Hash methods (keys, values, each) in Ruby - Time & Space Complexity

Time Complexity: Hash methods (keys, values, each)

O(n)

Understanding Time Complexity

When we use hash methods like keys, values, and each, we want to know how long these actions take as the hash grows.

We ask: How does the time to run these methods change when the hash has more items?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

my_hash = {a: 1, b: 2, c: 3, d: 4}

keys = my_hash.keys
values = my_hash.values
my_hash.each do |key, value|
  puts "Key: #{key}, Value: #{value}"
end

This code gets all keys, all values, and then prints each key-value pair from the hash.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Traversing all items in the hash.
• How many times: Once for each method call, it goes through every key-value pair.

How Execution Grows With Input

Each method looks at every item in the hash one time.

Input Size (n)	Approx. Operations
10	About 10 steps
100	About 100 steps
1000	About 1000 steps

Pattern observation: The work grows directly with the number of items. Double the items, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to run these methods grows in a straight line with the number of items in the hash.

Common Mistake

[X] Wrong: "Hash methods like keys or values are instant no matter how big the hash is."

[OK] Correct: These methods must look at every item to collect keys or values, so they take longer as the hash grows.

Interview Connect

Understanding how hash methods scale helps you write efficient code and explain your choices clearly in interviews.

Self-Check

"What if we used each_key instead of keys? How would the time complexity change?"

Practice

1. What does the keys method do when called on a Ruby hash?

easy

A. Deletes all keys from the hash

B. Returns an array of all the values in the hash

C. Returns an array of all the keys in the hash

D. Iterates over each key-value pair in the hash

Solution

1. Step 1: Understand the keys method

   The keys method extracts all keys from the hash and returns them as an array.

2. Step 2: Compare with other methods

   values returns values, each iterates pairs, so only keys returns keys array.

3. Final Answer:

   Returns an array of all the keys in the hash -> Option C

4. Quick Check:

   keys = keys array [OK]

Hint: Remember: keys method lists all keys as an array [OK]

Common Mistakes:

• Confusing keys with values method
• Thinking keys modifies the hash
• Mixing up keys with each method

2. Which of the following is the correct syntax to iterate over each key-value pair in a Ruby hash h?

easy

A. h.each(key, value) { puts key, value }

B. h.each { |key, value| puts key + value }

C. h.each do |key value| puts key, value end

D. h.each { key, value -> puts key + value }

Solution

1. Step 1: Identify correct block syntax for each

   The correct syntax uses curly braces or do-end with block parameters separated by a comma inside pipes: { |key, value| ... }.

2. Step 2: Check each option

   h.each { |key, value| puts key + value } uses correct syntax with pipes and comma. Others have syntax errors or wrong block format.

3. Final Answer:

   h.each { |key, value| puts key + value } -> Option B

4. Quick Check:

   Correct block syntax = h.each { |key, value| puts key + value } [OK]

Hint: Use pipes and commas inside block for key-value pairs [OK]

Common Mistakes:

• Omitting pipes around block parameters
• Missing comma between key and value
• Using arrow syntax incorrectly in blocks

3. What will be the output of the following Ruby code?

h = {a: 1, b: 2, c: 3}
h.keys

medium

A. [:1, :2, :3]

B. [1, 2, 3]

C. {a: 1, b: 2, c: 3}

D. [:a, :b, :c]

Solution

1. Step 1: Understand keys method on hash

   The keys method returns an array of all keys in the hash. Here keys are symbols :a, :b, :c.

2. Step 2: Match output with options

   [:a, :b, :c] matches the array of keys. [1, 2, 3] is values, C is the whole hash, D is invalid symbol array.

3. Final Answer:

   [:a, :b, :c] -> Option D

4. Quick Check:

   h.keys = [:a, :b, :c] [OK]

Hint: keys returns array of keys exactly as in hash [OK]

Common Mistakes:

• Confusing keys with values output
• Expecting hash instead of array
• Misreading symbols as strings

4. Identify the error in the following Ruby code snippet:

h = {x: 10, y: 20}
h.each do |k v|
  puts "#{k}: #{v}"
end

medium

A. Missing comma between block parameters k and v

B. Hash keys must be strings, not symbols

C. The each method cannot be used with do-end

D. The puts statement syntax is incorrect

Solution

1. Step 1: Check block parameter syntax

   Block parameters must be separated by a comma inside pipes: |k, v|. Here comma is missing.

2. Step 2: Verify other parts

   Keys as symbols are valid, each do-end is valid, and puts syntax is correct.

3. Final Answer:

   Missing comma between block parameters k and v -> Option A

4. Quick Check:

   Block params need comma [OK]

Hint: Always separate block parameters with commas inside pipes [OK]

Common Mistakes:

• Omitting comma between block variables
• Thinking symbols are invalid keys
• Confusing block syntax with method call

5. Given the hash h = {a: 1, b: 0, c: nil, d: 4}, which code snippet correctly returns an array of keys whose values are truthy (not false or nil)?

hard

A. h.select { |k, v| v }.keys

B. h.keys.select { |k| k }

C. h.values.select { |v| v }.keys

D. h.each { |k, v| k if v }.keys

Solution

1. Step 1: Understand filtering keys by truthy values

   We want keys where values are truthy (not false or nil). select filters pairs by value truthiness.

2. Step 2: Analyze each option

   h.select { |k, v| v }.keys filters hash pairs by truthy value, then gets keys -> [:a, :d] [check]. h.keys.select { |k| k } checks if key truthy (all symbols are), returns all keys. h.values.select { |v| v }.keys calls keys on array (NoMethodError). h.each { |k, v| k if v }.keys returns original hash, no filtering.

3. Final Answer:

   h.select { |k, v| v }.keys -> Option A

4. Quick Check:

   Filter pairs then get keys = h.select { |k, v| v }.keys [OK]

Hint: Use select on hash then call keys to filter by values [OK]

Common Mistakes:

• Trying to call keys on array of values
• Using each instead of select for filtering
• Selecting keys without checking values