The where Clause

A where clause adds an extra condition to something, like saying "loop through these, but only the ones that also pass this test."

In this page:

where in a for-in Loop
where in a switch Case

where in a for-in Loop

Adding where after the loop variable filters which elements actually execute the loop body, skipping ones that don't satisfy the condition.

Example: where in a for-in Loop

markup

for number in 1...10 where number % 3 == 0 {
    print("Multiple of 3: \(number)")
}

where in a switch Case

A case in a switch can add a where clause to require an additional condition beyond simply matching the value's shape.

Example: where in a switch Case

markup

let point = (2, 2)
switch point {
case let (x, y) where x == y:
    print("Point is on the diagonal")
case let (x, y):
    print("Point at (\(x), \(y))")
}

Common Mistakes

Confusing the loop-level where clause (which filters iterations) with switch case where (which adds a match condition) -- they look similar but apply differently.
Writing a separate if inside the loop body when a where clause on the loop itself would be cleaner and skip non-matching elements entirely.
Forgetting where in a for-loop still iterates every element -- it just skips the body for non-matching ones, it doesn't filter the underlying collection.

Chapter Summary

A where clause on a for-in loop filters which iterations run their body.
A where clause on a switch case adds an extra condition beyond just pattern matching.
where makes filtering conditions read naturally as part of the loop or case header.
It's especially useful with pattern matching, generics, and protocol constraints (seen more in later chapters).

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The where Clause

where in a for-in Loop

where in a switch Case

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