Array

Question 1

What is partition method?

Answer 1

Returns two arrays, the first containing the elements of enum for which the block evaluates to true, the second containing the rest.

(1..6).partition { |v| v.even? } #=> [[2, 4, 6], [1, 3, 5]]

Question 2

How to check that array contain only one item?

Answer 2

[‘fa’].one? #=> true
[1, 3, 2].one? #=> false

Question 3

Why do we need count method? 3 options

Answer 3

Returns the number of elements.

If an argument is given, counts the number of elements which equal obj using ==.

If a block is given, counts the number of elements for which the block returns a true value

ary = [1, 2, 4, 2]
ary.count #=> 4
ary.count(2) #=> 2
ary.count { |x| x%2 == 0 } #=> 3

Question 4

What is chunk method?

Answer 4

Enumerates over the items, chunking them together based on the return value of the block. Return enumerator. Breaks and start chunking into another array when a condition inside the block changed.

[3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5].chunk { |n|
n.even?
}.each { |even, ary|
p [even, ary]
}
#=> [false, [3, 1]]
# [true, [4]]
# [false, [1, 5, 9]]
# [true, [2, 6]]
# [false, [5, 3, 5]]

Question 5

How to use map with index?

Answer 5

array.map.with_index { |item, index| … }

[:a, :b, :c].map.with_index(2).to_a
#=> [[:a, 2], [:b, 3], [:c, 4]]

Question 6

How to sort an array by the length in 2 directions?

Answer 6

arr.sort_by(&:length)
arr.sort_by(&:length).reverse

Question 7

How to sort an array by several conditions?

Answer 7

You can pass the condition as an array to the block

arr.sort_by { |a| [-a.length, a] } # sort by length and by lexical order

Question 8

How to implement the method ‘each’?

Answer 8

=> 2

class Array
def each
index = 0
length = self.size

while index < length
  yield(self[index])
  index += 1
end   end end

[1,2,3].each { |a| puts a + 1 }

# => 3
# => 4

Question 9

How to implement the method ‘map’?

Answer 9

=> 2

class Array
def map
index = 0
new_array = []

while index < self.length
  new_array << yield(self[index])
  index += 1
end

new_array   end end

[1,2,3].map { |a| a + 1 }

# => 3
# => 4

Question 10

How to compare 2 objects of custom class?

Answer 10

include Comparable and define <=>

class Animal
include Comparable

attr_reader :age

def initialize(age)
@age = age
end

def <=>(value)
if self.age < value.age
-1
elsif self.age == value.age
0
elsif self.age > value.age
1
end
end
end

animal1 = Animal.new(10)
animal2 = Animal.new(20)

p animal1 <= animal2 #=> true

Question 11

How to add array functionality to your custom class?

Answer 11

include Enumerable module and define method ‘each’

class WordSplitter
include Enumerable

attr_reader :word

def initialize(word)
@word = word
end

def each
word.chars.each do |char|
yield(char)
end
end

end

WordSplitter.new(‘hello’).map { |a| a + ‘a’ }

Question 12

How to check that all values of the array are true for specif conditions?

Answer 12

array.all?(&:even?)

Question 13

How to group array elements by 2 with offset by 1?

Answer 13

[1,2,3,4].each_cons(2).each { |a| p a }
# =?
[1, 2]
[2, 3]
[3, 4]

Question 14

How to return first or last 3 elements of array?

Answer 14

arr.first(3)
arr.last(3)

Question 15

What does ‘splat operator’ mean?

Answer 15

numbers = *[1, 2], 3

Question 16

Why do we need ‘lazy’ method?

Answer 16

Never ends

Lazy method allows you not to calculate everything, but just finished when the condition won’t work anymore

(1..Float::INFINITY).select(&:prime?).first(10)

(1..Float::INFINITY).lazy.select(&:prime?).first(10)

Question 17

What is the alias ( or similar method ) for reject! and select! ?

Answer 17

keep_if # select!
delete_if # reject!

Question 18

What is ‘cycle’ method?

Answer 18

arr = [1, 2, 3]
arr.cycle(2) { |x| puts x } # 1, 2, 3, 1, 2, 3