Deletion iterators

By -
Often, deleting during iteration is problematic. So, in say Python, if you write:


for e in edges:
    if seen(e.v1) or seen(e.v2):
        delete e

you are in for some trouble.

And this is understandable. The default iterator is meant to get you through a sequence rapidly, and no one wants to drag along the baggage that would allow you to muck about with the sequence as you traverse it.

But why not have a second type of iterator that does allow deletion, which you would pull out just for the special cases where the above code is the sort of thing you need to do? Because I've had to "hand-roll" one of these every few months, it seems, and I bet others have as well.

Java has something like this with "fail-safe" iterators, but they seem to be for protection from another thread modifying the data while you iterate. In fact, the fail-safe iterator is guaranteed not to change while you are iterating.

Comments

  1. Ken B1:27 PM

    Look ahead one step.
    Alternatively, use an immutable collection but build a new winnowed collection. Unless there's some serious performance issue that approach will save you a lot of grief in the long run.

    ReplyDelete
  2. Ken B2:16 PM

    Btw, I recall the C++ STL having a variety of iterators for a variety of purposes.

    ReplyDelete

Post a Comment

Popular posts from this blog

Libertarians, My Libertarians!

By -
Cruel to be kind means that I love you . Because, while I think you are mistaken, your hearts are in the right place -- yes, even you, Silas --  unlike some people . This Breitbart fellow (discussed in the link above), by all appearances, deliberately doctored a video of Shirley Sherrod to make her remarks appear virulently racist, when they had, in fact, the opposite import. I heard that at a recent Austrian conference, some folks were talking about "Callahan's conservative turn." While that description is not entirely inaccurate, I must say that a lot of these people who today call themselves conservative give me the heebie-jeebies.
Read more

"Pre-Galilean" Foolishness

By -
 I am currently reading The Master and His Emissary , which appears to be an excellent book. ("Appears" because I don't know the neuroscience literature well enough to say for sure, yet.) But then on page 186 I find: "Asking cognition, however, to give a perspective on the relationship between cognition and affect is like asking astronomer in the pre-Galilean geocentric world, whether, in his opinion, the sun moves round the earth of the earth around the sun. To ask a question alone would be enough to label one as mad." OK, this is garbage. First of all, it should be pre-Copernican, not pre-Galilean. But much worse is that people have seriously been considering heliocentrism for many centuries before Copernicus. Aristarchus had proposed a heliocentric model in the 4th-century BC. It had generally been considered wrong, but not "mad." (And wrong for scientific reasons: Why, for instance, did we not observe stellar parallax?) And when Copernicus propose
Read more