Labeled statements for nested loops in Swift

Introduction

Labeled statements help you control which loop to stop or continue when you have loops inside loops.

When you want to exit an outer loop from inside an inner loop.

When you want to skip to the next iteration of an outer loop from inside an inner loop.

When you have multiple loops and need clear control over which one to break or continue.

When you want to avoid extra flags or complicated conditions to control nested loops.

Syntax

Swift

outerLabel: for item in collection {
    for innerItem in anotherCollection {
        if condition {
            break outerLabel
        }
    }
}

The label is a name followed by a colon before the loop.

You use break label or continue label to control the labeled loop.

Examples

This stops both loops when j equals 2 by breaking the outer loop.

Swift

outerLoop: for i in 1...3 {
    for j in 1...3 {
        if j == 2 {
            break outerLoop
        }
        print("i: \(i), j: \(j)")
    }
}

This skips to the next iteration of the outer loop when j equals 2.

Swift

outerLoop: for i in 1...3 {
    for j in 1...3 {
        if j == 2 {
            continue outerLoop
        }
        print("i: \(i), j: \(j)")
    }
}

Sample Program

This program prints pairs of i and j. When both i and j are 2, it breaks the outer loop and stops all looping.

Swift

outerLoop: for i in 1...3 {
    for j in 1...3 {
        if i == 2 && j == 2 {
            print("Breaking outer loop at i=2, j=2")
            break outerLoop
        }
        print("i: \(i), j: \(j)")
    }
}
print("Done")

Important Notes

Labels must be unique within the same scope.

Use labels to make your nested loops easier to control and read.

Summary

Labeled statements let you name loops to control them directly.

You can break or continue outer loops from inside inner loops using labels.

This helps avoid complicated flags or conditions in nested loops.

Practice

1. What is the main purpose of using labeled statements in nested loops in Swift?

easy

A. To speed up the loop execution

B. To declare variables inside loops

C. To directly control which loop to break or continue

D. To automatically optimize memory usage

Solution

1. Step 1: Understand labeled statements

   Labeled statements give a name to loops so you can refer to them directly.

2. Step 2: Control flow in nested loops

   This naming allows breaking or continuing an outer loop from inside an inner loop without extra flags.

3. Final Answer:

   To directly control which loop to break or continue -> Option C

4. Quick Check:

   Labeled loops control flow directly = C [OK]

Hint: Labels let you break/continue outer loops easily [OK]

Common Mistakes:

• Thinking labels declare variables
• Assuming labels improve speed
• Confusing labels with memory optimization

2. Which of the following is the correct syntax to label a loop named 'outerLoop' in Swift?

easy

A. for outerLoop in 1...3 { }

B. outerLoop: for i in 1...3 { }

C. for i in 1...3 outerLoop: { }

D. label outerLoop for i in 1...3 { }

Solution

1. Step 1: Recall label syntax in Swift

   Labels are written as a name followed by a colon before the loop statement.

2. Step 2: Match syntax to options

   outerLoop: for i in 1...3 { } correctly places 'outerLoop:' before the for loop.

3. Final Answer:

   outerLoop: for i in 1...3 { } -> Option B

4. Quick Check:

   Label name + colon before loop = A [OK]

Hint: Label name ends with colon before loop starts [OK]

Common Mistakes:

• Placing label after the loop
• Using 'label' keyword (not valid)
• Putting label inside loop braces

3. What will be the output of the following Swift code?

outerLoop: for i in 1...2 {
    for j in 1...3 {
        if j == 2 {
            break outerLoop
        }
        print("\(i),\(j)")
    }
}

medium

A. 1,1
1,2
1,3
2,1
2,2
2,3

B. 1,1
1,2

C. 1,1
2,1

D. 1,1

Solution

1. Step 1: Understand loop and break behavior

   The outer loop runs i from 1 to 2, inner loop j from 1 to 3. When j == 2, break outerLoop stops the outer loop entirely.

2. Step 2: Trace iterations

   First iteration: i=1, j=1 prints "1,1". When j=2, break outerLoop triggers, stopping all loops immediately.

3. Final Answer:

   1,1 -> Option D

4. Quick Check:

   Break outer loop at j=2 stops all = D [OK]

Hint: Break with label stops outer loop immediately [OK]

Common Mistakes:

• Thinking break only stops inner loop
• Expecting all prints before break
• Ignoring label effect on break

4. Identify the error in this Swift code using labeled statements:

outer: for i in 1...2 {
    for j in 1...2 {
        if j == 2 {
            break outerLoop
        }
        print(i, j)
    }
}

medium

A. Label 'outerLoop' is not defined

B. Syntax error: missing colon after label

C. Cannot use break inside nested loops

D. No error, code runs fine

Solution

1. Step 1: Check label definition

   The loop is labeled 'outer', but break tries to use 'outerLoop' which is undefined.

2. Step 2: Understand label usage

   Labels must match exactly. Using a different name causes a compile error.

3. Final Answer:

   Label 'outerLoop' is not defined -> Option A

4. Quick Check:

   Break label must match defined label = B [OK]

Hint: Break label must exactly match loop label name [OK]

Common Mistakes:

• Using different label names
• Forgetting colon after label
• Thinking break can't be used in nested loops

5. Given the task to print all pairs (i, j) where i and j range from 1 to 3, but stop all loops immediately when i + j equals 4, which Swift code correctly uses labeled statements to achieve this?

hard

A. outerLoop: for i in 1...3 {
for j in 1...3 {
if i + j == 4 {
break outerLoop
}
print("\(i),\(j)")
}
}

B. for i in 1...3 {
for j in 1...3 {
if i + j == 4 {
break
}
print(i, j)
}
}

C. outerLoop: for i in 1...3 {
for j in 1...3 {
if i + j == 4 {
continue outerLoop
}
print(i, j)
}
}

D. for i in 1...3 {
for j in 1...3 {
if i + j == 4 {
continue
}
print(i, j)
}
}

Solution

1. Step 1: Understand the stopping condition

   We want to stop all loops immediately when i + j == 4, so we must break the outer loop.

2. Step 2: Use labeled break correctly

   outerLoop: for i in 1...3 { for j in 1...3 { if i + j == 4 { break outerLoop } print("\(i),\(j)") } } uses 'break outerLoop' to stop the outer loop from inside the inner loop, which is correct.

3. Step 3: Check other options

   for i in 1...3 { for j in 1...3 { if i + j == 4 { break } print(i, j) } } breaks only inner loop; C uses continue which skips iterations but doesn't stop loops; D continues inner loop only.

4. Final Answer:

   Code that breaks outerLoop when i + j == 4 -> Option A

5. Quick Check:

   Labeled break stops all loops = A [OK]

Hint: Use labeled break to stop outer loop from inner loop [OK]

Common Mistakes:

• Using break without label stops only inner loop
• Using continue instead of break
• Not labeling the outer loop