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5 points by jimbokun 4158 days ago | link | parent

(incf (gethash k table 0))

table is a hash table, k is a key, gethash returns the value in table for k or 0 if no value for k is found. Think of incf as ++. It will increment the value by 1 and set that as the value for k in table.



7 points by pg 4158 days ago | link

And the problem with Arc is that currently the default value for an entry in a hash table is nil, rather than zero. If h is a hash table and you know (h 'foo) is 1, you can safely say

  (++ (h 'foo))  
But if you don't know whether (h 'foo) has a value yet you have to check explicitly:

  (= (h 'foo) (+ 1 (or (h 'foo) 0)))

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1 point by metageek 4158 days ago | link

How about if <code>h</code> takes an optional second argument, which is the default, and the macros are smart enough that you can do <code>(++ (h 'foo 0))</code>?

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1 point by greatness 4154 days ago | link

I agree, this is probably the best solution.

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1 point by reitzensteinm 4158 days ago | link

Perhaps (++ containsnil) should result in 1 anyway? Is there any case where that would break anything?

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2 points by simonb 4158 days ago | link

For one it breaks the expectation of a strongly typed language.

If something goes wrong and you want to fail as soon as possible not propagate the defect through the system.

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1 point by reitzensteinm 4158 days ago | link

Oh, it definitely throws strong typing right out of the window.

The reason I suggested it is because it would seem that almost all of the time where you go to do an increment on a nil value, you're working with an uninitialized element (not necessarily in a hash map) and treating that as 0 (as you're doing an increment) would in a certain sense be reasonable behaviour.

But I guess you're right, in the case where nil does represent an error, it'll be two steps backwards when you go to debug the thing.

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1 point by william42 4151 days ago | link

Or perhaps just set containsnil to 0 when you do that. (Knowing pg, this would probably work.)

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1 point by Tichy 4158 days ago | link

What is the usage scenario for that? I have never written such a code (incrementing values in hashtables) - maybe it is more common in LISP?

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1 point by bOR_ 4158 days ago | link

Happens when you want to categorize the frequency of items in a list, and I've been doing that all the time (categorizing gene frequencies in an agent-based model).

In ruby I'd extend the array class with this code

  class Array
    def categorize
      hash = Hash.new(0)
      self.each {|item| hash[item] += 1}
      return hash
    end
  end
although the other day I saw someone achieve the same thing using a hack on inject (the `; hash' part is only there because inject demands that, the work is done earlier.)

  array.inject(Hash.new(0)) {|hash,key| hash[key] += 1 ; hash}
Noticing that lisp / arc is more concise indeed. I'll have fun learning it.

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1 point by smallpaul 4155 days ago | link

Why would you extend rather than subclass the Array class? It kind of confirms all of my worst fears about Ruby's too-easy class reopening. (what happens when someone else defines an Array method called "categorize" for a totally unrelated purposes?)

I think that the Python syntax for this is

h[x] = h.get(x, 0) + 1

It isn't quite as concise as the Common Lisp but more so than Arc. I'd be curious to see what the Common Lisp looks like if you are doing something more complicated than an in-place increment. E.g. the equivalent of:

h[x] = h.get(x, 1) * 2

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2 points by bOR_ 4152 days ago | link

I'm a phd, working alone on projects, and the scripts I write a generally < 300 lines + 6 functions from a library I wrote. The agent-based models i write are ~ 200 lines, no libraries.

For me there's not much risk in redefining things.

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1 point by jsg 4155 days ago | link

(setf (gethash x h) (* (gethash x h 1) 2))

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1 point by ijoshua 4155 days ago | link

A hashtable containing integer values is a common implementation for the collection data structure known as a Bag or Counted Set. The value indicates how many instances of the key appear in the collection. Incrementing the value would be equivalent to adding a member instance. Giving a zero default is a shortcut to avoid having to check for membership.

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