Guard for early exit pattern in Swift - Mini Project: Build & Apply

Guard for early exit pattern

📖 Scenario: You are writing a simple Swift program that checks user input for a valid age before continuing. This is like checking if someone is old enough to enter a club. If the age is not valid, the program should stop early and show a message.

🎯 Goal: Build a Swift program that uses the guard statement to check if an age is valid (between 18 and 99). If the age is invalid, the program should exit early with a message. Otherwise, it should print a welcome message.

📋 What You'll Learn

Create a variable called ageInput with the value "25" (a string).

Create a variable called age that tries to convert ageInput to an integer using Int(ageInput).

Use a guard statement to check if age is not nil and between 18 and 99 inclusive.

If the guard condition fails, print "Invalid age. Access denied." and exit the function early.

If the guard condition passes, print "Welcome! You are allowed access.".

💡 Why This Matters

🌍 Real World

Checking user input early is common in apps to avoid errors and improve user experience, like verifying age for age-restricted content.

💼 Career

Using <code>guard</code> for early exit is a key Swift skill for writing safe, readable, and maintainable code in iOS app development.

1

Create the initial age input

Create a variable called ageInput and set it to the string "25".

Swift

# Create a variable called ageInput and set it to "25"
# Your code here

Hint

Use var ageInput = "25" to create the variable.

2

Convert the input string to an integer

Create a variable called age and set it to Int(ageInput) to convert the string to an integer.

Swift

var ageInput = "25"
# Create a variable called age and set it to Int(ageInput)
# Your code here

Hint

Use var age = Int(ageInput) to convert the string to an integer.

3

Use guard to check age validity

Write a guard statement to check if age is not nil and between 18 and 99 inclusive. Use else to print "Invalid age. Access denied." and return early.

Swift

var ageInput = "25"
var age = Int(ageInput)
# Use guard to check age is not nil and between 18 and 99
# If not, print "Invalid age. Access denied." and return
# Your code here

Hint

Use guard let validAge = age to unwrap the optional, then check the range with validAge >= 18 and validAge <= 99.

4

Print welcome message after guard

After the guard statement, print "Welcome! You are allowed access.".

Swift

var ageInput = "25"
var age = Int(ageInput)
guard let validAge = age, validAge >= 18, validAge <= 99 else {
    print("Invalid age. Access denied.")
    return
}
# Print "Welcome! You are allowed access." below
# Your code here

Hint

Use print("Welcome! You are allowed access.") after the guard block.

Practice

1. What is the main purpose of using guard statements in Swift?

easy

A. To define functions with multiple parameters

B. To create loops that run a fixed number of times

C. To check a condition and exit early if it is false

D. To declare constants and variables

Solution

1. Step 1: Understand the role of guard

   guard is used to check a condition and exit the current scope early if the condition is false.

2. Step 2: Compare with other options

   Options A, B, and C describe functions, loops, and variable declarations, which are unrelated to guard.

3. Final Answer:

   To check a condition and exit early if it is false -> Option C

4. Quick Check:

   Guard = early exit on false condition [OK]

Hint: Guard exits early if condition fails, keeping code clean [OK]

Common Mistakes:

• Confusing guard with if statements
• Thinking guard creates loops
• Using guard to declare variables

2. Which of the following is the correct syntax for a guard statement in Swift?

easy

A. guard condition { return }

B. guard (condition) { return }

C. guard else condition { return }

D. guard condition else { return }

Solution

1. Step 1: Recall the syntax of guard

   The correct syntax is guard condition else { ... } where the else block handles the early exit.

2. Step 2: Check each option

   guard condition else { return } matches the correct syntax. Options A, B, and D have missing else, extra parentheses, or misplaced keywords.

