w1d2 revisions

Question 1

When we want to select a certain number of items from an array, what method should we use?

Answer 1

Array#select

Question 2

What does #dup do? When is it useful?

Answer 2

It is an inherited method from the Object class, and returns a copy of the object. It is useful when we do not want to modify the original object and/or ‘self’

Question 3

What is the shorthand for reassigning values to multiple objects? How can we use it to swap the values of two variables?

Answer 3

var1, var2 = new_var1, new_var2

-‘var1’ now has the value of new_var1, var2 now has the value of new_var1

We can swap the values of variables like so:

var1, var2 = var2, var1

Question 4

What would be the code to quickly get an array of lines from a file?

Answer 4

lines = File.readlines(‘filename’).map(&:chomp)

Question 5

What is the following syntax short for?

arr.map(&:upcase)

Answer 5

arr.map do |elem|
elem.upcase
end

Question 6

T/F: Enumerator#each returns the original object that has been modified by the code block.

Answer 6

T

Question 7

As soon as we find ourselves adding a ‘!’ to a condition in an ‘if’ or ‘while’, what should we use instead?

Answer 7

‘unless’ or ‘until’, respectively. They are much cleaner.

Question 8

What would be the code to quickly retrieve the first element of each element of a multidimensional array?

Answer 8

starts = arr.map(&:first)

USE MAP TO RETURN A NEW ARRAY AND EACH TO EITHER RETURN THE ORIGINAL OBJECT OR THE ORIGINAL OBJECT AFTER IT HAS BEEN MUTATED

Question 9

What is a shorthand to sample a value from a range?

Answer 9

rand(start..end)

Question 10

Explain the ‘’ operator

Answer 10

It returns -1 if the left-hand operand is less than the right hand operator, 1 if the opposite, and 0 if they are equal.

Question 11

What does File::extname return?

Answer 11

The file extension

Question 12

How can File::basename return ONLY the filename and not the extension?

Answer 12

File.basename(‘filepath’, “.*”)

Question 13

When should we use Enumerator#any?

Answer 13

When we could use #each, but we only want to check if something occurs at least once:

def has_conflict?(new_course)
self.courses.any? do |enrolled_course|
new_course.conflicts_with?(enrolled_course)
end
end

Question 14

How can we raise a generic error?

Answer 14

raise “error message” if condition

Question 15

How can we raise a specific class of error?

Answer 15

raise ErrorClass.new(“error message”) if condition

Question 16

What is a case statement and when should we use it?

Answer 16

Basically a switch statement; declare something and check if it evaluates to each of the when statements:

case (statement)
when value
  actions
when another_value
  actions
else
  default_actions
end

Question 17

What is the method to round numbers correctly (i.e. not always down)

Answer 17

Fixnum#fdiv

Question 18

When should we use the ‘=~’ operator?

Answer 18

When we are checking to see if a string contains a certain regex.

Question 19

Explain the following code:

Array.new(3) {Array.new(3)}

Answer 19

Creates a multidimensional array of 3 by 3

Question 20

T/F: Putting ‘.join’ after the ‘end’ keyword will not do anything.

Answer 20

F; it serves to concatenate outputs.

Question 21

What is the difference between:

a, b = [1, 2, 3]

a, *b = [1, 2, 3]

Answer 21

In the first example, a = 1 and b = 2

In the second example, a = 1 and b = [2, 3]; the ‘*’ operator here means to catch all remaining elements left over after assignment

Question 22

Why bother with passing |x, y| in the following example:

[arr1, arr2].map do |arrs|
arrs.map{|x, y| newarr[x][y]
end

Answer 22

B/C there are two elements in the array passed to the first ‘map’, we need to account for the elements within the second map.

Question 23

Assuming rows, cols and diagonals are multidimensional arrays, what does the following code do?

def winner 
    (rows + cols + diagonals).each do |triple| 
       return :x if triple == [:x, :x, :x] 
       return :o if triple == [:o, :o, :o] 
     end

Answer 23

It puts the arrays together into one new array with 3 sub arrays. The each statement grabs the nth element of each sub array and passes them as a new array to ‘triple’