Ruby : Rubeque - difficult with *n or n - ruby

I'm doing http://www.rubeque.com/problems/queue-continuum/solutions/51a26923ba804b00020000df and I spent a while there. I can't realize why this code doesn't pass
def initialize(queue)
#q = queue
end
def pop(n=1)
#q.shift(n)
end
def push(arr)
arr.each { |x|
#q.push(x)
}
return true
end
def to_a
#q
end
but this works perfectly.
def initialize(queue)
#q = queue
end
def pop(*n)
#q.shift(*n)
end
def push(arr)
#q.push(*arr)
return true
end
def to_a
#q
end
i'm totally confused about
def pop(*n)
#q.shift(*n)
end
and
def push(arr)
#q.push(*arr)
end
why should I take (arr) as array and than change it into... *arr which is Array of array? I'm confused, please help!

The splat works in two ways.
When receiving arguments, it combines arguments into an array.
def foo *args; args end
foo(1) # => [1]
foo(1, 2, 3) # => [1, 2, 3]
When giving arguments, it decomposes an array into arguments.
def bar x, y, z; y end
bar(*[1, 2, 3]) # => 2
def baz x; x end
baz(1) # => [1]
baz(1, 2, 3) # => Error
The *arr you are wondering is the latter case. It is not an object like [1, 2, 3] (hence, not an array of arrays). It is a part of arguments (like 1, 2, 3) passed to a method.
There are other uses of splats (as in array literals, case statements, etc.), but their function is either of the two uses above.

Related

Why index self instead of instance variable in bracket method

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>'

How do I make a function that accepts an anonymous function using 'do' keyword syntax?

When using array.each you can specify the function in two forms:
Curly Braces:
a = [1,2,3]
a.each { |x| puts x * x }
Output:
1
4
9
=> [1, 2, 3]
'do' Syntax:
a = [1,2,3]
a.each do |x|
puts (x * x)
end
Output:
1
4
9
=> [1, 2, 3]
Question:
How can I replicate the 'do' syntax style with my own custom function? The closest to the curly brace style I can get is:
What I've tried:
def PutWith2Arg(proc)
puts proc.call(2)
end
PutWith2Arg(Proc.new { |x| x + 100 })
Output:
102
=> nil
The do |foo| … end and { |foo| … } syntaxes are equivalent. These are 'blocks' in Ruby, and any method can get them. To call them you need to either:
def my_method # No need to declare that the method will get a block
yield(42) if block_given? # Pass 42 to the block, if supplied
end
my_method do |n|
puts "#{n} times 2 equals #{n*2}"
end
#=> "42 times 2 equals 84"
my_method{ |n| puts "#{n} times 2 equals #{n*2}" }
#=> "42 times 2 equals 84"
my_method # does nothing because no block was passed
or, for more sophisticated uses:
def my_method( &blk ) # convert the passed block to a Proc named blk
blk.call( 42 ) if blk
end
# Same results when you call my_method, with or without a block
The latter style is useful when you need to pass the block on to another method. If you have a Proc or Lambda referenced by a variable, you can pass it to a method as the block for that method using the & syntax:
def my_method( &blk ) # convert the passed block to a Proc named blk
[1,2,3].each( &blk ) # invoke 'each' using the same block passed to me
end
my_method{ |x| p x=>x**2 }
#=> {1=>1}
#=> {2=>4}
#=> {3=>9}
For more details, this webpage is fairly instructive.

Return Enumerator without using existing iterators

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

Pass parameters to passed-in block in Ruby

I want to pass a block to a function, and then call that block with some additional parameters as follows:
def foo(&block)
some_array = (1..3).to_a
x = 7 # Simplified
result = some_array.map &block # Need some way to pass in 'x' here
end
def a_usage_that_works
foo do |value|
value
end
end
def a_usage_that_doesnt_work
foo do |value, x|
x # How do I pass in x?
end
end
# rspec to demonstrate problem / required result
describe "spike" do
it "works" do
a_usage_that_works.should == [1,2,3]
end
it "doesn't work" do
a_usage_that_doesnt_work.should == [7, 7, 7]
end
end
How can I pass in the additional parameter to the block?
Create another block and call first one from it.
def foo(&block)
some_array = (1..3).to_a
x = 7 # Simplified
result = some_array.map {|elem| block.call(elem, x)}
end
You pass to the block by yielding to it.
def foo(&block)
some_array = [1,2,3]
x = 7
some_array.map{|el| yield el, x}
end
p foo{|p1, p2| p2} #=>[7,7,7]
p foo{|p1, p2| p1} #=>[1,2,3]
You can use a higher-order function to generate a simplified function:
Let's assume that the block we pass to foo will accept value, x.
Naive strategy, using an inline-defined x:
def foo(&block)
some_array = (1..3).to_a
x = 7
simple_func = proc {|value| block.call(value, x) }
result = some_array.map &simple_func
end
Strategy using separation of concerns:
def get_simple_func(block)
# This assumes x won't change per iteration.
# If it can change, you can move the calculation inside the proc.
# Moving it inside also allows the calculation to depend on "value", in case you want that.
x = complex_calculation_for_x()
proc {|value| block.call(value, x) }
end
def foo(&block)
some_array = (1..3).to_a
simple_func = get_simple_func(block)
result = some_array.map &simple_func
end
Obviously you shouldn't use this when x is a literal value because it would be over-engineering. But as the calculation of x becomes more complex, separating it out makes the code more readable. Also, foo can focus on the specific task of applying the function to some_array.

How do I call a method, given its name, on an element of an array?

How do I call a method, given its name, on an element of an array?
For example, I could have:
thing = "each"
I want to be able to do something like:
def do_thing(thing)
array = [object1,object2]
array[0].thing
end
so that do_thing(to_s), for example, would run object1.to_s.
You can use public_send or send. public_send only sends to public methods while send can see public and private methods.
def do_thing(thing)
array = [1,2,3]
array.public_send(thing)
end
do_thing('first')
# => 1
do_thing(:last)
# => 3
Update A more general version:
def do_thing(array, index, method, *args)
array[index].public_send(method, *args)
end
do_thing([1, 2, 3], 0, :to_s)
# => "1"
do_thing([[1,2], [3, 4]], 0, :fetch, 0)
# => 1
require 'ostruct'
o = OpenStruct.new(attribute: 'foo')
do_thing([o], 0, :attribute=, 'bar')
o.attribute == 'bar'
# => true
Object#send
thing = "each"
def do_thing(thing)
array = [1,2,3]
array.send(thing)
end
From the doc:
class Klass
def hello(*args)
"Hello " + args.join(' ')
end
end
k = Klass.new
k.send :hello, "gentle", "readers" #=> "Hello gentle readers"
Here is an example to help you out although I don't have any idea what objects are residing inside your array:
arr = [Array.new(2,10),"abc" ]
arr.each{|i| p i.send(:length)}
#>>2
#>>3

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