For-in with where clause in Swift - Step-by-Step Execution

Concept Flow - For-in with where clause

Start loop over collection

↓

Check where condition

↓

Execute loop body

↓

Next element or end loop

The loop goes through each item, checks the condition after 'where', runs the body only if true, otherwise skips to next.

Execution Sample

Swift

let numbers = [1, 2, 3, 4, 5]
for num in numbers where num % 2 == 0 {
    print(num)
}

Loops over numbers, printing only even ones (where condition filters items).

Execution Table

Iteration	num	Condition (num % 2 == 0)	Action	Output
1	1	False	Skip iteration
2	2	True	Execute loop body	2
3	3	False	Skip iteration
4	4	True	Execute loop body	4
5	5	False	Skip iteration

💡 All elements checked; loop ends after last element.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	Final
num	nil	1	2	3	4	5	5

Key Moments - 2 Insights

Why does the loop skip some numbers instead of printing them?

Because the where clause filters items. Only numbers where num % 2 == 0 (even numbers) run the loop body, as shown in execution_table rows 1,3,5 where condition is False and action is skip.

Does the loop stop early when the condition is false?

No, the loop continues checking all elements. It only skips executing the body for elements that don't meet the condition, as seen in execution_table where all iterations run but some skip the body.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'num' at iteration 4?

A 3

B 2

C 4

D 5

💡 Hint

Check the 'num' column in execution_table row for iteration 4.

At which iteration does the condition 'num % 2 == 0' become true for the first time?

A Iteration 2

B Iteration 1

C Iteration 3

D Iteration 5

💡 Hint

Look at the 'Condition' column in execution_table to find first True.

If the where clause was removed, what would change in the execution table?

A The loop would skip all iterations.

B All iterations would execute the loop body and print all numbers.

C Only odd numbers would be printed.

D The loop would run zero times.

💡 Hint

Without where clause, no filtering happens, so all elements run the loop body.

Concept Snapshot

for item in collection where condition {
    // code runs only if condition true
}

- Loops over collection
- 'where' filters items
- Body runs only if condition true
- Skips items failing condition
- Loop continues through all elements

Full Transcript

This visual execution shows a Swift for-in loop with a where clause. The loop goes through each number in the array. For each number, it checks if the number is even (num % 2 == 0). If true, it runs the loop body and prints the number. If false, it skips printing and moves to the next number. The variable 'num' changes each iteration to the current number. The loop does not stop early; it checks all numbers. This filtering behavior is controlled by the where clause. The execution table tracks each step, showing which numbers print and which skip. This helps beginners see how the where clause filters items in a loop.