I'm working on creating a method that pads an array, and accepts 1. a desired value and 2. an optional string/integer value. Desired_size reflects the desired number of elements in the array. If a string/integer is passed in as the second value, this value is used to pad the array with extra elements. I understand there is a 'fill' method that can shortcut this - but that would be cheating for the homework I'm doing.
The issue: no matter what I do, only the original array is returned. I started here:
class Array
def pad(desired_size, value = nil)
desired_size >= self.length ? return self : (desired_size - self.length).times.do { |x| self << value }
end
end
test_array = [1, 2, 3]
test_array.pad(5)
From what I researched the issue seemed to be around trying to alter self's array, so I learned about .inject and gave that a whirl:
class Array
def pad(desired_size, value = nil)
if desired_size >= self.length
return self
else
(desired_size - self.length).times.inject { |array, x| array << value }
return array
end
end
end
test_array = [1, 2, 3]
test_array.pad(5)
The interwebs tell me the problem might be with any reference to self so I wiped that out altogether:
class Array
def pad(desired_size, value = nil)
array = []
self.each { |x| array << x }
if desired_size >= array.length
return array
else
(desired_size - array.length).times.inject { |array, x| array << value }
return array
end
end
end
test_array = [1, 2, 3]
test_array.pad(5)
I'm very new to classes and still trying to learn about them. Maybe I'm not even testing them the right way with my test_array? Otherwise, I think the issue is I get the method to recognize the desired_size value that's being passed in. I don't know where to go next. Any advice would be appreciated.
Thanks in advance for your time.
In all 3 of your tries, you are returning the original array if desired_size is greater than the original array size. You have that backwards. In other words, you just return instead of padding.
Your first attempt was close. You need to:
1) Fix your conditional check.
2) It's OK to modify the self array, so the more complicated tries are not necessary.
3) Make sure you return self no matter what you do.
By modifying self, not only do you return the modified array, but you also change the array held by the variable test_array. So if you were to do:
test_array = [1, 2, 3]
puts test_array.pad(5, 4).inspect // prints [1, 2, 3, 4, 4]
puts test_array // prints [1, 2, 3, 4, 4]
In Ruby, when a function modifies self, the function name ends with a !, so if you were to write it modifying self, it would be better to name it pad!.
If you want to write it so that it doesn't modify self, you could start with:
array = self.dup
and then do all of your operations on array.
Related
I have built a version of mastermind that checks a user's input and provides feedback based on how close the user's guess was to the winning sequence. If you're not familiar with the game, you get feedback indicating how many of your characters were guessed correctly at the same index and how many characters guessed are in the sequence, but at the wrong index. If there are duplicates in the guess, then you would not count the extra values unless they correspond to the same number of duplicates in the secret code.
Example: If the sequence is ["G","G","G","Y"] and the user guesses ["G", "Y","G","G"] then you'd want to return 2 for items at the same index and 2 for items at different indexes that are included in the secret sequence.
Another example: If the sequence is ["X","R","Y","T"] and the user guesses ["T","T","Y","Y"] then you'd return 1 for items at the same index 1 for the character guessed that is in the sequence but at the wrong index.
Anyway, to me this is not a simple problem to solve. Here's the code I used to get it to work, but it's not elegant. There must be a better way. I was hoping someone can tell me what I'm missing here?? New to Ruby...
def index_checker(input_array, sequence_array)
count = 0
leftover_input = []
leftover_sequence = []
input.each_with_index do |char, idx|
if char == sequence[idx]
count += 1
else
leftover_input << char
leftover_sequence << sequence[idx]
end
end
diff_index_checker(leftover_input, leftover_sequence, count)
end
def diff_index_checker(input, sequence, count)
count2 = 0
already_counted = []
input.each do |char|
if sequence.include?(char) && !already_counted.include?(char)
count2 += 1
already_counted << char
end
end
[count, count2]
end
Here's a clean Ruby solution, written in idiomatic Ruby object-oriented style:
class Mastermind
def initialize(input_array, sequence_array)
#input_array = input_array
#sequence_array = sequence_array
end
def matches
[index_matches, other_matches]
end
def results
[index_matches.size, other_matches.size]
end
private
attr_reader :input_array, :sequence_array
def index_matches
input_array.select.with_index { |e, i| e == sequence_array[i] }
end
def other_matches
non_exact_input & non_exact_sequence
end
def non_exact_input
array_difference(input_array, index_matches)
end
def non_exact_sequence
array_difference(sequence_array, index_matches)
end
# This method is based on https://stackoverflow.com/a/3852809/5961578
def array_difference(array_1, array_2)
counts = array_2.inject(Hash.new(0)) { |h, v| h[v] += 1; h }
array_1.reject { |e| counts[e] -= 1 unless counts[e].zero? }
end
end
You would use this class as follows:
>> input_array = ["G","G","G","Y"]
>> sequence_array = ["G", "Y","G","G"]
>> guess = Mastermind.new(input_array, sequence_array)
>> guess.results
#> [2, 2]
>> guess.matches
#> [["G", "G"], ["G", "Y"]]
Here's how it works. First everything goes into a class called Mastermind. We create a constructor for the class (which in Ruby is a method called initialize) and we have it accept two arguments: input array (the user guess), and sequence array (the answer).
