Our Daily Method #19: Time#method_missing

Geplaatst door Michiel de Mare wo, 05 maa 2008 13:43:00 GMT

Our stock of cool tricks is depleted, but inspiration still strikes once in a while.

Are you using strftime a lot? Do you like writing strfime? Have you ever spelled it as strtime? strf_time? Thought it was frmtstr?

And if you spell it right, you still have to type those stupid %-signs. All in all, room for improvement.

class Time
  def method_missing(name,*args)
    if name.to_s =~ /f_/
      c = args[0] || ' '
      x = name.to_s[2..-1].split(//).
          map {|f| f =~ /[a-z]/i ? "%"+f : c}.

So, how does it work?

It handles every method starting with f_. What follows are the characters used within strftime: Y for 4-digit year, d for day of month, etc. Underscores are converted to spaces, or to the first argument, if you provide one.


# standard lib
Time.now.strftime("%Y%j")       #=> "2008065" 
Time.now.strftime("s")          #=> "1204725800" 
Time.now.strftime("%d %m %Y")   #=> "05 03 2008" 
Time.now.strftime("%d-%m-%Y")   #=> "05-03-2008" 
Time.now.strftime("%d %b %Y")   #=> "05 Mar 2008" 

# new and improved
Time.now.f_Yj           #=> "2008065" 
Time.now.f_s            #=> "1204725800" 
Time.now.f_d_m_Y        #=> "05 03 2008" 
Time.now.f_d_m_Y('-')   #=> "05-03-2008" 
Time.now.f_d_b_Y        #=> "05 Mar 2008" 

Cool? Stupid? Why don’t you play with it ?

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Our Daily Method #18: NilClass#method_missing

Geplaatst door Remco van 't Veer vr, 29 feb 2008 07:40:00 GMT

Warning: only apply the following under parental guidance!

We know the whiny nil, as applied in Rails, but what about an obedient nil:

class NilClass
  def method_missing(*args)

No more monkeying like:

person && person.name && person.name.upcase

Not DRY at all! Let’s use the obedient nil instead:


This concludes our last daily method. Ode to Michiel for providing a vast amount of nice tricks!

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Our Daily Method #17: Enumerable#make_index

Geplaatst door Michiel de Mare wo, 27 feb 2008 08:03:00 GMT

Do you know what I like so much about relational databases? And what I really miss in object-models? That databases make it so effortless to index data.

Imagine, you’ve got a huge pile of data, unordered, continually changing, but because your database keeps a few indexes, you can access any item you wish within a few milliseconds.

Wouldn’t it be fun if you could do that in Ruby? Especially the effortless part? It happens regularly that I’ve got an array with hundreds of items in which I have to search. That quicky gets slow, so I create a few hashes that I use as indexes. But what a difference with a real database!

Let make a first step in the right direction, and implement the make_index method.

module Enumerable
  def make_index(*names)
    Struct.new(*names).new(*names.map{|n| group_by(&n)})

index = User.find(:all).make_index(:login, :id, :nickname)

index.login['mdemare'] => [<#User ...>]
index.nickname['Michiel'] => [<#User ...>]

I admit, not a huge improvement yet. But we’re getting there. For another time, indexes on more than one attribute, unique indexes and indexing attributes with a natural order. (index.created_at > Time.now.at_midnight)

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Our Daily Method #16: Array#bsearch

Geplaatst door Michiel de Mare di, 26 feb 2008 08:00:00 GMT

Searching in arrays is pretty slow. Better use a hash.

But if the array is already sorted, you can use a binary search, which works by dividing the array in two and searching in the left orthe right part, until there’s only one element left.

class Array
  def bsearch(k)
    x,y = -1,size
    while y != x + 1 do
      m = (x + y) / 2
      if self[m] <= k
        x = m
        y = m
    x >= 0 && self[x] == k && x

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Our Daily Method #15: Kernel#levenshtein

Geplaatst door Michiel de Mare ma, 25 feb 2008 08:00:00 GMT

Not in any way invented by us, but still extremely useful and not a part of the standard library: the Levenshtein Distance

What does it calculate? The number of characters to add, delete or switch to turn the first string into the second. Ideal for detecting typos.

# example: 
levenshtein('levenstine', 'levenshtein') # => 3
Or how about instead of a LIKE-clause? Not quite as fast (cough) but much more accurate.

