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So I need to create an instance method for Array that takes two arguments, the size of an array and an optional object that will be appended to an array.
If the the size argument is less than or equal to the Array.length or the size argument is equal to 0, then just return the array. If the optional argument is left blank, then it inputs nil.
Example output:
array = [1,2,3]
array.class_meth(0) => [1,2,3]
array.class_meth(2) => [1,2,3]
array.class_meth(5) => [1,2,3,nil,nil]
array.class_meth(5, "string") => [1,2,3,"string","string"]
Here is my code that I've been working on:
class Array
def class_meth(a ,b=nil)
self_copy = self
diff = a - self_copy.length
if diff <= 0
self_copy
elsif diff > 0
a.times {self_copy.push b}
end
self_copy
end
def class_meth!(a ,b=nil)
# self_copy = self
diff = a - self.length
if diff <= 0
self
elsif diff > 0
a.times {self.push b}
end
self
end
end
I've been able to create the destructive method, class_meth!, but can't seem to figure out a way to make it non-destructive.
Here's (IMHO) a cleaner solution:
class Array
def class_meth(a, b = nil)
clone.fill(b, size, a - size)
end
def class_meth!(a, b = nil)
fill(b, size, a - size)
end
end
I think it should meet all your needs. To avoid code duplication, you can make either method call the other one (but not both simulaneously, of course):
def class_meth(a, b = nil)
clone.class_meth!(a, b)
end
or:
def class_meth!(a, b = nil)
replace(class_meth(a, b))
end
As you problem has been diagnosed, I will just offer a suggestion for how you might do it. I assume you want to pass two and optionally three, not one and optionally two, parameters to the method.
Code
class Array
def self.class_meth(n, arr, str=nil)
arr + (str ? ([str] : [nil]) * [n-arr.size,0].max)
end
end
Examples
Array.class_meth(0, [1,2,3])
#=> [1,2,3]
Array.class_meth(2, [1,2,3])
#=> [1,2,3]
Array.class_meth(5, [1,2,3])
#=> [1,2,3,nil,nil]
Array.class_meth(5, [1,2,3], "string")
#=> [1,2,3,"string","string"]
Array.class_meth(5, ["dog","cat","pig"])
#=> [1,2,3,"string","string"]
Array.class_meth(5, ["dog","cat","pig"], "string")
#=> [1,2,3,"string","string"]
Array.class_meth(5, ["dog","cat","pig"])
#=> ["dog", "cat", "pig", nil, nil]
Array.class_meth(5, ["dog","cat","pig"], "string")
#=> ["dog", "cat", "pig", "string", "string"]
Before withdrawing his answer, #PatriceGahide suggested using Array#fill. That would be an improvement here; i.e., replace the operative line with:
arr.fill(str ? str : nil, arr.size, [n-arr.size,0].max)
self_copy = self does not make a new object - assignment in Ruby never "copies" or creates a new object implicitly.
Thus the non-destructive case works on the same object (the instance the method was invoked upon) as in the destructive case, with a different variable bound to the same object - that is self.equal? self_copy is true.
The simplest solution is to merely use #clone, keeping in mind it is a shallow clone operation:
def class_meth(a ,b=nil)
self_copy = self.clone # NOW we have a new object ..
# .. so we can modify the duplicate object (self_copy)
# down here without affecting the original (self) object.
end
If #clone cannot be used other solutions involve create a new array or obtain an array #slice (returns a new array) or even append (returning a new array) with #+; however, unlike #clone, these generally lock-into returning an Array and not any sub-type as may be derived.
After the above change is made it should also be apparent that it can written as so:
def class_meth(a ,b=nil)
clone.class_meth!(a, b) # create a NEW object; modify it; return it
# (assumes class_meth! returns the object)
end
A more appropriate implementation of #class_meth!, or #class_meth using one of the other forms to avoid modification of the current instance, is left as an exercise.
FWIW: Those are instance methods, which is appropriate, and not "class meth[ods]"; don't be confused by the ill-naming.
I can't tell what's wrong with my code:
def morse_code(str)
string = []
string.push(str.split(' '))
puts string
puts string[2]
end
What I'm expecting is if I use "what is the dog" for str, I would get the following results:
=> ["what", "is", "the", "dog"]
=> "the"
But what I get instead is nil. If I do string[0], it just gives me the entire string again. Does the .split function not break them up into different elements? If anyone could help, that would be great. Thank you for taking the time to read this.
