Archive for December, 2009

Ricerca sales manager in Sourcesense

Tuesday, December 22nd, 2009

Summary: position in Sourcesense for a Sales Manager

Sourcesense sta cercando un commerciale per assunzione. Questa persona si dovrà occupare (fra l’altro) di sviluppare il business per il team Agile. Se pensi di essere in grado di spiegare ai potenziali clienti perché è vantaggioso comprare sviluppo e formazione da un team che è in grado di sviluppare software in incrementi settimanali, con il controllo e la sicurezza che ne conseguono, allora sei la persona giusta per noi!

Lettori di questo blog: se conoscete una persona adatta, per favore diteglielo.

Tutte le informazioni qui:

Work with asymmetric bounds

Tuesday, December 22nd, 2009

Let’s talk about ranges of integers. Quick: how many numbers are in the range 16 ≤ x ≤ 37, that is in the set {16, 17, …, 37}?

If you answered 21 then you got it wrong: the correct answer is 22. When you express a range inclusive of both ends, the formula to compute the number of elements is

size of [lower, upper] = upper - lower + 1

The “+1” in the above formula is the source of many “off-by-one” or fenceposts errors. Can we get rid of the “+1”? Yes, there is a way. Use asymmetric bounds, including the lower bound and excluding the upper bound. The formula becomes

size of [lower, upper) = upper - lower

I’m using square and round brackets as a shorthand notation for ranges with inclusive and exclusive bounds:

  [a, b] = { x | a ≤ x ≤ b }
  [a, b) = { x | a ≤ x < b }

Another reason in favor of asymmetric bounds: how do you express an empty range? With symmetric inclusive ranges, you can’t: the smallest range you can express contains one element. For instance [3, 3] contains only the number 3. With asymmetric bounds, you can express an empty range with, for instance, [3, 3), which is empty.

Asymmetric bounds work very well in conjunction with counting from zero. Java and C arrays are indexed from 0, so that the index range is [0, N). The upper bound is now equal to the number of elements! Expressing it with symmetric bounds gives [0, N-1], which is ugly because of the “-1”.

If you need to iterate on a C or Java array, the standard pattern is

  int[] array = new int[N];
  for (int i=0; i<N; i++) { ... }

You see how the body is executed when i==0, through i==N-1, but not when i==N, since we are using i<N as a boundary condition. How many times is the loop is executed? Exactly N! It’s now easy to see that the iteration is performed the correct number of times.

Asymmetric bounds can be concatenated easily. Suppose you have a function that paginates results on a web page. You are on the page which contains results 30, 31, … 39. If you express the range with two parameters from and to, the page url would look like

And the links to the next and previous page would be

  <a href="/results?from=20&to=30">previous page</a>
  <a href="/results?from=40&to=50">next page</a>

where the bounds for the next or previous page are easily computed by adding or subtracting the page size from the current bounds. Compare with
  <a href="/results?from=20&to=29">previous page</a>
  <a href="/results?from=40&to=49">next page</a>

it’s more difficult to check that the bounds are correct here: you have to think where the “-1” must be applied.

So, if you define “+” as the operation of concatenation of ranges, you get this nice little law:

  [a, b) + [b, c) = [a, c)

It’s this sort of properties that make it easy to combine software objects together. This helps towards the goal of programming with lego-like bricks.

In conclusion: express ranges with asymmetric bounds! You will avoid off-by-one errors, get rid of “+1″s from your code, and make your programs more modular.


The book C Traps and Pitfalls by Andrew Koenig has a good explanation of asymmetric bounds.

XP Day London was, indeed, a blast!

Thursday, December 10th, 2009

It was very, very, inspirational and fun. I ran another session of Birthday Greetings Kata, facilitated an open space on the Hexagonal Architecture. London is a city I love to visit, and the Church House location, is fantastic: just a few steps from Westminster Abbey.

And I had the authors sign my copy of GOOS (I believe every good book should have an acronym… and I don’t like GOOSGBT :-) )

My signed copy of GOOS

Thank you guys! I have a feeling this book will be a good companion in my re-discovery of OOP.

Entity-relationship-oriented programming

Thursday, December 10th, 2009

How I agree with Dafydd Rees when he writes:

Most Java and C# programmers have no idea how to do object-oriented programming.

They’re really doing entity-relationship-oriented programming. Trouble is, they’re in the majority so they form a self-reinforcing group that take for granted knowledge and skills that they don’t even realise that they don’t have.

Aaand… I’m afraid I was in the E-R-oriented camp until not long ago. Now I’m beginning to see how how objects should work.