Default values for missing keys in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code with default hash values?

Consider the following Ruby code that uses a hash with a default value. What will be printed?

Ruby

h = Hash.new(0)
h[:a] += 1
h[:b] += 2
puts h[:a]
puts h[:b]
puts h[:c]

A

1
2
0

B

1
2
nil

C

0
0
0

D

1
2
2

💡 Hint

Remember that the default value is returned for missing keys but does not get stored in the hash unless assigned.

The hash h is created with a default value of 0. When h[:a] += 1 runs, it reads the default 0, adds 1, and assigns 1 to :a. Similarly for :b. For :c, it was never assigned, so it returns the default 0.

❓ Predict Output

intermediate

What happens when using a block for default values in Ruby hash?

What will this Ruby code print?

Ruby

h = Hash.new { |hash, key| hash[key] = [] }
h[:x] << 1
h[:y] << 2
puts h[:x].inspect
puts h[:y].inspect
puts h[:z].inspect

A

[1]
[2]
[1]

B

[1]
[2]
[]

C

[1]
[2]
nil

D

[]
[]
[]

💡 Hint

Using a block for default values allows the hash to assign a new object for missing keys.

The block sets the default value to a new empty array and assigns it to the missing key. So when h[:x] << 1 runs, it creates h[:x] = [] then appends 1. Same for :y. For :z, it creates an empty array assigned to :z.

🔧 Debug

advanced

Why does this Ruby hash with default array behave unexpectedly?

Look at this Ruby code:

h = Hash.new([])
h[:a] << 1
h[:b] << 2
puts h[:a].inspect
puts h[:b].inspect
puts h.inspect

What is the output and why?

A

[1, 2]
[1, 2]
{}
Because the default array is shared for all missing keys.

B

[]
[]
{}
Because the default array is empty and not modified.

C

[1]
[2]
{:a=>[1], :b=>[2]}
Because each key gets its own array.

D

[1]
[1]
{:a=>[1], :b=>[1]}
Because the default array is copied for each key.

💡 Hint

Think about whether the default object is shared or duplicated for each key.

Using Hash.new([]) sets the same array as the default object for all missing keys. So appending to h[:a] actually modifies the shared default array. The hash itself remains empty because no keys were assigned explicitly.

📝 Syntax

advanced

Which Ruby hash default value syntax is correct?

Which of the following Ruby hash initializations correctly sets a default value of 10 for missing keys?

A h = Hash.new { |h, k| 10 }

B h = Hash.new { 10 }

C h = Hash.new(10)

D h = Hash.new { |h, k| h[k] = 10 }

💡 Hint

Only one option assigns the default value to the key in the hash.

Option D uses a block that assigns 10 to the missing key, so the key is stored with value 10. Options A, B, and D return 10 as default but do not store it in the hash.

🚀 Application

expert

How many keys are in the hash after this Ruby code runs?

Given this Ruby code:

h = Hash.new { |hash, key| hash[key] = key * 2 }
h[1]
h[2]
h[3]
count = h.size

What is the value of count after running this code?

A 1

B 0

C 3

D 2

💡 Hint

Accessing a missing key with a block default assigns a value to that key.

Each time h[1], h[2], and h[3] is accessed, the block assigns key * 2 to that key in the hash. So after these three accesses, the hash has 3 keys.