Your code should be :
def morse_code(str)
string = []
string.push(*str.split(' '))
puts string
p string[2]
end
morse_code("what is the dog" )
# >> what
# >> is
# >> the
# >> dog
# >> "the"
str.split(' ') is giving ["what", "is", "the", "dog"], and you are pushing this array object to the array string. Thus string became [["what", "is", "the", "dog"]]. Thus string is an array of size 1. Thus if you want to access any index like 1, 2 so on.., you will get nil. You can debug it using p(it calls #inspect on the array), BUT NOT puts.
def morse_code(str)
string = []
string.push(str.split(' '))
p string
end
morse_code("what is the dog" )
# >> [["what", "is", "the", "dog"]]
With Array, puts works completely different way than p. I am not good to read MRI code always, thus I take a look at sometime Rubinious code. Look how they defined IO::puts, which is same as MRI. Now look the specs for the code
it "flattens a nested array before writing it" do
#io.should_receive(:write).with("1")
#io.should_receive(:write).with("2")
#io.should_receive(:write).with("3")
#io.should_receive(:write).with("\n").exactly(3).times
#io.puts([1, 2, [3]]).should == nil
end
it "writes nothing for an empty array" do
x = []
#io.should_receive(:write).exactly(0).times
#io.puts(x).should == nil
end
it "writes [...] for a recursive array arg" do
x = []
x << 2 << x
#io.should_receive(:write).with("2")
#io.should_receive(:write).with("[...]")
#io.should_receive(:write).with("\n").exactly(2).times
#io.puts(x).should == nil
end
We can now be sure that, IO::puts or Kernel::puts behaves with array just the way, as Rubinious people implemented it. You can now take a look at the MRI code also. I just found the MRI one, look the below test
def test_puts_recursive_array
a = ["foo"]
a << a
pipe(proc do |w|
w.puts a
w.close
end, proc do |r|
assert_equal("foo\n[...]\n", r.read)
end)
end
Trying to port some old PHP code to Ruby and missing some key info on creating arrays in Ruby.
The PHP code:
foreach ($results as $r) {
$array[$r['column']][] = $r
}
Is this the simplest way to do it in Ruby? Do I have to initialize the second array?
#array = []
result.each do |r|
#array[r.cron_column] = []
#array[r.cron_column] << r
end
I figure this is a simple syntax issue, but my Googling has turned up empty. Thanks for your help!
You are indexing into an empty array, so that will always return nil. nil does not define the << operator, so you get an error. You need to initialize the value at array[index] if you want to use the << operator.
I am assuming you want an array of arrays, so you can use this instead which will initialize the value at items[index] to an empty array if it is nil before pushing the value onto it
items = []
array.each do |r|
(items[r.column] ||= []) << r
end
The only change here is that, if items[r.column] returns nil it will be set equal to an empty array, otherwise nothing will be done. If you really just want to set the value at items[index] to r, just use the = operator.
Are you sure you need an array as output? it would appear a hash would be more convenient; moreover, it's way easier to build in your scenario (which is usually a sign you are in the correct path):
# example
results = [
OpenStruct.new(:x => 1, :cron_column => 0),
OpenStruct.new(:x => 2, :cron_column => 1),
OpenStruct.new(:x => 3, :cron_column => 1),
]
#array = results.group_by(&:cron_column)
# {0=>[#<OpenStruct x=1, cron_column=0>],
# 1=>[#<OpenStruct x=2, cron_column=1>, #<OpenStruct x=3, cron_column=1>]}
If cron_column "has no holes" (that's it, you have values from 0 to N), you can easily create an array with this same idea: results.group_by(&:cron_column).sort.map { |k, v| v } or results.group_by(&:cron_column).sort.map(&:last), as you prefer.
array = results.inject([]) { |m, r| m[r.column] = r; m }
Update: oh, e1[] = e2 adds a new array element in PHP, so tokland is right, in which case:
array = results.inject([]) { |m, r| (m[r.column] ||= []) << r; m }
As mentioned in this answer, Array.new(size, object) creates an array with size references to the same object.
hash = Hash.new
a = Array.new(2, hash)
a[0]['cat'] = 'feline'
a # => [{"cat"=>"feline"},{"cat"=>"feline"}]
a[1]['cat'] = 'Felix'
a # => [{"cat"=>"Felix"},{"cat"=>"Felix"}]
Why does Ruby do this, rather than doing a dup or clone of object?
Because that's what the documentation says it does. Note that Hash.new is only being called once, so of course there's only one Hash
If you want to create a new object for every element in the array, pass a block to the Array.new method, and that block will be called for each new element:
>> a = Array.new(2) { Hash.new }
=> [{}, {}]
>> a[0]['cat'] = 'feline'
=> "feline"
>> a
=> [{"cat"=>"feline"}, {}]
>> a[1]['cat'] = 'Felix'
=> "Felix"
>> a
=> [{"cat"=>"feline"}, {"cat"=>"Felix"}]
For certain classes that can't be modified in-place (like Fixnums) the Array.new(X, object) form works as expected and is probably more efficient (it just calls memfill instead of rb_ary_store and yielding to the block):
For more complicated objects you always have the block form (e.g. Array.new(5) { Hash.new }).
