Written Assessment Study Guide

Question 1

method definition

Answer 1

A method definition consists of the keyword def followed by the method name, optional parameter, optional body, and the end keyword. A method invocation involves an object calling the method and (optionally) passing in arguments.

Question 2

Implicit Return Value of Method Invocations and Blocks

Answer 2

Methods and block implicitly return the last evaluated expression in the given method or block, unless we use explicit return.

Question 3

Array#sort

Answer 3

The #sort method implements the spaceship operator, <=> to compare and sort elements in a collection. By default, the sort method will order elements in non-descending order (least to greatest). If you pass in a block, you can change the ordering of the elements.

<=> is a method that accepts two operands, the calling object and an argument, e.g. 1 and 2 in 1 <=> 2. The spaceship operator returns one of 4 values when comparing operands:
-1 When the calling object (left operand) is less than the right, e.g. 1 <=> 2
0, when the calling object and right operand have the same value.e.g., 1<=>1
1, when the calling object has a greater value than the right operand. 2<=>1
nil, when the calling object and the right operand are of different data types and uncomparable, e.g. 31 <=> ‘12’, we are unable to use the spaceship operator to compare a number with a string*. *unless you override the spaceship operator and redefine the operator.

Question 4

Is Array#sort mutating?

Answer 4

No. This sort method will return a new array containing elements from the calling object that we’ve sorted according to the value of the block.

Question 5

Truthiness

Answer 5

Truthiness is a quality that describes whether an object will evaluate to true or false in a boolean context.

Question 6

Which values are truthy?

Answer 6

All values are truthy, except for false or nil.

Question 7

How do truthy/falsy variables evaluate in a boolean context?

Answer 7

Variables that are truthy evaluate to true in a boolean context.
Variables that are falsy evaluate to false in a boolean context.

Question 8

Is truthy the same as true?

Answer 8

No. For instance, the value 1 is truthy, but 1 == true would evaluate to false.

Question 9

Is falsy the same as false?

Answer 9

No. Falsy is not the same as false. For instance, nil is falsy, but nil == false would evaluate to false because nil and false are different objects.

Question 10

Variables as pointers

Answer 10

Variables store pointers, not values, to an address spaces in memory.

a = “hi there”
b = a
a = “not there”
We reassign a to “not there”. However, evaluating b will still return “hi there” because b contains a pointer to “hi there”.

Question 11

Mutating strings

a = “hi”
b = a
a &laquo_space;”, Bob”

Answer 11

We mutate the string referenced by a, which is also referenced by b. Therefore, evaluating b will return hi, Bob

Question 12

What’s a, b, and c? What if the last line was c = a.uniq!?

a = [1, 2, 3, 3]
b = a
c = a.uniq

Answer 12

a and b point to [1,2,3,3]. c points to [1,2,3]. If the last line was c = a.uniq!, then a, b, and c would point to [1,2,3].

Question 13

What is a after the test method returns? Did the method modify the value of a?

def test(b)
b.map {|letter| “I like the letter: #{letter}”}
end

a = [‘a’, ‘b’, ‘c’]
test(a)

Answer 13

When we use variables to pass arguments to a method, we’re essentially assigning the value of the original variable (a in this case) to a variable inside the method (b). This is equivalent to executing b = a. Inside the method, the operations we perform on the b variable determine whether the value of a will change. Some operations, like map, will have no affect on a. Others, like map! will mutate the value assigned to a.

Question 14

puts vs return

Answer 14

Ruby methods ALWAYS return the evaluated result of the last line of the expression unless an explicit return comes before it.

Question 15

What happens when we add an explicit return to this code?

def add_three(number)
return number + 3
number + 4
end

returned_value = add_three(4)
puts returned_value

Answer 15

We output 7 because we return the value of number + 3 out of the add_three method using the return keyword. We don’t execute number + 4.

Question 16

When are hashes better than arrays?

Answer 16

When you want to quickly access specific data without having to know the exact index or searching through the pile of data one by one.

Question 17

Array#map

Answer 17

The map method takes each element of an array, passes [the element] to a block, and builds a new array out of the values the block returns.

The map method adds the block’s return value itself to a new array.

Question 18

def map
new_arr = []
self.each do |element|
new_arr &laquo_space;yield(element)
end
new_arr
end

some_array.map do |element|
# does something
end

Answer 18

An implementation for map could look like the above. What’s happening is that map will iterate over each element in the collection we invoked map on and pass in the element to a do..end block. Within this block, we yield control to a block and pass in the element as an argument to the block. The block then returns a value back to yield, and we add this return value to the new_arr. At the end, map returns a new array consisting of the elements returned by the block passed into map.

Question 19

select method

Answer 19

select evaluates the return value of the block. The block returns a value on each iteration, which then gets evaluated by select. select only cares about the truthiness of the block’s return value. If the return value of the block is “truthy”, then the element during that iteration will be selected. If the return value of the block is “false” then the element will not be selected.

Question 20

each method

Answer 20

each sends the value of the current element to the block in the form of an argument. In this block, the argument to the block is num and it represents the value of the current element in the array. The code within the block is executed for each iteration.