,3]
3> MyBucket = <<"numbers">>.
<<"numbers">>
4> Obj1 = riakc_obj:new(MyBucket,<<"ott">>,Val1).
5> riakc_pb_socket:put(Pid,Obj1).
ok
6> {ok, Fetched} = riakc_pb_socket:get(Pid,MyBucket,<<"ott">>).
{ok,{riakc_obj,<<"numbers">>,<<"ott">>,
<<107,206,97,96,96,96,204,96,202,5,82,28,202,156,255,126,
6,251,159,184,148,193,148,...>>,
[{{dict,3,16,16,8,80,48,
{[],[],[],[],[],[],[],[],[],[],[],[],...},
{{[],[],[],[],[],[],[],[],[],[],...}}},
<<131,107,0,3,1,2,3>>}],
undefined,undefined}}
7> Val1 == binary_to_term(riakc_obj:get_value(Fetched)).
true
8> binary_to_term(riakc_obj:get_value(Fetched)).
[1,2,3]
Now everything was fine , i want to extract the values from the list [1,2,3] that where only divisible by 2 from a mapreduce through erlang.
for that i have done the following,
9>Res = fun(Fetched,none)-> [X || X <- binary_to_term(riakc_obj:get_value(Fetched)), X rem 2 == 0] end.
I don't know exactly about what should write in the function parameters,as i am new to erlang and riak, so please correct me if am wrong.
Now when i going for mapreduce i have written this,
10>{ok , [{X}]} = riakc_pb_socket:mapred(Pid,[{<<"numbers">>,<<"ott">>}],[{map,{qfun,Res},none,true}]).
It was throwing the below exception..please suggest me a good way to understand what's going on and how to run mapreduce.
** exception exit: {noproc,{gen_server,call,
[<0.34.0>,
{req,{rpbmapredreq,<<131,108,0,0,0,3,104,2,100,0,6,105,
110,112,117,116,115,108,...>>,
<<"application/x-erlang-binary">>},
60100,
{132578423,<0.49.0>}},
infinity]}}
in function gen_server:call/3 (gen_server.erl, line 188)
in call from riakc_pb_socket:mapred/5 (src/riakc_pb_socket.erl, line 643)
Please,help me and suggest me, i hjave followed the links but i am not getting clearly that how to write on my own for a mapreduce so please guide me..it will help me.
You are getting the noproc error because you are passing an arity-2 function. The map phase function should be arity-3. Also, specifying none in the function header will cause that clause to only be executed when the corresponding argument explicitly matches, use _ in place of arguments you don't care about, like:
Res = fun(Fetched,_,_)-> [X || X <- binary_to_term(riakc_obj:get_value(Fetched)), X rem 2 == 0] end.
Related
Hi first of all I'm a beginner regarding Erlang,
I'm trying to extract a tuple from a list of tuples and assign it to a variable.
Consider this dataset.
[{0,25075,-2},{0,0,-2},{0,376100,-4}]
For example, I want to assign {0,25075,-2} to something like Var1 and so on.
I've tried the following in the shell for testing;
Tuples = [{0,25075,-2},{0,0,-2},{0,376100,-4}].
{Var1, Var2, Var3} = Tuples.
But I get this error;
ฐ** exception error: no match of right hand side value
[{0,25075,-2},{0,0,-2},{0,376100,-4}]
Any help would be appreciated, thanks.
In the shell, if you have already assigned a value to a variable then you must clear the variable before reusing it. Use the command f(). to clear all current variables, or f(Var1). to just clear Var1.
Secondly, your syntax is wrong. It should be:
1> Tuples = [{0,25075,-2},{0,0,-2},{0,376100,-4}].
[{0,25075,-2},{0,0,-2},{0,376100,-4}]
2> [Var1, Var2, Var3] = Tuples.
[{0,25075,-2},{0,0,-2},{0,376100,-4}]
3> Var1.
{0,25075,-2}
4> Var2.
{0,0,-2}
5> Var3.
{0,376100,-4}
6> Var1 = "something else".
** exception error: no match of right hand side value "something else"
7> f(Var1).
ok
8> Var1 = "something else".
"something else"
9> Var1.
"something else"
You will not do assignments like this over lists very often, though, typically you will iterate and/or use list operations.
I work in erlang
Now, I have a big problem
I want to have a log from a table mnesia and this log should be write in excel file
So the goal is write data from table mnesia to the excel file
I think and this is related to some code find in this forum that the best way is to write .txt file then transfer data from .txt file to excel file
I find this code in this forum in this link.
exporttxt()->
F = fun(T) -> mnesia:foldl(fun(X,Acc) -> [X|Acc] end, [],T) end,
{atomic,L} = mnesia:transaction(F(user)),
file:write_file("test.txt",[io_lib:format("~p\t~p\t~p~n",[F1,F2,F3]) ||
#user{id = F1,adress = F2,birthday = F3} <- L]).