*Edit:* Modified according to the comments. Sorry for the stupid example, I was tired when I wrote that.
I came up with this answer, very short and simple solution
c_hash = Hash.new
["a","b","c","d","e","f"].each do |o|
tmp = Hash.new
[1,2,3].map {|r| tmp[r] = Array.new}
c_hash[o] = tmp
end
c_hash['a'][1] << 10
p c_hash
I've got a Ruby method like the following:
# Retrieve all fruits from basket that are of the specified kind.
def fruits_of_kind(kind)
basket.select { |f| f.fruit_type == kind.to_s }
end
Right now, you can call this like:
fruits_of_kind(:apple) # => all apples in basket
fruits_of_kind('banana') # => all bananas in basket
and so on.
How do I change the method so that it will correctly handle iterable inputs as well as no inputs and nil inputs? For example, I'd like to be able to support:
fruits_of_kind(nil) # => nil
fruits_of_kind(:apple, :banana) # => all apples and bananas in basket
fruits_of_kind([:apple, 'banana']) # => likewise
Is this possible to do idiomatically? If so, what's the best way to write methods so that they can accept zero, one, or many inputs?
You need to use the Ruby splat operator, which wraps all remaining arguments into an Array and passes them in:
def foo (a, b, *c)
#do stuff
end
foo(1, 2) # a = 1, b = 2, c = []
foo(1, 2, 3, 4, 5) #a = 1, b = 2, c = [3, 4, 5]
In your case, something like this should work:
def fruits_of_kind(*kinds)
kinds.flatten!
basket.select do |fruit|
kinds.each do |kind|
break true if fruit.fruit_type == kind.to_s
end == true #if no match is found, each returns the whole array, so == true returns false
end
end
EDIT
I changed the code to flatten kinds so that you can send in a list. This code will handle entering no kinds at all, but if you want to expressly input nil, add the line kinds = [] if kinds.nil? at the beginning.
Use the VARARGS feature of Ruby.
# Retrieve all fruits from basket that are of the specified kind.
# notice the * prefix used for method parameter
def fruits_of_kind(*kind)
kind.each do |x|
puts x
end
end
fruits_of_kind(:apple, :orange)
fruits_of_kind()
fruits_of_kind(nil)
-sasuke
def fruits_of_kind(kind)
return nil if kind.nil?
result = []
([] << kind).flatten.each{|k| result << basket.select{|f| f.fruit_type == k.to_s }}
result
end
The 'splat' operator is probably the best way to go, but there are two things to watch out for: passing in nil or lists. To modify Pesto's solution for the input/output you'd like, you should do something like this:
def fruits_of_kind(*kinds)
return nil if kinds.compact.empty?
basket.select do |fruit|
kinds.flatten.each do |kind|
break true if fruit.fruit_type == kind.to_s
end == true #if no match is found, each returns the whole array, so == true returns false
end
end
If you pass in nil, the * converts it to [nil]. If you want to return nil instead of an empty list, you have to compact it (remove nulls) to [], then return nil if it's empty.
If you pass in a list, like [:apple, 'banana'], the * converts it to [[:apple, 'banana']]. It's a subtle difference, but it's a one-element list containing another list, so you need to flatten kinds before doing the "each" loop. Flattening will convert it to [:apple, 'banana'], like you expect, and give you the results you're looking for.
EDIT: Even better, thanks to Greg Campbell:
def fruits_of_kind(basket, kind)
return nil if kind.nil?
kind_list = ([] << kind).flatten.map{|kind| kind.to_s}
basket.select{|fruit| kind_list.include?(fruit) }
end
OR (using splat)
def fruits_of_kind(*kinds)
return nil if kinds.compact.empty?
kind_list = kinds.flatten.map{|kind| kind.to_s}
basket.select{|fruit| kind_list.include?(fruit.fruit_type) }
end
There's a nicely expressive use of splat as an argument to array creation that handles your last example:
def foo(may_or_may_not_be_enumerable_arg)
arrayified = [*may_or_may_not_be_enumerable_arg]
arrayified.each do |item|
puts item
end
end
obj = "one thing"
objs = ["multiple", "things", 1, 2, 3]
foo(obj)
# one thing
# => ["one thing"]
foo(objs)
# multiple
# things
# 1
# 2
# 3
# => ["multiple", "things", 1, 2, 3]