For-in loop with collections in Swift - Step-by-Step Execution

Concept Flow - For-in loop with collections

Start with collection

↓

Pick first item

↓

Execute loop body with item

↓

More items?

No→Exit loop

Yes↓

Pick next item

↩Back to Execute loop body

The loop picks each item from the collection one by one and runs the code inside the loop for each item until all items are done.

Execution Sample

Swift

let fruits = ["Apple", "Banana", "Cherry"]
for fruit in fruits {
    print(fruit)
}

This code goes through each fruit in the list and prints its name.

Execution Table

Iteration	fruit value	Action	Output
1	"Apple"	Print fruit	Apple
2	"Banana"	Print fruit	Banana
3	"Cherry"	Print fruit	Cherry
-	-	No more items, exit loop	-

💡 All items in the collection have been processed, loop ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
fruit	nil	"Apple"	"Banana"	"Cherry"	nil

Key Moments - 2 Insights

Why does the loop variable 'fruit' change each time?

Because the loop picks the next item from the collection each iteration, as shown in the execution_table rows 1 to 3.

Does the original collection 'fruits' change during the loop?

No, the collection stays the same; only the loop variable 'fruit' changes to each item, as seen in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'fruit' in iteration 2?

A "Banana"

B "Apple"

C "Cherry"

D nil

💡 Hint

Check the 'fruit value' column in row 2 of execution_table.

At which iteration does the loop finish processing all items?

A After iteration 2

B After iteration 3

C After iteration 1

D Never ends

💡 Hint

Look at the exit_note and the last iteration row in execution_table.

If the collection had 4 items, how many times would the loop run?

A 3 times

B 5 times

C 4 times

D Depends on the code inside the loop

💡 Hint

The loop runs once for each item in the collection, as shown in the concept_flow.

Concept Snapshot

For-in loop syntax:
for item in collection {
    // code using item
}

Behavior:
- Loops over each item in the collection
- 'item' changes each iteration
- Loop ends after last item

Key rule: The collection is not changed by the loop.

Full Transcript

This visual shows how a for-in loop works with collections in Swift. The loop starts by picking the first item from the collection. Then it runs the code inside the loop using that item. After that, it checks if there are more items. If yes, it picks the next item and repeats. If no, it stops. The variable 'fruit' holds the current item and changes each time. The original collection stays the same. The loop runs once for each item in the collection.

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