3. Final Answer:

   guard condition else { return } -> Option D

4. Quick Check:

   Guard syntax = guard condition else { } [OK]

Hint: Remember: guard condition else { exit code } [OK]

Common Mistakes:

• Putting condition inside parentheses incorrectly
• Omitting else keyword
• Placing else before condition

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

func checkAge(_ age: Int?) {
    guard let age = age, age >= 18 else {
        print("Access denied")
        return
    }
    print("Access granted")
}

checkAge(nil)
checkAge(16)
checkAge(21)

medium

A. Access denied
Access denied
Access granted

B. Access granted
Access granted
Access granted

C. Access denied
Access granted
Access denied

D. Access granted
Access denied
Access denied

Solution

1. Step 1: Analyze the guard condition

   The guard checks if age is not nil and at least 18. If false, it prints "Access denied" and returns early.

2. Step 2: Evaluate each function call

   For checkAge(nil), age is nil -> prints "Access denied".
   For checkAge(16), age is 16 < 18 -> prints "Access denied".
   For checkAge(21), age is 21 ≥ 18 -> prints "Access granted".

3. Final Answer:

   Access denied Access denied Access granted -> Option A

4. Quick Check:

   Guard filters nil or under 18 = Access denied [OK]

Hint: Guard lets only valid age (18+) pass, else denies [OK]

Common Mistakes:

• Ignoring nil check in guard
• Assuming all calls print Access granted
• Confusing guard with if

4. Identify the error in this Swift code using guard:

func greet(name: String?) {
    guard name != nil else {
        print("No name provided")
    }
    print("Hello, \(name!)")
}

medium

A. Incorrect use of optional binding in guard

B. Missing return or exit after guard else block

C. Using force unwrap (!) inside print

D. Guard condition should be inside parentheses

Solution

1. Step 1: Understand guard requirements

   The else block of a guard must exit the current scope (return, break, continue, or throw).

2. Step 2: Check the code

   The else block prints a message but does not return or exit, so the function continues and force unwraps a nil value, causing a runtime error.

3. Final Answer:

   Missing return or exit after guard else block -> Option B

4. Quick Check:

   Guard else must exit scope [OK]

Hint: Always exit after guard else block (return, throw, etc.) [OK]

Common Mistakes:

• Forgetting return after guard else
• Assuming print alone exits function
• Ignoring force unwrap risks

5. You want to write a function that takes an optional string and prints its uppercase version only if the string is non-empty. Using guard, which code snippet correctly implements this behavior?

hard

A. func printUpper(_ text: String?) {
guard let text = text, !text.isEmpty else {
print("Empty or nil string")
return
}
print(text.uppercased())
}

B. func printUpper(_ text: String?) {
if let text = text, !text.isEmpty {
print(text.uppercased())
} else {
print("Empty or nil string")
}
}

C. func printUpper(_ text: String?) {
guard text != nil else {
print("Empty or nil string")
return
}
print(text!.uppercased())
}

D. func printUpper(_ text: String?) {
guard let text = text else {
print("Empty or nil string")
return
}
if text.isEmpty {
print("Empty or nil string")
}
print(text.uppercased())
}

Solution

1. Step 1: Understand the requirements

   The function must print uppercase only if the string is non-nil and not empty, else print a message and exit early.

2. Step 2: Evaluate each option

   func printUpper(_ text: String?) { guard let text = text, !text.isEmpty else { print("Empty or nil string") return } print(text.uppercased()) } uses a single guard with optional binding and emptiness check, then prints uppercase.
   func printUpper(_ text: String?) { if let text = text, !text.isEmpty { print(text.uppercased()) } else { print("Empty or nil string") } } uses if-else, not guard.
   func printUpper(_ text: String?) { guard text != nil else { print("Empty or nil string") return } print(text!.uppercased()) } only checks nil, not empty.
   func printUpper(_ text: String?) { guard let text = text else { print("Empty or nil string") return } if text.isEmpty { print("Empty or nil string") } print(text.uppercased()) } uses guard for nil, then if for empty that prints message but lacks return, so continues to print uppercased empty string, incorrect.

3. Final Answer:

   guard let text = text, !text.isEmpty else { print("Empty or nil string"); return } print(text.uppercased()) -> Option A

4. Quick Check:

   Guard with let and !isEmpty = correct early exit [OK]

Hint: Combine let binding and condition in one guard for clean code [OK]

Common Mistakes:

• Checking only nil but not empty
• Using if instead of guard for early exit
• Splitting checks into multiple statements unnecessarily