Case/when statement in Ruby - Step-by-Step Execution

Concept Flow - Case/when statement

Start

↓

Evaluate expression

↓

Compare with when conditions

↓

Match found?

No→Else block or nil

↓

Execute matched block

↓

End

The case expression evaluates once, then compares to each when condition until a match is found, executing that block or else block if no match.

Execution Sample

Ruby

grade = 'B'
case grade
when 'A'
  puts 'Excellent'
when 'B'
  puts 'Good'
else
  puts 'Needs Improvement'
end

This code checks the value of grade and prints a message based on its value.

Execution Table

Step	Expression Value	When Condition	Match?	Action	Output
1	'B'	'A'	No	Check next when
2	'B'	'B'	Yes	Execute puts 'Good'	Good
3	N/A	N/A	N/A	Exit case

💡 Match found at step 2, so case ends after executing that block.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
grade	nil	'B'	'B'	'B'

Key Moments - 2 Insights

Why does the case statement stop checking after the first match?

Because once a when condition matches the expression value (see step 2 in execution_table), Ruby executes that block and exits the case immediately.

What happens if no when condition matches and there is no else block?

The case expression returns nil and no code inside case runs, as no match was found and no else block exists.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 2?

A Good

B Excellent

C Needs Improvement

D No output

💡 Hint

Check the 'Output' column at step 2 in the execution_table.

At which step does the case statement find a match?

A Step 1

B Step 2

C Step 3

D No match found

💡 Hint

Look at the 'Match?' column in the execution_table.

If grade was 'C' and no else block was present, what would happen?

A It would print 'Needs Improvement'

B It would print 'Good'

C It would print nothing and return nil

D It would cause an error

💡 Hint

Refer to key_moments about no match and no else block behavior.

Concept Snapshot

case expression
case variable
when value1
  # code if match
when value2
  # code if match
else
  # code if no match
end

Evaluates variable once, compares to when values, runs first matching block or else block.

Full Transcript

The case/when statement in Ruby evaluates an expression once and compares it to each when condition in order. If a match is found, it executes that block and exits the case. If no match is found, it runs the else block if present, otherwise returns nil. In the example, grade is 'B'. The case checks 'A' first (no match), then 'B' (match), so it prints 'Good' and stops. This prevents checking further conditions after a match. If no when matches and no else exists, nothing runs and nil is returned.