Collection slicing and indices in Swift - Step-by-Step Execution

Concept Flow - Collection slicing and indices

Start with a Collection

↓

Choose startIndex and endIndex

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Create slice using collection[startIndex..<endIndex

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Access elements via slice indices

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Use slice as a view, not a copy

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End

This flow shows how to select a part of a collection using indices to create a slice, which acts as a view into the original collection.

Execution Sample

Swift

let numbers = [10, 20, 30, 40, 50]
let slice = numbers[1..<4]
print(slice)

This code creates a slice of the array 'numbers' from index 1 up to but not including index 4, then prints the slice.

Execution Table

Step	Action	Indices Used	Slice Content	Output
1	Define array 'numbers'	-	[10, 20, 30, 40, 50]	-
2	Select slice from index 1 to 4 (exclusive)	1..<4	[20, 30, 40]	-
3	Print slice	-	[20, 30, 40]	[20, 30, 40]
4	End	-	-	-

💡 Slice created from indices 1 to 3; index 4 is exclusive, so slice ends before it.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
numbers	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]	[10, 20, 30, 40, 50]
slice	nil	[20, 30, 40]	[20, 30, 40]	[20, 30, 40]

Key Moments - 2 Insights

Why does the slice end before index 4 and not include the element at index 4?

Because the slice uses a half-open range 1..<4, which includes the start index 1 but excludes the end index 4, as shown in execution_table step 2.

Is the slice a new copy of the elements or a view into the original collection?

The slice is a view into the original collection, not a copy. Changes to the original collection can affect the slice, as implied by the concept flow.

Visual Quiz - 3 Questions

Look at the execution table, what is the content of 'slice' after step 2?

A [10, 20, 30]

B [30, 40, 50]

C [20, 30, 40]

D [10, 20, 30, 40]

💡 Hint

Check the 'Slice Content' column in row for step 2 in the execution_table.

At which step is the slice printed to the output?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look for the 'Print slice' action in the execution_table.

If the slice was created with numbers[2..<5], what would be the slice content?

A [20, 30, 40]

B [30, 40, 50]

C [10, 20, 30]

D [40, 50]

💡 Hint

Recall that the slice includes start index but excludes end index; check variable_tracker for original array.

Concept Snapshot

Collection slicing uses half-open ranges like startIndex..<endIndex.
Slices are views into the original collection, not copies.
Indices must be valid and within collection bounds.
Access slice elements using the same indices as the original collection.
Slices help work with parts of collections efficiently.

Full Transcript

This visual execution trace shows how to create and use slices in Swift collections. Starting with an array of numbers, we select a slice using a half-open range of indices. The slice includes elements from the start index up to but not including the end index. The slice acts as a view into the original array, not a copy. We print the slice to see the selected elements. Key points include understanding half-open ranges and that slices share storage with the original collection.