CompactMap for optional unwrapping in Swift - Step-by-Step Execution

Concept Flow - CompactMap for optional unwrapping

Start with array of optionals

↓

Apply compactMap

↓

Check each element

↓

Not nil

↓

Keep value

↓

Return array without nils

compactMap takes an array with optional values, removes all nils, and returns an array of only the unwrapped values.

Execution Sample

Swift

let numbers: [Int?] = [1, nil, 3, nil, 5]
let unwrapped = numbers.compactMap { $0 }
print(unwrapped)

This code removes nil values from the array and prints only the non-nil numbers.

Execution Table

Step	Element	Is nil?	Action	Resulting Array
1	1	No	Keep 1	[1]
2	nil	Yes	Discard nil	[1]
3	3	No	Keep 3	[1, 3]
4	nil	Yes	Discard nil	[1, 3]
5	5	No	Keep 5	[1, 3, 5]
End	-	-	All elements processed	[1, 3, 5]

💡 All elements checked; nil values removed; final array contains only unwrapped Ints.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
numbers	[1, nil, 3, nil, 5]	[1, nil, 3, nil, 5]	[1, nil, 3, nil, 5]	[1, nil, 3, nil, 5]	[1, nil, 3, nil, 5]	[1, nil, 3, nil, 5]	[1, nil, 3, nil, 5]
unwrapped	[]	[1]	[1]	[1, 3]	[1, 3]	[1, 3, 5]	[1, 3, 5]

Key Moments - 2 Insights

Why does compactMap remove nil values but map does not?

compactMap unwraps optionals and discards nils, while map keeps nils as they are. See execution_table rows 2 and 4 where nils are discarded.

What happens if all elements are nil?

compactMap returns an empty array because all nils are discarded. This is shown by the exit_note where only non-nil values remain.

Visual Quiz - 3 Questions

Look at the execution_table, what is the resulting array after step 3?

A [1, nil, 3]

B [1]

C [1, 3]

D [1, 3, nil]

💡 Hint

Check the 'Resulting Array' column at step 3 in the execution_table.

At which step does compactMap discard the first nil value?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'Is nil?' and 'Action' columns in the execution_table.

If the input array had no nils, how would the final unwrapped array compare to the original?

A It would be empty

B It would be the same as the original array without optionals

C It would contain only nils

D It would contain fewer elements

💡 Hint

Refer to variable_tracker and concept_flow about how compactMap works with non-nil values.

Concept Snapshot

compactMap removes nils from an array of optionals.
Syntax: array.compactMap { $0 }
Returns an array of unwrapped non-nil values.
Useful to clean arrays with optional elements.
map keeps nils; compactMap removes them.

Full Transcript

We start with an array of optional integers, some are nil. Using compactMap, we check each element. If it is not nil, we keep it; if it is nil, we discard it. Step by step, the resulting array grows only with non-nil values. At the end, we have an array without any nils. This is different from map, which keeps nils. compactMap is useful to unwrap optionals and clean arrays in one step.