Case/when statement in Ruby - Time & Space Complexity

Time Complexity: Case/when statement

O(n)

Understanding Time Complexity

We want to understand how long a case/when statement takes to run as the input changes.

Specifically, how does the number of checks grow when we have more conditions?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

case value
when 1
  puts "One"
when 2
  puts "Two"
when 3
  puts "Three"
else
  puts "Other"
end
    

This code checks the value against several conditions and prints a message for the first match.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking each condition one by one until a match is found.
• How many times: Up to the number of conditions (here 3), but stops early if matched.

How Execution Grows With Input

As the number of conditions grows, the checks increase roughly one by one.

Input Size (n)	Approx. Operations
3	Up to 3 checks
10	Up to 10 checks
100	Up to 100 checks

Pattern observation: The number of checks grows directly with the number of conditions.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a match grows linearly with the number of conditions.

Common Mistake

[X] Wrong: "The case statement always runs in constant time no matter how many conditions there are."

[OK] Correct: The case statement checks conditions one by one, so more conditions mean more checks and longer time.

Interview Connect

Understanding how case/when scales helps you explain how your code behaves with many conditions, a useful skill in real projects.

Self-Check

"What if we replaced the case/when with a hash lookup? How would the time complexity change?"

Practice

1.

What does the case/when statement do in Ruby?

easy

A. It chooses which code to run based on matching a value.

B. It repeats code multiple times.

C. It defines a new method.

D. It creates a new variable.

Solution

1. Step 1: Understand the purpose of case/when

   The case/when statement compares a value to different options to decide which code to run.

2. Step 2: Compare with other options

   Repeating code is done by loops, defining methods uses def, and variables are created by assignment, so these are incorrect.

3. Final Answer:

   It chooses which code to run based on matching a value. -> Option A

4. Quick Check:

   case/when selects code by value match [OK]

Hint: Remember: case/when picks code by matching values [OK]

Common Mistakes:

• Confusing case/when with loops
• Thinking case/when defines methods
• Believing case/when creates variables

2.

Which of the following is the correct syntax for a case/when statement in Ruby?

case x
  when 1
    puts "One"
  when 2
    puts "Two"
  else
    puts "Other"
end

easy

A. case x
when 1
puts "One"
when 2
puts "Two"
else
puts "Other"
end

B. case x {
when 1:
puts "One"
when 2:
puts "Two"
else:
puts "Other"
}

C. switch(x) {
case 1:
puts "One"
case 2:
puts "Two"
default:
puts "Other"
}

D. if x == 1 then puts "One" elsif x == 2 then puts "Two" else puts "Other" end

Solution

1. Step 1: Check Ruby case/when syntax

   Ruby uses case followed by when clauses and ends with end. No braces or colons are used.

2. Step 2: Compare options

   case x when 1 puts "One" when 2 puts "Two" else puts "Other" end matches Ruby syntax exactly. Options B and C use braces and colons like other languages. if x == 1 then puts "One" elsif x == 2 then puts "Two" else puts "Other" end is an if/elsif, not case/when.

3. Final Answer:

   case x when 1 puts "One" when 2 puts "Two" else puts "Other" end -> Option A

4. Quick Check:

   Ruby case/when uses no braces or colons [OK]

Hint: Ruby case/when uses no braces or colons, just end [OK]

Common Mistakes:

• Using braces or colons like other languages
• Confusing case/when with switch/case from other languages
• Using if/elsif instead of case/when

3.

What is the output of this Ruby code?

grade = 'B'
case grade
when 'A'
  puts "Excellent"
when 'B', 'C'
  puts "Good"
when 'D'
  puts "Pass"
else
  puts "Fail"
end

medium

A. Fail

B. Excellent

C. Pass

D. Good

Solution

1. Step 1: Identify the value of grade

   The variable grade is set to 'B'.

2. Step 2: Match grade in case/when

   The when 'B', 'C' line matches 'B', so it runs puts "Good".

3. Final Answer:

   Good -> Option D

4. Quick Check:

   grade 'B' matches when 'B', 'C' [OK]

Hint: Check all values in when line, including multiple matches [OK]

Common Mistakes:

• Only checking first value in when list
• Confusing else output
• Ignoring multiple values in one when

4.

Find the error in this Ruby code snippet:

score = 85
case score
when 90..100
  puts "A"
when 80..89
  puts "B"
when 70..79
  puts "C"
else
  puts "F"
end

Why does it not print "B" for score 85?

medium

A. Ranges in when must be strings, not numbers.

B. The case statement compares score with ranges incorrectly; use case with when and ===.

C. The code is correct; it will print "B" as expected.

D. The ranges overlap and cause confusion.

Solution

1. Step 1: Understand how case/when works with ranges

   Ruby's case/when uses the === operator, which works with ranges to check if a value is inside.

2. Step 2: Check the given score and ranges

   Score 85 falls inside 80..89, so puts "B" will run correctly.

3. Final Answer:

   The code is correct; it will print "B" as expected. -> Option C

4. Quick Check:

   Ranges work with case/when using === [OK]

Hint: Ranges work naturally in when clauses with case [OK]

Common Mistakes:

• Thinking ranges must be strings
• Believing case/when can't handle ranges
• Assuming overlapping ranges cause errors here

5.

You want to write a Ruby case/when statement that prints "Weekend" for Saturday and Sunday, "Weekday" for Monday to Friday, and "Invalid" for any other input. Which code correctly does this?

hard

A. case day
when 'Saturday', 'Sunday'
puts "Weekend"
when 'Monday'..'Friday'
puts "Weekday"
else
puts "Invalid"
end

B. case day
when 'Saturday', 'Sunday'
puts "Weekend"
when 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'
puts "Weekday"
else
puts "Invalid"
end

C. case day
when 'Saturday'..'Sunday'
puts "Weekend"
when 'Monday'..'Friday'
puts "Weekday"
else
puts "Invalid"
end

D. case day
when ['Saturday', 'Sunday']
puts "Weekend"
when ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']
puts "Weekday"
else
puts "Invalid"
end

Solution

1. Step 1: Understand how to list multiple values in when

   Use commas to separate multiple values in one when line, like when 'Saturday', 'Sunday'.

2. Step 2: Check each option

   case day when 'Saturday', 'Sunday' puts "Weekend" when 'Monday'..'Friday' puts "Weekday" else puts "Invalid" end incorrectly uses a range of strings 'Monday'..'Friday' which does not work as expected for days. case day when ['Saturday', 'Sunday'] puts "Weekend" when ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday'] puts "Weekday" else puts "Invalid" end uses arrays in when, which does not match values correctly. case day when 'Saturday'..'Sunday' puts "Weekend" when 'Monday'..'Friday' puts "Weekday" else puts "Invalid" end uses string ranges like 'Saturday'..'Sunday' which is not meaningful for days. case day when 'Saturday', 'Sunday' puts "Weekend" when 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday' puts "Weekday" else puts "Invalid" end correctly lists each weekday separately with commas.

3. Final Answer:

   case day when 'Saturday', 'Sunday' puts "Weekend" when 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday' puts "Weekday" else puts "Invalid" end -> Option B

4. Quick Check:

   Use commas for multiple values in when [OK]

Hint: Use commas, not ranges or arrays, for multiple when values [OK]

Common Mistakes:

• Using ranges of strings for days
• Using arrays inside when clauses
• Trying to use string ranges that don't work