I'm trying to call a method I defined earlier in the code and it gives me this error:
"undefined method `filter' for 0:Integer (NoMethodError)"
This is the code:
def descending_order(n)
#arrNum = n.to_s.chars.map {|i| i.to_i }
#final = [#arrNum[0]]
def self.filter(arr, ind)
if self > arr[ind]
#final.insert(ind, self)
else
self.filter(arr, ind+1)
end
end
#arrNum.each { |i| i.filter(#final, 0) }
return #final
end
I tried everything I could think of! Thanks
Your approach is wrong!
If your intention is to return a reverse sorted array of a given number then the code snippet should be:
def descending_order(n)
n.digits.sort.reverse
end
The above snippet returns following output
puts descending_order(352614)
=> [6, 5, 4, 3, 2, 1]
Related
When I try to call a method with select and num.even? as follows,
def selection(array)
puts "This is inside the method"
return yield(array)
end
collection = [1,2,3,4,5]
selection(collection.select) { |num| num.even? }
I get a no defined Method error:
undefined method `even?' for #<Enumerator: [1, 2, 3, 4, 5]:select>
I'm looking for a return of even numbers in the array. I can get the select even? combo work in other examples of an array.
Array#select returns an Enumerator instance if no block was given to it
then you call selection method passing the result of call to collection.select as an argument and { |num| num.even? } as block
inside your selection function you yield the argument (an Enumerator instance) to the block
in the block you call even? on the block argument, resulting in the error message you receive.
I am unsure what’s wrong with collection.select(&:even?), but if you want to re-implement it yourself, here you go:
def selection(array)
# convention: return enumerator unless block is given
return enum_for(:selection) unless block_given?
enumerator = array.each
result = []
loop do
(value = enumerator.next) rescue return result
result.push(value) if yield value
end
end
selection([1,2,3,4,5]) { |num| num.even? }
#⇒ [2, 4]
You are seeing this error because you are passing an enumerator object in to your "selection" method ... that is, the result of "collection.select" is an Enumerator and enumerators do not implement an "even" method.
I believe that you are trying to implement your own version of "select". The following is one way to achieve your stated intent: "I'm looking for a return of even numbers in the array."
def selection(array)
results = []
for item in array do
results << item if yield item
end
results
end
collection = [1,2,3,4,5]
puts selection(collection) { |num| num.even? }
# => [2,4]
https://mixandgo.com/learn/mastering-ruby-blocks-in-less-than-5-minutes is a nice reference
yield(array) is passing the whole array in one go to the block given to the method, so it is trying to call even? on the array.
I have:
class Thing
def initialize
#array = [[0, 0, 0], [1, 1, 1]]
end
end
thing = Thing.new
The normal way to access an element in #array is to use [] as in:
#array[0][1] # => 0
I am trying to overwrite [] so as to get results like this:
position_array = [0, 1]
#array[position_array] # => 0
This is my attempt:
class Thing
def [](position_array)
index_row, index_col = position_array
#array[index_row][index_col]
end
def get_value(position_array)
#array[position_array] # doesn't work
# self[position_array] # does work
end
end
thing.get_value([0, 1])
# >> 'get_value': no implicit conversion of Array into Integer (TypeError)
Why do I need to index the Thing object in order to index #array?
Just think of message and receiver.
#array[position_array] sends the message [] to the receiver #array. #array is an instance of Array, so the method Array#[] gets invoked.
self[position_array] sends the message [] to the receiver self. Within instance methods, self refers to that instance. And because self is an instance of Thing, the method Thing#[] gets invoked.
Since Thing is a subclass of Object and not a subclass of Array (nothing wrong here, you shouldn't subclass Array anyway), your implementation of [] does not override Array#[]. Both methods are totally independent of each other, just like String#[] or Hash#[].
This is how I would approach it:
class Thing
def initialize
#array = [[1, 2, 3], [4, 5, 6]]
end
def [](i, j)
#array[i][j]
end
end
thing = Thing.new
thing[0, 1] #=> 2
thing[1, 1] #=> 5
You could use a prepended method to non-invasively override the [] method in Array by duck-typing the parameter passed to the [] method, and then calling the original if its not what you expect. Then you don't need a Thing object at all.