But this code produces an error
As commented, the problem is clearly explained in the link itself. If you want the code then here it is. But please understand before directly jumping into the code.
exporttxt()->
F = fun() -> mnesia:foldl(fun(X,Acc) -> [X|Acc] end, [],user) end,
{atomic,L} = mnesia:transaction(F),
file:write_file("test.txt",[io_lib:format("~p\t~p\t~p~n",[F1,F2,F3]) ||
#user{id = F1,adress = F2,birthday = F3} <- L]).
in the subject you mention, I said that I didn't test the code, and of course there was a syntax error.
Here is a code that run.
1> ok = mnesia:create_schema([node()]).
ok
2> rd(my_user,{firstname,lastname,age}).
my_user
3> ok =application:start(mnesia).
ok
4> {atomic,ok} = mnesia:create_table(my_user,[{attributes,record_info(fields,my_user)},{disc_copies,[node()]},{type,bag}]).
{atomic,ok}
5> Add_user = fun(Fn,Ln,Ag) ->
5> F = fun() -> mnesia:write(my_user,#my_user{firstname=Fn,lastname=Ln,age=Ag},write) end,
5> mnesia:activity(transaction,F)
5> end.
#Fun<erl_eval.18.82930912>
6> ok = Add_user("Georges","Boy",25).
ok
7> ok = Add_user("Joe","Armstrong",52).
ok
8> ok = Add_user("Pascal","Me",55).
ok
9> F = fun(T) -> mnesia:foldl(fun(X,Acc) -> [X|Acc] end, [],T) end.
#Fun<erl_eval.6.82930912>
10> {atomic,L} = mnesia:transaction(F,[my_user]).
{atomic,[#my_user{firstname = "Pascal",lastname = "Me",
age = 55},
#my_user{firstname = "Joe",lastname = "Armstrong",age = 52},
#my_user{firstname = "Georges",lastname = "Boy",age = 25}]}
11> ok = file:write_file("test.txt",[io_lib:format("~p\t~p\t~p~n",[F1,F2,F3]) || #my_user{firstname = F1, lastname = F2, age = F3} <- L]).
ok
12>
you will have in your working directory a file named test.txt containing
"Pascal" "Me" 55
"Joe" "Armstrong" 52
"Georges" "Boy" 25
and if you open it with excel you will get
But this is not a code sequence you should use directly.
line 1 should take place in a code use for the deployment of your application.
line 2 is a record definition, necessary for the shell to understand the next lines. It should be replaced by a -record(...) in a module or an included file.
lines 3 and 4 should take place in the init function of one of the higher level supervisor (with some test to check already started application, existing table...)
line 5 should be in the interface definition of a server
line 6,7,8 should be generated by a user interface in some client
and last line 9,10,11 in another interface (for admin?).
Message to Lost_with_coding and his fellows,
If I may give you my opinion, you are burning steps. You should focus on mastering the Erlang syntax, the concept of pattern matching and variable bounding... after you should look at more advance construction such as list and binary comprehensions. Take time to look at error messages and use them to solve simple problems. The official Erlang documentation is great for this purpose. I always have it open in my browser and sometimes I also use this link http://erldocs.com/R15B/ when I look for function I don't know.
Next will come higher order functions, error handling, processes, concurrency, OTP... plus the usage of the efficient but not sexy Erlang tools (tv, appmon, debugger...).
I recommend it very often, but use the fantastic Fred Hebert's web site http://learnyousomeerlang.com/ and follow it step by step, rewriting the code, not copy/paste; it really worths the effort.
When reading rabbitmq's rabbit.erl，it contain hipe compilation related code.
hipe_compile() ->
Count = length(?HIPE_WORTHY),
io:format("HiPE compiling: |~s|~n |",
[string:copies("-", Count)]),
T1 = erlang:now(),
PidMRefs = [spawn_monitor(fun () -> [begin
{ok, M} = hipe:c(M, [o3]),
io:format("#")
end || M <- Ms]
end) ||
Ms <- split(?HIPE_WORTHY, ?HIPE_PROCESSES)],
[receive
{'DOWN', MRef, process, _, normal} -> ok;
{'DOWN', MRef, process, _, Reason} -> exit(Reason)
end || {_Pid, MRef} <- PidMRefs],
T2 = erlang:now(),
io:format("|~n~nCompiled ~B modules in ~Bs~n",
[Count, timer:now_diff(T2, T1) div 1000000]).
But there is no explanation about hipe in the erlang's reference doc. What's the meaning of 'o3'?
(emacs#chen-yumatoMacBook-Pro.local)51> hipe:c(xx_reader,[o3]).
{ok,xx_reader}
After I use hipe:c as above, No new compiled file can be found the in the pwd() directory?