We set each of these arguments to an instance variable, which is indicated by its beginning with #. Then we use attr_reader to create getter methods for #input_array and #sequence_array, which allows us to get the values by calling input_array and sequence_array from any instance method within the class.
We then define two public methods: matches (which returns an array of exact matches and an array of other matches (the ones that match but at the wrong index), and results (which returns a count of each of these two arrays).
Now, within the private portion of our class, we can define the guts of the logic. Each method has a specific job, and each is named to (hopefully) help a reader understand what it is doing.
index_matches returns a subset of the input_array whose elements match the sequence_array exactly.
other_matches returns a subset of the input_array whose elements do not match the sequence_array exactly, but do match at the wrong index.
other_matches relies on non_exact_input and non_exact_sequence, each of which is computed using the array_difference method, which I copied from another SO answer. (There is no convenient Ruby method that allows us to subtract one array from another without deleting duplicates).
Code
def matches(hidden, guess)
indices_wo_match = hidden.each_index.reject { |i| hidden[i] == guess[i] }
hidden_counts = counting_hash(hidden.values_at *indices_wo_match)
guess_counts = counting_hash(guess.values_at *indices_wo_match)
[hidden.size - indices_wo_match.size, guess_counts.reduce(0) { |tot, (k, cnt)|
tot + [hidden_counts[k], cnt].min }]
end
def counting_hash(arr)
arr.each_with_object(Hash.new(0)) { |s, h| h[s] += 1 }
end
Examples
matches ["G","G","G","Y"], ["G", "Y","G","G"]
#=> [2, 2]
matches ["X","R","Y","T"] , ["T","T","Y","Y"]
#=> [1, 1]
Explanation
The steps are as follows.
hidden = ["G","G","G","Y"]
guess = ["G", "Y","G","G"]
Save the indices i for which hidden[i] != guess[i].
indices_wo_match = hidden.each_index.reject { |i| hidden[i] == guess[i] }
#=> [1, 3]
Note that the number of indices for which the values are equal is as follows.
hidden.size - indices_wo_match.size
#=> 2
Now compute the numbers of remaining elements of guess that pair with one of the remaining values of hidden by having the same value. Begin by counting the numbers of instances of each unique element of hidden and then do the same for guess.
hidden_counts = counting_hash(hidden.values_at *indices_wo_match)
#=> {"G"=>1, "Y"=>1}
guess_counts = counting_hash(guess.values_at *indices_wo_match)
#=> {"Y"=>1, "G"=>1}
To understand how counting_hash works, see Hash::new, especially the explanation of the effect of providing a default value as an argument of new. In brief, if a hash is defined h = Hash.new(3), then if h does not have a key k, h[k] returns the default value, here 3 (the hash is not changed).
Now compute the numbers of matches of elements of guess that were not equal to the value of hidden at the same index and which pair with an element of hidden that have the same value.
val_matches = guess_counts.reduce(0) do |tot, (k, cnt)|
tot + [hidden_counts[k], cnt].min
end
#=> 2
Lastly, return the values of interest.
[hidden.size - indices_wo_match.size, val_matches]
#=> [2, 2]
In the code presented above I have substituted out the variable val_matches.
With Ruby 2.4+ one can use Enumerable#sum to replace
guess_counts.reduce(0) { |tot, (k, cnt)| tot + [hidden_counts[k], cnt].min }
with
guess_counts.sum { |k, cnt| [hidden_counts[k], cnt].min }
def judge(secret, guess)
full = secret.zip(guess).count { |s, g| s == g }
semi = secret.uniq.sum { |s| [secret.count(s), guess.count(s)].min } - full
[full, semi]
end
Demo:
> judge(["G","G","G","Y"], ["G","Y","G","G"])
=> [2, 2]
> judge(["X","R","Y","T"], ["T","T","Y","Y"])
=> [1, 1]
A shorter alternative, though I find it less clear:
full = secret.zip(guess).count(&:uniq!)
I prefer my other answer for its simplicity, but this one would be faster if someone wanted to use this for arrays larger than Mastermind's.
def judge(secret, guess)
full = secret.zip(guess).count { |s, g| s == g }
pool = secret.group_by(&:itself)
[full, guess.count { |g| pool[g]&.pop } - full]
end
Demo:
> judge(["G","G","G","Y"], ["G","Y","G","G"])
=> [2, 2]
> judge(["X","R","Y","T"], ["T","T","Y","Y"])
=> [1, 1]
I am iterating over an array, and I'm wondering if there's a shorthand to refer to the receiver of #each (or #each_with_index) method from within the iteration.
self returns main.