# In the old days:
    :conditions => ['name like ?', params[:name] + '%']])

# Now:
    sort_by {|u| levenshtein(params[:name], u.name)}.

def levenshtein(s1, s2)
  d = {}
  (0..s1.size).each { |row| d[[row, 0]] = row }
  (0..s2.size).each { |col| d[[0, col]] = col }
  (1..s1.size).each do |i|
    (1..s2.size).each do |j|
      i1,j1 = i-1,j-1
      cost = s1[i1] != s2[j1] ? 1 : 0
      d[[i, j]] = [d[[i1, j1]] + cost,
                   d[[i1, j]] + 1,
                   d[[i, j1]] + 1].min
      if i1*j1 > 0 and s1[i1] == s2[j-2] and s1[i-2] == s2[j1]
        d[[i, j]] = [d[[i,j]] , d[[i-2, j-2]] + cost].min
  d[[s1.size, s2.size]]

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Our Daily Method #14: Time#warp

Geplaatst door Remco van 't Veer vr, 22 feb 2008 08:00:00 GMT

Time travel can be very convenient but unfortunately it isn’t very easy. Reality won’t allow it but we can bend the Time class!

class Time
  def self.now_with_warping
    @warptime || now_without_warping

  class << self
    alias_method :now_without_warping, :now
    alias_method :now, :now_with_warping

  def warp
    self.class.instance_variable_set('@warptime', self)
    self.class.instance_variable_set('@warptime', nil)
Now we can travel back to “Unix Epoch”:

Time.at(0).warp do
  puts "The current time is: #{Time.now}" 
Or just before the end of time as we know it:

Time.at(2 ** 31 - 1).warp do
  Time.now + 1

What’s the use? It makes testing time dependent code very easy!

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Our Daily Method #13: Object#mymethods

Geplaatst door Michiel de Mare do, 21 feb 2008 08:00:00 GMT

I always have an irb or rails console session open, and I love methods to see what an object is capable of, but it certainly returns an exceptional amount of unsorted crap, especially in Rails.

Therefore, mymethods:

class Object
  def mymethods
    (methods - Object.instance_methods).sort

(1..2).methods.size # => 150
(1..2).mymethods.size # => 46

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Our Daily Method #12: Comparable#at_least

Geplaatst door Michiel de Mare wo, 20 feb 2008 08:00:00 GMT

It took me a while before I discovered the accepted Ruby idiom to find the bigger of two values. I expected a method in Kernel or perhaps in Comparable. Instead, the Ruby Way is to create an array and ask the maximum value in the array:

Well, it’s certainly short, but I don’t like it, for three reasons:
  • You create an array to compare two integers? That has to be inefficient, right? Of course, worrying about efficiency is not The Ruby Way.
  • I don’t like the name. When I say max it feels as if I’m declaring an upper bound, a maximum, when in fact, I’m declaring a lower bound, a minimum.
  • By listing both values in the array, you’re placing them on equal footing, when often that’s not really the case. Often it’s more an afterthought: you’re saying: I want value foo (and by the way, it must be at least 7.25).
Hence, at_most and at_least:

total_time / total_tries.at_least(1)

Implementation too trivial to list.

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Our Daily Method #11: Hash#inv_multi

Geplaatst door Michiel de Mare di, 19 feb 2008 08:00:00 GMT

Hash has an invert-method, but it often happens that you’re dealing with a hash with arrays as values. Naturally, you don’t want arrays as keys – you want every element in the array to turn into a key, with an array of all original keys with that element as the value.

Simple example

{1 => [3,4]}.inv_multi
# => {3=>[1], 4=>[1]}

Complex example

{1 => [3,4,5,6], 2 => [3,4], 4 => 11}.inv_multi
# => {5=>[1], 11=>[4], 6=>[1], 3=>[1, 2], 4=>[1, 2]}
The method itself:

class Hash
  def inv_multi
    # there's inject! again. daily method #5
    inject!({}) do |h,(k,v)|  # Is this obvious? If not, say so!
      # this lambda binds h and k.
      l = lambda {|x| (h[x] ||= []) << k}
      if Array === v
      else # value doesn't have to be an array

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Our Daily Method #10: Object#r[rs][ds]s

Geplaatst door Michiel de Mare ma, 18 feb 2008 08:00:00 GMT

Ruby 1.9 has Object#tap. Danny introduced Object#with a long time ago. Then there’s Object#instance_eval.

Do these methods have anything in common, except for the inscrutability of their names?

It turns out that they do. These methods fall in the family of methods that:
  1. Are defined for any Object
  2. Take a block and always execute it once
  3. Don’t do anything else

So it may seem amazing that there are as much as three methods in this category. Actually, there’s even a fourth!

You see, even within these strict specs there are two decisions that we have to take.
  1. What do we return? The object itself? Or the result of the block?
  2. Within the block, what is self? Is it the object? Or is it unchanged (so we need to pass on the object as an argument)?
How do the methods we referred to earlier stack up?
  • tap: returns result, same self in block
  • with: return self, same self in block
  • instance_eval: returns result, different self in block.
And the Missing Method:
  • switch: returns self, different self in block.

Let’s see these methods in action!

Lees verder...

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