Map, filter, reduce patterns in Swift - Step-by-Step Execution

Concept Flow - Map, filter, reduce patterns

Start with Array

↓

Apply Map

→Transform each element

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Apply Filter

→Keep elements matching condition

↓

Apply Reduce

→Combine elements into one value

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Result

This flow shows how an array is transformed step-by-step by map, filter, and reduce functions.

Execution Sample

Swift

let numbers = [1, 2, 3, 4, 5]
let doubled = numbers.map { $0 * 2 }
let even = doubled.filter { $0 % 2 == 0 }
let sum = even.reduce(0) { $0 + $1 }

This code doubles numbers, keeps even ones, then sums them.

Execution Table

Step	Operation	Input	Output	Explanation
1	Map	[1, 2, 3, 4, 5]	[2, 4, 6, 8, 10]	Each number multiplied by 2
2	Filter	[2, 4, 6, 8, 10]	[2, 4, 6, 8, 10]	All are even, so all kept
3	Reduce	[2, 4, 6, 8, 10]	30	Sum of all elements
4	End	-	-	No more operations

💡 All steps completed; final result is 30

Variable Tracker

Variable	Start	After Map	After Filter	After Reduce
numbers	[1, 2, 3, 4, 5]	[1, 2, 3, 4, 5]	[1, 2, 3, 4, 5]	[1, 2, 3, 4, 5]
doubled	-	[2, 4, 6, 8, 10]	[2, 4, 6, 8, 10]	[2, 4, 6, 8, 10]
even	-	-	[2, 4, 6, 8, 10]	[2, 4, 6, 8, 10]
sum	-	-	-	30

Key Moments - 3 Insights

Why does filter keep all elements after map in this example?

Because after map, all numbers are even (2,4,6,8,10), so the filter condition ($0 % 2 == 0) is true for all, as shown in execution_table row 2.

What does reduce(0) { $0 + $1 } mean?

It means start with 0, then add each element one by one to get the total sum, as shown in execution_table row 3.

Does map change the original array?

No, map creates a new array with transformed elements; the original 'numbers' stays the same, as seen in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 1, what is the output of map?

A [2, 4, 6, 8, 10]

B [1, 2, 3, 4, 5]

C [1, 4, 9, 16, 25]

D [0, 2, 4, 6, 8]

💡 Hint

Check the 'Output' column in execution_table row 1.

At which step does the array get reduced to a single value?

A Step 1 (Map)

B Step 2 (Filter)

C Step 3 (Reduce)

D Step 4 (End)

💡 Hint

Look at execution_table row 3 where output is a single number.

If the filter condition changed to keep only numbers greater than 5, what would be the output after filter?

A [2, 4]

B [6, 8, 10]

C [1, 2, 3]

D []

💡 Hint

Check variable_tracker 'doubled' after map and apply new filter condition.

Concept Snapshot

Map, filter, reduce are array helpers.
Map changes each element.
Filter keeps elements matching a test.
Reduce combines all into one value.
Use them to write clear, simple code.

Full Transcript

We start with an array of numbers. First, map multiplies each number by 2, creating a new array. Then, filter keeps only even numbers, which in this case are all of them. Finally, reduce adds all numbers together to get the sum. Variables change step-by-step but original array stays the same. This pattern helps process data clearly and efficiently.