You should be able to just reference it:
my_thing.each {|one_thing| puts my_thing }
This is pretty similar to the answer I gave here https://stackoverflow.com/a/45421168/2981429 but slightly different.
First off, you can create a scope with self bound to the array, and then execute the each in that scope:
[1].instance_exec do
# in this scope, self is the array
# thus we can use just 'each' because the self is inferred
each do |x|
# note that since 'class' is a special keyword,
# it needs to be explicitly namespaced on self
puts self.class, x
end
end
# => prints Array, 1
You can create a utility function to do this, if you want:
def bound_each(enumerable, &blk)
enumerable.instance_exec { each &blk }
end
bound_each([1]) { |x| puts self.class, x }
# prints Array, 1
You can call your each method within an Object#tap block and reference the original receiver like that.
[1, 2, 3].tap { |i| i.each { |j| p i.dup << j } }
# [1, 2, 3, 1]
# [1, 2, 3, 2]
# [1, 2, 3, 3]
#=> [1, 2, 3]
Here the receiving object is [1, 2, 3] and is passed to the block-variable i which we can use locally or in nested scopes such as each's block.
Avoid modifying the receiving object else you may end up with undesired results such as an infinite array. Using dup could allay this possibility.
This is an interesting question. As far as I know it's not possible – the closest I can come up with would be to use inject (or reduce) and explicitly pass the receiver as an argument. A bit pointless, but there might be a use-case for it that I'm not seeing:
a = [1,2,3]
a.inject(a) do |this, element|
this == a #=> true
this.include?(element) #=> true
this
end
Apart from looking a bit redundant, you have to be very sure to return this at the end of each iteration, as the return value will become this in the next iteration. For that reason (and the fact that you could just reference your collection in an each block, as in David's answer) I don't recommend using this.
Edit - as Simple Lime pointed out in the comments – I missed the obvious Enumerator#with_object, which has the same (rather pointless) effect, but without the drawback of having to return this at the end of each iteration. For example:
a = [1,2,3]
a.map.with_object(a) do |element, this|
this == a #=> true, for each iteration
end
I still don't recommend that you use this though.
Is there a way to return the new array?
I try to return the array of squared value [1, 4, 9] but it keeps returning [1, 2, 3] (the original array) Here is my code:
def square_array(array)
array.each do |number|
number *= number
puts number
end
end
square_array([1, 2, 3])
A much simpler version that does what you want:
def square_array(array)
array.map do |number|
number*number
end
end
The problem with your code is that when you assign something to number, you're just assigning a value to a local variable, not some magic reference into an array.
Try this out:
Using Each method
def square_array(array)
temp = []
array.each do |number|
temp << (number * number)
end
temp
end
When writing semantic ruby, it is best to use proper enumerable methods. There is no need for the temporary variable in this case... we can use the #map method to return a new array that is the result of applying a function to each value in turn. This is a core concept of the functional programming paradigm:
def square_array numbers
numbers.map { |x| x ** 2 }
end
I fund a way, I just needed to push the result in a new array
def square_array(array)
new_array = []
array.each do |num|
squared_num = num ** 2
new_array.push(squared_num)
end
return new_array
end
Ok, reviewing Procs, lambdas, and blocks via this link.
Question on this code:
class Array
def iterate!
self.each_with_index do |n, i|
self[i] = yield(n)
end
end
end
array = [1, 2, 3, 4]
array.iterate! do |n|
n ** 2
end
puts array.inspect
Conceptually, I understand almost everything, except one line which is this:
self[i] = yield(n)
I get that this self in this line self.each_with_index do |n, i| means that it's a class method, right?
But why do we need to assign the parameters in yield(n) to self[i]?
Please explain in super basic way if you can.
(in other words, please be nice - which people generally are for most part here - just a little extra nervous that I'm not getting this which is making me feel stupid)
The method is iterate!, which is an instance method. self in self.each_with_index is the receiver of the method Enumerable#each_with_instance. Since self is the current instance of Array ([1,2,3,4] in your example), self. is not needed; i.e., you could (and imo, should) just write each_with_index do |n, i|.... In other words, self is the implied receiver when no explicit receiver is specified.
Regarding the line:
self[i] = yield(n)
for your example array = [1,2,3,4] your enumerator is:
enum = [1,2,3,4].each_with_index
#=> #<Enumerator: [1, 2, 3, 4]:each_with_index>
with elements
enum.to_a
#=> [[1, 0], [2, 1], [3, 2], [4, 3]]
The first element passed into block by Array#each is therefore [1,0], which is assigned to the block variables:
n = 1
i = 0
resulting in
self[0] = yield(1) => 1**2 => 1
and so on.
