Symbol keys vs string keys decision in Ruby - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of hash key access with symbol vs string

What is the output of this Ruby code?

Ruby

h = { foo: 'bar' }
puts h[:foo]
puts h['foo']

A

bar

B

bar

nil

C

nil
bar

D

nil
nil

💡 Hint

Remember that symbol keys and string keys are different in Ruby hashes.

In Ruby, a hash with symbol keys does not recognize string keys as the same. So h[:foo] returns 'bar', but h['foo'] returns nil.

🧠 Conceptual

intermediate

Why choose symbol keys over string keys?

Which of the following is the main reason to prefer symbol keys over string keys in Ruby hashes?

A Symbols are immutable and reused, saving memory compared to strings.

B Symbols are mutable and can be changed at runtime.

C Symbols automatically convert to strings when used as keys.

D Symbols allow faster string concatenation.

💡 Hint

Think about memory and performance when using symbols as keys.

Symbols are immutable and reused internally, so using them as hash keys saves memory and can improve performance compared to strings.

❓ Predict Output

advanced

Effect of string keys mutation on hash keys

What is the output of this Ruby code?

Ruby

key = 'name'
h = { key => 'Alice' }
key << '!' 
puts h['name']
puts h['name!']

A

Alice

nil

B

nil

Alice

C

nil

nil

D

Alice

Alice

💡 Hint

Consider what happens when you mutate the string used as a key after the hash is created.

The hash key is the original string 'name'. Mutating the string variable after adding it as a key does not change the existing key in the hash. So h['name'] returns 'Alice', but h['name!'] returns nil.

🔧 Debug

advanced

Why does this hash key lookup fail?

Given this code, why does h[:name] return nil?

Ruby

h = { 'name' => 'Bob' }
puts h[:name]

A Because the key :name is misspelled.

B Because the hash key is a symbol, but lookup uses a string key.

C Because the hash is empty.

D Because the hash key is a string, but lookup uses a symbol key.

💡 Hint

Check the type of the key used when creating the hash and when accessing it.

The hash key is the string 'name', but the lookup uses the symbol :name. These are different keys in Ruby hashes, so h[:name] returns nil.

🚀 Application

expert

Count keys that are symbols in a mixed hash

Given this hash, how many keys are symbols?

Ruby

h = { foo: 1, 'bar' => 2, :baz => 3, 'qux' => 4 }

A 1

B 3

C 2

D 4

💡 Hint

Look carefully at each key's type: symbol or string.

The keys foo: and :baz are symbols. The keys 'bar' and 'qux' are strings. So there are 2 symbol keys.