Switch with where clauses in Swift - Step-by-Step Execution

Concept Flow - Switch with where clauses

Start with a value

↓

Enter switch statement

↓

Check case 1

↓

Check where clause?

↓

Default case or end

↓

Evaluate where condition

↓

Execute case

↓

End switch

The switch checks each case and if it has a where clause, it evaluates that condition before executing the case.

Execution Sample

Swift

let number = 15
switch number {
case let x where x % 3 == 0:
    print("Divisible by 3")
default:
    print("Other")
}

This code checks if number is divisible by 3 using a where clause in the switch case.

Execution Table

Step	Case Checked	Where Clause Condition	Condition Result	Branch Taken	Output
1	case let x where x % 3 == 0	15 % 3 == 0	true	Yes	Divisible by 3
2	default	N/A	N/A	No

💡 First case matches with where clause true, so switch ends after executing that case.

Variable Tracker

Variable	Start	After Step 1	Final
number	15	15	15
x	N/A	15	15

Key Moments - 2 Insights

Why does the switch check the where clause condition after matching the case pattern?

Because the where clause adds an extra condition to decide if the matched case should run. See execution_table step 1 where the case matches number but only runs because where condition is true.

What happens if the where clause condition is false?

The switch skips that case and checks the next one. In the execution_table, if the condition was false, it would move to the default case.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'x' when the where clause is checked?

A 0

B 3

C 15

D N/A

💡 Hint

Check variable_tracker row for 'x' after Step 1.

At which step does the switch decide to execute the case with the where clause?

A Step 1

B Step 2

C No step, it skips

D Before Step 1

💡 Hint

Look at execution_table row where Branch Taken is 'Yes'.

If the number was 14, how would the execution_table change?

A Step 1 where clause condition would be true, case runs

B Step 1 where clause condition would be false, default case runs

C No cases would run

D Error occurs

💡 Hint

Think about 14 % 3 == 0 and how where clause affects case execution.

Concept Snapshot

switch value {
  case let x where condition:
    // runs if pattern matches and condition is true
  default:
    // runs if no other case matches
}

Use where to add extra checks to cases.
Switch stops at first matching case with true where clause.

Full Transcript

This visual trace shows how a Swift switch statement works when using where clauses. The switch starts by checking each case pattern. If a case has a where clause, it evaluates that condition next. Only if the where clause is true does the switch execute that case. Otherwise, it moves to the next case. In the example, the number 15 matches the case pattern and passes the where clause check (divisible by 3), so it prints 'Divisible by 3' and ends. Variables like 'x' get assigned during pattern matching and are used in the where clause. If the where clause was false, the switch would continue to the default case. This helps add extra conditions to switch cases beyond simple pattern matching.

Practice

1.

What does a where clause do in a Swift switch statement?

easy

A. It defines the default case in the switch statement.

B. It adds an extra condition to a case to check before matching.

C. It repeats the case multiple times automatically.

D. It changes the type of the value being switched.

Solution

1. Step 1: Understand the role of where clause

   The where clause adds an extra condition to a case in a switch statement.

2. Step 2: Compare with other options

   Other options describe unrelated behaviors like default case or type changes, which are not what where does.

3. Final Answer:

   It adds an extra condition to a case to check before matching. -> Option B

4. Quick Check:

   where clause = extra condition [OK]

Hint: Remember: where adds conditions to cases [OK]

Common Mistakes:

• Thinking where defines default case
• Confusing where with case repetition
• Assuming where changes variable types

2.

Which of the following is the correct syntax for a switch case with a where clause in Swift?

let number = 10
switch number {
    case let x where x > 5:
        print("Greater than 5")
    default:
        print("5 or less")
}

easy

A. case let x if x > 5:

B. case x where x > 5:

C. case let x where x > 5:

D. case x if x > 5:

Solution

1. Step 1: Identify correct pattern binding syntax

   In Swift, to bind a value in a case, use let x before the where clause.

2. Step 2: Check the where clause usage

   The where clause follows the pattern binding, so case let x where x > 5: is correct.

3. Final Answer:

   case let x where x > 5: -> Option C

4. Quick Check:

   Pattern binding with let + where clause = case let x where x > 5: [OK]

Hint: Use 'let' before variable, then 'where' for condition [OK]

Common Mistakes:

• Omitting 'let' before variable
• Using 'if' instead of 'where'
• Placing 'where' before 'let'

3.

What will be printed by this Swift code?

let age = 20
switch age {
case let x where x < 13:
    print("Child")
case let x where x >= 13 && x < 20:
    print("Teenager")
case let x where x >= 20:
    print("Adult")
default:
    print("Unknown")
}

medium

A. Adult

B. Teenager

C. Child

D. Unknown

Solution

1. Step 1: Check the value of age

   The variable age is 20.

2. Step 2: Evaluate each case condition

   First case: x < 13? No (20 < 13 is false). Second case: 13 <= x < 20? No (20 < 20 is false). Third case: x >= 20? Yes (20 >= 20 is true).

3. Final Answer:

   Adult -> Option A

4. Quick Check:

   age 20 matches 'Adult' case [OK]

Hint: Check conditions in order; first true case runs [OK]

Common Mistakes:

• Choosing 'Teenager' because 20 is close to 19
• Ignoring >= and < operators
• Forgetting switch stops at first match

4.

Identify the error in this Swift switch statement using where clauses:

let score = 85
switch score {
case let x where x > 90:
    print("Excellent")
case let x where x >= 80:
    print("Good")
case let x where x >= 70:
    print("Average")
case _:
    print("Poor")
}

medium

A. Using let binding is not allowed with where.

B. Missing default case for unmatched values.

C. Incorrect syntax for where clause.

D. The cases overlap and order causes wrong matching.

Solution

1. Step 1: Analyze case conditions and order

   The cases check if score > 90, then >= 80, then >= 70. Since 85 >= 80 and also >= 70, the second case matches first.

2. Step 2: Understand overlapping conditions effect

   Because cases overlap, order matters. If order was different, output might change. This can cause confusion or wrong matching.

3. Final Answer:

   The cases overlap and order causes wrong matching. -> Option D

4. Quick Check:

   Overlapping where clauses need careful order [OK]

Hint: Order cases from most specific to least specific [OK]

Common Mistakes:

• Ignoring overlapping conditions
• Assuming default case is mandatory
• Thinking let binding disallowed with where

5.

You want to categorize a number as "Small", "Medium", or "Large" using a switch with where clauses. The rules are:

• Small: less than 10
• Medium: between 10 and 100 inclusive
• Large: greater than 100

Which switch statement correctly implements this?

let value = 50
switch value {
    case let x where x < 10:
        print("Small")
    case let x where x >= 10 && x <= 100:
        print("Medium")
    case let x where x > 100:
        print("Large")
    default:
        print("Unknown")
}

hard

A. Correctly categorizes all values as per rules.

B. Fails to categorize values equal to 10 or 100.

C. Has syntax errors in where clauses.

D. Default case will never be reached.

Solution

1. Step 1: Check each case condition against rules

   First case: x < 10 matches "Small". Second case: x >= 10 and x <= 100 matches "Medium". Third case: x > 100 matches "Large".

2. Step 2: Verify coverage and syntax

   All values fit one of these cases. Syntax is correct with let x where clauses. Default is fallback but not needed here.

3. Final Answer:

   Correctly categorizes all values as per rules. -> Option A

4. Quick Check:

   Conditions cover all ranges correctly [OK]

Hint: Use clear, non-overlapping where conditions [OK]

Common Mistakes:

• Overlapping ranges causing wrong matches
• Missing <= or >= in conditions
• Forgetting to bind variable with let