I'll try to explain in a super basic way.
I get that this self in this line self.each_with_index do |n, i| means
that it's a class method, right?
Nope. The meaning of self depends on the context. If self was in the class, it would refer to the class. But here self is in an instance method, so it refers to the instance (so each_with_index is also an instance method).
But why do we need to assign the parameters in yield(n) to self[i]?
The goal of iterate! is to modify the array in place. Since self refers to the instance, self[i] accesses the elements of the array that iterate! is being called on, thus modifying the array in place.
Also, I'm not sure what you mean by "parameters" here. yield(n) passes n to the block, runs the block, and returns the value.
self[i] = yield(n) reassigns the values in the array, to the block that was specified in
array.iterate! do |n|
n ** 2
end
which basically means, take the value of the array, and square it, save that value in the element of the array. So [1, 2, 3 , 4] becomes [1 ** 2, 2 ** 2, 3 ** 2, 4 ** 2] => [2, 4, 9, 16]
Self changes with(and actually is) the current context or surrounding object.
Since
self.each_with_index do |n, i|
...
is monkey patching the Array class and is within an instance method iterate!, self refers to the instance itself: in this case the array [1, 2, 3, 4].
You're probably thinking of this:
class some_class
def self.a_class_method
...
which is defined in the context of a class. So self is the class itself(which is also an object), not an instance of that class.
Since self is just the array [1, 2, 3, 4]
self[i] = yield(n)
is replacing each element of the array with results of the sent in block.
Here iterate! is an instance function of Array class and you have an array object.When you do
array.iterate! do |n|
n ** 2
end
You are passing a block 'do |n| n**2 end' to iterate! function.In the function you can access this block using yield.But as you can see block is expecting one parameter through |n| so you need to pass one parameter and the block code will return the square of it.
self[i] = yield(n)
self is being used in Array instance context.So it is modifying the values of array.
For more information please check this article:
http://geekdirt.com/blog/blocks-lambda-and-procs-in-ruby/
I have a class Test:
class Test
attr_accessor :data
def initialize
#data = [0, 1, 2, 3]
end
def map
#data.map!{|i| i = yield i }
end
end
and I attempt to use it like:
a = Test.new
a.map{|i|
if(i==2)
i+=1
break i #<--- -This line is the focus
else
1
end
}
puts a.data
The output I expect is [1, 1, 3, 3]. Instead, I get [1, 1, 2, 3]. The last iteration of the block in map doesn't return the modified i.
I replaced break i with next i. This performed as I expected, and produced the output [1, 1, 3, 1].
How can I modify this piece of code (or, ideally the line I point out in my second code-snippet) so that I would get the output [1, 1, 3, 3]? In other words, how can I make the block finish, but pass one last value back to map? Is there a neat and readable way to do this (besides, say, toggling a boolean flag break_now)?
I'm assuming you're asking how to leave a block and make use of the last value that was calculated rather than how to calculate a specific set of numbers; for the latter, there is probably a clever one-liner.
How about something like this:
class Test
attr_accessor :data
def initialize
#data = [0, 1, 2, 3]
end
def modify
#data.map! {|i| yield i }
end
end
a = Test.new
a.modify do |i|
break i if #done
#done = i == 2
#done ? (i + 1) : 1
end
puts a.data
An additional thought—#map is an important method in Ruby with a specific interface. In your example you're violating the interface by modifying a field in Test. For this reason I've used the name #modify instead.
In general, you could get away with this by modifying the yielded values in place. For example, if your array consisted of Strings instead of Fixnums:
class Test
attr_accessor :data
def initialize
#data = %w{a b c d}
end
def map
#data.map! { |i| yield i }
end
end
a = Test.new
a.map do |i|
if i == 'c'
i.next!
break
else
'b'
end
end
p a.data #=> ["b", "b", "d", "d"]
The problem with your example is this:
Fixnum objects have immediate value. This means that when they are assigned or passed as parameters, the actual object is passed, rather than a reference to that object. Assignment does not alias Fixnum objects. There is effectively only one Fixnum object instance for any given integer value…
Fixnums can't be altered in-place, so your expression i += 1 in the lower block doesn't affect the value of i in the upper block. That's why you get 2 in your example but d in my example.
You have to do this:
a.map{ |i| (i % 2 == 0) ? i + 1 : i }
When you use map function you don't change 'a' variable, if you want change 'a' variable do this:
a.map!{ |i| (i % 2 == 0) ? i + 1 : i }
The new value of 'i' is the value return by the block, so don't do something like:
a.map{|i| i = 1 }
because if you do:
a.map{|i| i = 1; 5 }
the result will be:
[5, 5, 5, 5]