For-in loop with collections in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a for-in loop used for in Swift?

A for-in loop is used to repeat a block of code for each item in a collection, like an array or dictionary.

beginner

How do you loop through all elements in an array named fruits using a for-in loop?

You write:
for fruit in fruits {
print(fruit)
}
This prints each fruit in the array one by one.

intermediate

Can you use a for-in loop to iterate over a dictionary in Swift? How?

Yes! You loop over key-value pairs like this:
for (key, value) in dictionary {
print("\(key): \(value)")
}

beginner

What happens if you use a for-in loop on an empty collection?

The loop body does not run at all because there are no items to go through.

intermediate

Why is a for-in loop preferred over a traditional while loop for collections?

Because it is simpler, safer, and less error-prone. It automatically goes through each item without needing to manage indexes.

Which of the following is the correct way to loop through an array numbers in Swift?

A for number; numbers { print(number) }

B for number in numbers { print(number) }

C for (number in numbers) { print(number) }

D loop number in numbers { print(number) }

Option B uses the correct Swift syntax for a for-in loop.

What does the variable in a for-in loop represent when looping over a dictionary?

A The index of each item

B Only the keys

C Only the values

D Each key-value pair as a tuple

In Swift, looping over a dictionary gives you each key-value pair as a tuple.

What will happen if you run a for-in loop on an empty array?

A The loop body does not execute

B The loop runs infinitely

C The loop runs once with a nil value

D The program crashes

The loop body does not run because there are no items to iterate over.

Which collection types can you use with a for-in loop in Swift?

A Arrays, dictionaries, sets, and ranges

B Only arrays

C Only dictionaries

D Only strings

Swift's for-in loop works with any collection type like arrays, dictionaries, sets, and ranges.

Why might you choose a for-in loop over a while loop when working with collections?

A For-in loops are more complex

B While loops are easier to read

C For-in loops automatically handle iteration safely

D While loops are faster

For-in loops automatically handle going through each item safely without manual index management.

Explain how a for-in loop works with an array in Swift. Include an example.

Think about how you say 'for each item in the list, do something'.

Describe how to use a for-in loop to iterate over a dictionary and access both keys and values.

Remember dictionaries store pairs, so you get two things each time.

Practice

1. What does the following Swift code do?

let numbers = [1, 2, 3]
for number in numbers {
    print(number)
}

easy

A. Prints each number in the array one by one

B. Prints the entire array once

C. Causes a syntax error

D. Prints the sum of the numbers

Solution

1. Step 1: Understand the for-in loop structure

   The loop goes through each element in the array named 'numbers'.

2. Step 2: Observe the print statement inside the loop

   Each element is printed on its own line as the loop runs.

3. Final Answer:

   Prints each number in the array one by one -> Option A

4. Quick Check:

   for number in numbers prints each number [OK]

Hint: For-in loops print items one at a time [OK]

Common Mistakes:

• Thinking it prints the whole array at once
• Confusing the loop variable with the collection
• Expecting a sum instead of individual prints

2. Which of the following is the correct syntax for a for-in loop iterating over an array named fruits in Swift?

easy

A. for fruit in fruits { print(fruit) }

B. for (fruit : fruits) { print(fruit) }

C. for fruit to fruits { print(fruit) }

D. foreach fruit in fruits { print(fruit) }

Solution

1. Step 1: Recall Swift for-in loop syntax

   The correct syntax uses 'for variable in collection { }'.

2. Step 2: Compare options to syntax

   Only for fruit in fruits { print(fruit) } matches this syntax exactly.

3. Final Answer:

   for fruit in fruits { print(fruit) } -> Option A

4. Quick Check:

   Correct for-in syntax uses 'in' keyword [OK]

Hint: Remember: for item in collection { } is correct [OK]

Common Mistakes:

• Using 'to' instead of 'in'
• Using parentheses incorrectly
• Using 'foreach' which is not Swift syntax

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

let names = ["Anna", "Bob", "Cara"]
for name in names {
    print(name.count)
}

medium

A. 3 3 4

B. 3 4 3

C. 4 3 3

D. 4 3 4

Solution

1. Step 1: Identify the array elements and their lengths

   "Anna" has 4 letters, "Bob" has 3, "Cara" has 4.

2. Step 2: Understand the loop prints each name's character count

   The loop prints 4, then 3, then 4 on separate lines.

3. Final Answer:

   4 3 4 -> Option D

4. Quick Check:

   name.count matches each string length [OK]

Hint: Count letters of each name inside the loop [OK]

Common Mistakes:

• Mixing up the order of counts
• Counting letters incorrectly
• Assuming print outputs on same line

4. Find the error in this Swift code snippet:

let colors = ["red", "green", "blue"]
for color in colors
    print(color)
}

medium

A. Array declaration syntax is wrong

B. Variable name 'color' is invalid

C. Missing opening brace '{' after for loop

D. print statement should be outside the loop

Solution

1. Step 1: Check for proper loop syntax

   The for-in loop requires braces '{' and '}' to enclose the loop body.

2. Step 2: Identify missing opening brace

   The code has a closing brace '}' but no opening brace '{' after the for line.

3. Final Answer:

   Missing opening brace '{' after for loop -> Option C

4. Quick Check:

   for loops need braces around their body [OK]

Hint: Always use { } to wrap for loop code block [OK]

Common Mistakes:

• Forgetting opening brace
• Misplacing braces
• Assuming print can be outside without braces

5. You have a dictionary of student names and their scores:

let scores = ["Alice": 85, "Bob": 92, "Cara": 78]

Using a for-in loop, how do you print only the names of students who scored above 80?

hard

A. for name in scores.keys {
if scores[name] > 80 {
print(name)
}
}

B. for (name, score) in scores {
if score > 80 {
print(name)
}
}

C. for score in scores.values {
if score > 80 {
print(score)
}
}

D. for name in scores {
if scores[name] > 80 {
print(name)
}
}

Solution

1. Step 1: Understand dictionary iteration with key-value pairs

   Using 'for (name, score) in scores' lets you access both keys and values directly.

2. Step 2: Apply condition to print names with score above 80

   Inside the loop, check if score > 80, then print the name.

3. Final Answer:

   for (name, score) in scores { if score > 80 { print(name) } } -> Option B

4. Quick Check:

   Use (key, value) pairs to filter dictionary [OK]

Hint: Use (key, value) in dictionary to access both [OK]

Common Mistakes:

• Trying to loop only keys or values without pairing
• Forgetting to unwrap optional values
• Printing scores instead of names