🔰for...in vs. forEach

Swift ⟩ flow control ⟩ loop ⟩ for...in vs. .forEach()

flow control	for ... in	.forEach()
continue	✅ skip to the next iteration.	⛔ not allowed
break	✅ break the iteration completely.	⛔ not allowed
return	❌ invalid outside of a function.	❗skip to the next iteration.

🧨 雷區

• 👉 replit

⛔ error: return invalid outside of a func.

for n in 1...10 {
    if n == 3 { return }    // ⛔ error
    //          ^^^^^^
    // ⛔ error: return invalid outside of a func
}

• 👉 replit

⛔ error: continue is only allowed inside a loop.

(1...10).forEach { n in 
    if n > 2 { continue }    // ⭐ `continue` is inside a closure❗
    //         ^^^^^^^^
    // ⛔ error: 'continue' is only allowed inside a loop
}

• 👉 replit

⛔ error: unlabeled break is only allowed inside a loop or switch, a labeled break is required to exit an if or do.

(1...10).forEach { n in 
    if n > 2 { break }    // ⭐ `break` is inside a "closure"
    //         ^^^^^
    // ⛔ error: 
    //  • unlabeled 'break' is only allowed inside a loop or switch
    //  • labeled 'break' is required to exit an `if` or `do`
}

💈範例

• 👉 replit

// ------------ for...in ------------------------------

for n in 1...10 {
    if n == 3 { continue }    // ⭐ skip to next iteration
    if n > 6 { break }        // ⭐ break loop completely❗
    a1.append(n)              // [1, 2, 4, 5, 6]
}

// ------------ .forEach ------------------------------

(1...10).forEach { n in 
    if n > 2 { return }     // ⭐ early return ( skip to next iteration❗)
    a2.append(n)            // [1, 2]     (n > 2 NOT appended)
}

(1...10).forEach { n in 
    a3.append(n)            // ⭐ number appended
    if n > 2 { return }     // ⭐ late return ( has no effect❗)
}                           // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

⭐️ 重點

👥 相關

• seq.forEach()

• for...in

📗 參考

• Advanced Swift, p.

• Sundell ⟩ Picking between a for loop and forEach