Arrays

Question 1

What does “each” method return on an array?

Answer 1

It always returns the original array.
e.g.
x = [1, 2, 3, 4, 5]
x.each do |a|
a + 1
end

=>[1, 2, 3, 4, 5]

Question 2

What do “array.first” and “array.last” do? (+examples)

Answer 2

They return the first and the last element of the array.

e.g.
array = [1,2,3,4,5]

array.first
=>1
array.last
=>5

Question 3

What are the two result

options after a method is evaluated?

Answer 3

It either returns new data or modifies the original data.

new data:(does not mutate the caller)
.sort (arranges numbers in the array in order)

modifies: (mutate the caller)
.pop (takes the last element of the array)

Question 4

How do you add an argument to the beginning and take one away from the end of a(n) array/list?

Answer 4

a = [3,4,5,6]

a.unshift(10)
=> [10,3,4,5,6]

a.pop
=>[10,3,4,5]

Question 5

How to add an item back to the array permanently?

Answer 5

a = [2,5,6]

a.push(3)
=>[2,5,6,3]

Question 6

What is the alternative to the .pop method?

Answer 6

«

a = [2,3,4,6]

a &laquo_space;6
=>[2,3,4]

Question 7

What is the difference between .map and .collect methods and how do they work?

Answer 7

They are the same.

a = [1,2,3,4]

a.collect {|num| num*5}
=> [5,10,15,20]

a.map {|num| num*5}
=> [5,10,15,20]

Question 8

What is the difference between the “delete” and “delete_at” methods?

Answer 8

Both can delete an element
from the array, the “delete_at” targets it by its index (array[2]) while the “delete” targets it by its value (array[“lemon”]).
Both mutate the caller.

Question 9

How can you iterate through an array and delete the duplicated values?

Answer 9

With the .uniq method.
e.g.
array = [1,2,3,4,2,1,3,2,1]
array.uniq
=> [1,2,3,4]

It doesn’t mutate the caller.

Question 10

Write a method that iterates through an array and return those numbers that are less than 8!
array = [2,6,9,8,12,1,4,7,25]

Answer 10

array.select {|num| num < 8}

Question 11

What is the alternative to the .pop method?

Answer 11

«

a = [2,3,4,6]

a &laquo_space;6
=>[2,3,4]

Question 12

What is the difference between .map and .collect methods and how do they work?

Answer 12

They are the same.

a = [1,2,3,4]

a.collect {|num| num*5}
=> [5,10,15,20]

a.map {|num| num*5}
=> [5,10,15,20]

Question 13

What is the difference between the “delete” and “delete_at” methods?

Answer 13

Both can delete an element
from the array, the “delete_at” targets it by its index (array[2]) while the “delete” targets it by its value (array[“lemon”]).
Both mutate the caller.

Question 14

How can you iterate through an array and delete the duplicated values?

Answer 14

With the .uniq method.
e.g.
array = [1,2,3,4,2,1,3,2,1]
array.uniq
=> [1,2,3,4]

It doesn’t mutate the caller.

Question 15

Write a method that iterates through an array and return those numbers that are less than 8!
array = [2,6,9,8,12,1,4,7,25]

Answer 15

array.select {|num| num < 8}

=> [2,6,1,4,7]

Question 16

What does the bang suffix (!) indicate usually?

Answer 16

It indicates that the method will change (or mutate) the caller permanently.

Question 17

How do you compare arrays?

Answer 17

With the equality operator (==)
The result is a boolean value.

e.g.
a = [1,2,3]
b = [2,5,6]
=> false

Question 18

What do you use “include?” method for?

Answer 18

It checks to see if the argument given is included in the array.
It returns a boolean value.

e.g.
a = [2,6,8,9]
a.include?(8)
=> true

Question 19

How do you turn an array containing nested arrays into a one-dimensional array?

Answer 19

Use the ".flatten" method!
e.g.
a = [1,2, [3,4,5], [2,9]
a.flatten
=>[1,2,3,4,5,2,9]

It doesn’t mutate the caller.

Question 20

What is the difference between “.each”, “.each_index” and “.each_with_index”?

Answer 20

They all iterate through an array.
“.each” represents the value at each index
“.each_index” represents the index number
“.each_with_index” can manipulate both the value and the index by passing two parameters to the block of code.

e.g.
a = ["a", "b", "c"]
a.each_with_index {|val, idx| puts "#{idx+1}. #{val}" }
1. a
2. b
3. c

Question 21

What does the .sort array do?

Answer 21

It is a handy way to order an array.
a = [2,9,3,5,4,6]
a.sort
=> [2,3,4,5,6,9]

It mutates the caller if we add (!).
It works with strings and floats as well.

Question 22

Which method combines two arrays and returns their combination of all elements as an array?

Answer 22

The “.product” method.
e.g.
[“a”, “d”].product([1,3,8])
=> [[“a”, 1], [“a”, 3], [“a”, 8], [“b”, 1], [“b.”, 3], [“b”, 8]]

Question 23

What is the difference between “.each” and “.map”?

Answer 23

Both iterate over a collection.
".each" it does the operation and returns the original value.
".map" creates and returns a new array containing the values by the block.
e.g.
a = [1,2,3]
a.each {|e| print e}
1,2,3 
=> [1,2,3]

a = [1,2,3]
a.map {|e| e**2}
=>[1,4,9]

Question 24

Insert 3 into the following array, after 1!

array = [1,2,3,6,9]

Answer 24

puts array.insert(1, 3)
=> [1,3,2,3,6,9]

(inserts before the element’s index, in this case we want it after 1 which is index 0, next element (3) is index 1 that’s why we use it)

Question 25

What is the value of a and why?

a = 7
array = [1,2,3]

array.each do |a|
a += 1
end

puts a

Answer 25

7 because the second a is initialized inside the block and executes on the array and never sees the first a that is initialized in the outer scope, that’s why a stays 7.
this is called shadowing.
(when a block argument hides a local variable that is defined outside the block)

Question 26

What does .each method return?

Answer 26

It returns the original array it was called on.

e.g.
array = [1,2,3]

array.each do |a|
a += 1
end

puts array
=> [1,2,3]

Question 27

What does %w() represents?

Answer 27

It represents an array!
e.g.
%w(y n) is the same as [‘y’, ‘n’]

Question 28

What does the sample method do on an array?

Answer 28

It chooses a random element from the array.

a = [1,2,3,4,5,6,7,8,9]
a.sample
=> 7

Question 29

What does “select” method do?

Answer 29

Returns a new array containing all elements of array for which the given block returns a true value.
e.g.
numbers = [5,9,21,26,39]
div_by_three = numbers.select do |number|
number % 3 == 0
end

p div_by_three
=> [9,21,39]

Question 30

What is the difference between “select” and “map” methods?

Answer 30

It is the way the new element’s value is chosen.
“map” returns a new array with each element transformed based on the block’s return value.
“select” returns a new array containing elements selected only if the block’s return value evaluates to true.