module MyArrayExtension
def [] (*param)
if param.size == 2
row, col = param
raise ArgumentError, 'Row must be an integer' if row.class != Integer
raise ArgumentError, 'Column must be an integer' if col.class != Integer
raise ArgumentError, "Element at row #{row} is not an array" if self[row].class != Array
self[row][col]
else
super
end
end
end
class Array
prepend MyArrayExtension
end
thing = [[1,2,3],[4,5,6]]
puts "The 2D array is: #{thing}"
puts "Extension used on the thing to get at element 1 of first array:"
puts thing[0,1]
puts '-' * 20
normal = [1,2,:blah,4,5]
puts "Normal array is #{normal}"
puts "Original [] method used to get the 3rd element:"
puts normal[2]
puts '-' * 20
puts "Using the extension on the non-2D array:"
puts normal[0,1]
The output of this program is:
The 2D array is: [[1, 2, 3], [4, 5, 6]]
Extension used on the thing to get at element 1 of first array:
2
--------------------
Normal array is [1, 2, :blah, 4, 5]
Original [] method used to get the 3rd element:
blah
--------------------
Using the extension on the non-2D array:
./test.rb:9:in `[]': Element at row 0 is not an array (ArgumentError)
from ./test.rb:35:in `<main>'
When trying to pass a block to the sum method:
def sum(list, &block)
list.find_all{yield}.reduce(0, :+)
end
sum([12, 14, 0, 7, 56, 0]) {|i| i % 2 == 0}
I get this error:
NoMethodError: undefined method `%' for nil:NilClass
My method couldn't recognize the i as an element in my list. I don't know how fix it. Any suggestion?
list.find_all { |i| yield i }.reduce(0, :+)
or equivalently
list.find_all(&block).reduce(0, :+)
What you originally wrote searched through the list so that for each element you yield - this would invoke the block with no parameters, i gets assigned nil, and nil % 2 is bad.
How do I return Enumerator from my array wrapper without using already existing array iterators?
class MyArray
def initialize
#inner = []
end
def each
index = 0
while index < #inner.size
yield #inner[index] if block_given?
index += 1
end
end
end
I can't figure out how to avoid calling things like #inner.each at the end of the each method.
Given:
#inner = [1, 2, 3]
Code
#inner.to_enum
will return an enumerator.
enum = #inner.to_enum
enum.each{|e| p e}
# => 1, 2, 3
I am new to Ruby, is there a way to yield values from Ruby functions? If yes, how? If not, what are my options to write lazy code?
Ruby's yield keyword is something very different from the Python keyword with the same name, so don't be confused by it. Ruby's yield keyword is syntactic sugar for calling a block associated with a method.
The closest equivalent is Ruby's Enumerator class. For example, the equivalent of the Python:
def eternal_sequence():
i = 0
while True:
yield i
i += 1
is this:
def eternal_sequence
Enumerator.new do |enum|
i = 0
while true
enum.yield i # <- Notice that this is the yield method of the enumerator, not the yield keyword
i +=1
end
end
end
You can also create Enumerators for existing enumeration methods with enum_for. For example, ('a'..'z').enum_for(:each_with_index) gives you an enumerator of the lowercase letters along with their place in the alphabet. You get this for free with the standard Enumerable methods like each_with_index in 1.9, so you can just write ('a'..'z').each_with_index to get the enumerator.
I've seen Fibers used in that way, look at an example from this article:
fib = Fiber.new do
x, y = 0, 1
loop do
Fiber.yield y
x,y = y,x+y
end
end
20.times { puts fib.resume }
If you are looking to lazily generate values, #Chuck's answer is the correct one.
If you are looking to lazily iterate over a collection, Ruby 2.0 introduced the new .lazy enumerator.
range = 1..Float::INFINITY
puts range.map { |x| x+1 }.first(10) # infinite loop
puts range.lazy.map { |x| x+1 }.first(10) # [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
Ruby supports generators out of the box using Enumerable::Generator:
require 'generator'
# Generator from an Enumerable object
g = Generator.new(['A', 'B', 'C', 'Z'])
while g.next?
puts g.next
end
# Generator from a block
g = Generator.new { |g|
for i in 'A'..'C'
g.yield i
end
g.yield 'Z'
}
# The same result as above
while g.next?
puts g.next
end
https://ruby-doc.org/stdlib-1.8.7/libdoc/generator/rdoc/Generator.html
Class Enumerator and its method next behave similar
https://docs.ruby-lang.org/en/3.1/Enumerator.html#method-i-next
range = 1..Float::INFINITY
enumerator = range.each
puts enumerator.class # => Enumerator
puts enumerator.next # => 1
puts enumerator.next # => 2
puts enumerator.next # => 3