blog.teemu.im

I could not find any decent examples from the web on how to generate a random string with a certain set of characters and length in Erlang. The basic idea for such a method is to take a string of allowed characters and loop N times where the N is the length of the resulting string. Then at each loop we take some random character from the string that contains the required set of characters. Sounds relatively simple, right? Next we have to write this in Erlang. This is what I came up with…

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get_random_string(Length, AllowedChars) ->
    lists:foldl(fun(_, Acc) ->
                        [lists:nth(random:uniform(length(AllowedChars)),
                                   AllowedChars)]
                            ++ Acc
                end, [], lists:seq(1, Length)).

Ok, Erlang is not the most readable language in the world and a simple thing such as generating a random string can look pretty tedious. No worries. I’ll go through the method line by line.

I’m using the lists:foldl method here. What it does is that it goes through a list (from left to right) and calls a function that has as it’s parameter a value from that list and the result form the previous iteration. The result of the method is the result of the last call to the function. The list I give as a parameter to lists:foldl is a sequence of numbers from one to the length of the resulting random string. For that I use the lists:seq method. This is how we define how many times we loop.

I’ll explain the fun() that is the first parameter of lists:foldl. Here is what it looks like separate from the whole code.

fun(_, Acc) ->
     [lists:nth(random:uniform(length(AllowedChars)), AllowedChars)]
          ++ Acc
end

The first parameter of the function is the value from the given list ([1, 2, 3, 4,..., N]) and we don’t use it (hence the underscore). The second parameter Acc is called the accumulator that is the result from the previous iteration. To achieve our goal of producing random strings we use lists:nth and random:uniform method calls to pick a random character from the AllowedChars string. Note that the lists:nth returns the integer value of that character so that is why the method call is wrapped in square brackets making the result a string (in Erlang strings are lists of integers). What we do then is that we add the Acc (the result of the previous iteration) to the result and this way build our random string.

There is also a third parameter for the lists:foldl method that you probably have guessed already. Naturally, you also have to give the value of the accumulator for the first iteration, which in this case is empty list [] or empty string since strings in Erlang are actually lists.

Here is an example of the result that the method produces.

test:get_random_string(32, "qwertyQWERTY1234567890").     
"8qttW01wQET1qRTt1r4tr2T392QY94Re"

I’ve recently got back to coding Erlang and noticed a neat module that I didn’t know existed that is probably worth writing a blog entry about. I’ve started developing a PubSubHubbub hub in Erlang called Hubbabubba and I’m using the great Mochiweb HTTP library as the HTTP server implementation. I discovered the reloader.erl module that comes with Mochiweb. It automatically reloads the code when you have the application running and you modify the code (remember to compile as well). This is something that I’ve found very useful when developing with Django or AppEngine and I’m really satisfied that there is a similar solution for Erlang as well.

Python for Nokia’s Series 60 platform has been around for four years now and to be honest, not much has happened on that front during the past year. However, suddenly on 24th of December Nokia releases a version 1.9.0 of the Python for Series 60 that is a major rewrite of the whole thing and comes now with Python 2.5 version of the core language. As usual, the odd-number version branch means that the release is for testing purposes only and the even-numbered 2.0 version branch should be released once it becomes more stable. Blog at Croozeus.com has done pretty nice wrap up of the new release and here are my thoughts.

First of all I should mention that as I write this blog entry I’ve not yet had time to test the new Python for Series 60 release. So I apologize beforehand for all false comments I’m about to make.

It seems that they are giving a lot more focus on the ease of development in this new release. Which is a really good thing I must say. What pleases me the most is that they are planning on improving the Python runtime deployment so that the end-user or the guy that is installing Python applications should not have to worry too much about having or not having the Python runtime installed on his phone. Definitely a good thing. The new release includes a packaging tool that is not part of the S60 SDK and is basically ensymble with added GUI. Ensymble is an excellent tool and it is very nice to see it included in the official PyS60 release.

Like mentioned already, the new release includes the version 2.5 of the Python interpreter and most of it’s standard libraries. Ok, the word “most” does not sound good here. I’ll focus first on the good things. First, the new release has Expat XML parser in it. Definitely a good thing since XML is something that pretty much every application out there uses and so far people have had to make ugly regexp XML parsers or use 3rd party package of expat to be able to parse XML in their PyS60 applications. However, I would say that including JSON parser as the official Python 2.6 release did lately, would probably be a good idea too.

Also, inclusion of asyncore and more compliant socket module sounds nice. I can’t wait to try Twisted on PyS60 and see how it works. One could do some crazy things with that on a mobile phone.

Then to the things that are not included from the standard Python 2.5 libraries. Now, I don’t have too much information on this since, like said before, I haven’t tried the new release yet. However, to my disappointment I noticed that sqlite3 is not included. SQLite should probably be a platform component in S60 because it is kind of becoming a de facto standard in mobile platforms since both iPhone and Android platforms use it. I don’t know what is the equivalent in S60 or does such exist but having some kind of storage other than just plain text file for PyS60 applications would be extremely nice thing to have.

Ok, I have to try out the new PyS60 release soon. I’m hoping that I will be pleasantly surprised. I’ll definitely write more about it later.