Where it is?
o3 indicates the optimization level used by the compiler. There're also levels o0, o1, o2. Details of the levels are as follows:
o1 = [inline_fp,pmatch,peephole],
o2 = [icode_range,icode_ssa_const_prop,icode_ssa_copy_prop,icode_type,
icode_inline_bifs,rtl_lcm,rtl_ssa,rtl_ssa_const_prop,spillmin_color,
use_indexing,remove_comments,concurrent_comp,binary_opt] ++ o1,
o3 = [{regalloc,coalescing},icode_range] ++ o2.
You can use hipe:help_option(Option) to further investigate the meanings of different options. For example,
3> hipe:help_option(regalloc).
regalloc - Select register allocation algorithm. Used as {regalloc, METHOD}.
Currently available methods:
naive - spills everything (for debugging and testing)
linear_scan - fast; not so good if few registers available
graph_color - slow, but gives OK performance
coalescing - slower, tries hard to use registers
optimistic - another variant of a coalescing allocator
ok
4> hipe:help_option(icode_range).
icode_range - Performs integer range analysis on the Icode level
ok
I think HiPE is JIT compilation, just as the one used in Java. The native parts are available only in runtime, so there should be no explicit representation in your file system.
Also, hipe:c do require a .beam file is present. For example, if you create a test.erl with some stuff, and without compiling it to a .beam file, call hipe:c directly will lead to an error:
1> hipe:c(test, [o3]).
<HiPE (v 3.9.3)> EXITED with reason {cant_find_beam_file,test} #hipe:419
=ERROR REPORT==== 29-Nov-2012::17:03:02 ===
<HiPE (v 3.9.3)> Error: [hipe:418]: Cannot find test.beam file.** exception error: {hipe,419,{cant_find_beam_file,test}}
in function hipe:beam_file/1 (hipe.erl, line 419)
in call from hipe:c/2 (hipe.erl, line 313)
2> c(test).
{ok,test}
3> hipe:c(test, [o3]).
{ok,test}
There is some in erlang's doc. See here. But the doc is not much indeed. The index page of HiPE only updated recently.
Also, you can check some help in erlang shell.
> hipe:help().
> hipe:help_options().
> hipe:help_option(Option).
I've found in module rpc function pmap. And I've stuck when tried to use it.
The first question - does it requires connection with other nodes, or it would performs as lists:map if there is no connection to other nodes?
I've tried to execute pmap without connection to other nodes, but got exception:
7> rpc:pmap( { erlang, '+' }, [], [[1,1],[2,3]] ).
** exception exit: badrpc
in function rpc:check/2 (rpc.erl, line 745)
After that - I've launched another local node and connected it with current. But still get the same error.
Please provide me how to use rpc:pmap correctly.
Thanks
P.S.
Following code works expectedly (returns result 3):
70> erlang:apply( erlang, '+', [1,2] ).
3
71> erlang:'+'(1,2).
3
The rpc:map({Module, Function},ExtraArgs,List1) evaluates apply(Module, Function, [Elem|ExtraArgs]), for every element Elem in List1, in parallel.
So the right syntax using erlang:'+' is:
1> rpc:pmap( { erlang, '+' }, [2], [1,2,3] ).
[3,4,5]
2>
which evaluates [1+2,2+2,3+2]
the syntax rpc:pmap( { lists, sum }, [], [[1,2],[4,5]] ) will evaluate [1+2,4+5] and return
[3,9]
Pascal
Is it possible to know the name of a function in itself ?
a_function()->
io:format("I am ~p!", [????]). % What to use here?
Use the macro ?FUNCTION_NAME to get the name of the current function as an atom, and ?FUNCTION_ARITY to get the arity as an integer.
Example:
-module(test).
-export([a_function/2]).
a_function(_Foo, _Bar) ->
io:write("I am ~p/~p!",[?FUNCTION_NAME, ?FUNCTION_ARITY]).
1> c(test).
{ok,test}
2> test:a_function(a, b).
I am a_function/2!
This was implemented in EEP-0045.
For Erlang Versions 18 and Older
In older Erlang versions, there's no simple way to get the current function name at compile time. You can however retrieve it at runtime:
{current_function, {M, F, A}} = process_info(self(), current_function)
Where A is the arity (number of arguments), not the actual arguments. The first argument to process_info/2 is a process ID which can be either the current process (self()) or an other process. For example:
1> process_info(self(), current_function).
{current_function,{erl_eval,do_apply,5}}
Note however, that while this would be functionally equivalent to the ?FUNCTION_NAME macro, it's much slower because it is evaluated in runtime.
at runtime, you could throw an exception and check the top of the stacktrace.
foo() ->
catch throw(away),
[{Module, Fun, Arity} | _] = erlang:get_stacktrace(),
io:format("I am ~p:~p/~p!~n",[Module, Fun, Arity]).