For-in loop with ranges in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a for-in loop with a closed range

What is the output of this Swift code?

for i in 1...3 {
    print(i, terminator: " ")
}

Swift

for i in 1...3 {
    print(i, terminator: " ")
}

A 1 2 3

B 1 2

C 0 1 2 3

D 2 3 4

💡 Hint

The closed range operator '...' includes both the start and end values.

The for-in loop iterates over the range 1 to 3 inclusive, printing each number with a space.

❓ Predict Output

intermediate

Output of a for-in loop with a half-open range

What does this Swift code print?

for i in 0..<4 {
    print(i, terminator: ",")
}

Swift

for i in 0..<4 {
    print(i, terminator: ",")
}

A 0,1,2,

B 0,1,2,3,4,

C 1,2,3,4,

D 0,1,2,3,

💡 Hint

The half-open range operator '..<' excludes the upper bound.

The loop runs from 0 up to but not including 4, printing each number followed by a comma.

🔧 Debug

advanced

Identify the error in the for-in loop with range

What error does this Swift code produce?

for i in 1...5 {
    if i = 3 {
        print("Three")
    }
}

Swift

for i in 1...5 {
    if i == 3 {
        print("Three")
    }
}

A Syntax error: Cannot assign to value: 'i' is a 'let' constant

B Runtime error: Index out of range

C Syntax error: Expected '==' in conditional

D No error, prints 'Three' once

💡 Hint

Check the condition inside the if statement for correct comparison syntax.

The code uses '=' which is assignment, but in conditions you must use '==' for comparison.

🧠 Conceptual

advanced

Number of iterations in a for-in loop with stride

How many times does this Swift loop run?

for i in stride(from: 0, to: 10, by: 3) {
    print(i)
}

Swift

for i in stride(from: 0, to: 10, by: 3) {
    print(i)
}

A 10 times

B 4 times

C 5 times

D 3 times

💡 Hint

The stride goes from 0 up to but not including 10, stepping by 3 each time.

The values are 0, 3, 6, 9 — four iterations before reaching 10.

❓ Predict Output

expert

Output of nested for-in loops with ranges

What is the output of this Swift code?

for i in 1...2 {
    for j in 1...2 {
        print(i * j, terminator: " ")
    }
}

Swift

for i in 1...2 {
    for j in 1...2 {
        print(i * j, terminator: " ")
    }
}

A 1 2 2 4

B 1 1 2 2

C 2 4 1 2

D 1 2 3 4

💡 Hint

Multiply each pair of i and j values in the nested loops.

The outer loop runs i=1,2; inner loop runs j=1,2; products are 1*1=1, 1*2=2, 2*1=2, 2*2=4.

Practice

1. What does the following Swift code print?
for i in 1...3 { print(i) }

easy

A. 1 2

B. 1 2 3

C. 0 1 2 3

D. 2 3 4

Solution

1. Step 1: Understand the range operator

   The range 1...3 is a closed range including 1, 2, and 3.

2. Step 2: Loop prints each number in the range

   The loop prints 1, then 2, then 3 on separate lines.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Closed range includes last number = 1 2 3 [OK]

Hint: Closed range includes last number, so count all numbers [OK]

Common Mistakes:

• Thinking the last number is excluded
• Starting from 0 instead of 1
• Confusing closed and half-open ranges

2. Which of the following is the correct syntax for a for-in loop iterating from 0 to 4 (excluding 4) in Swift?

easy

A. for i in 0...4 { }

B. for i in 0 to 4 { }

C. for i in 0..<4 { }

D. for i = 0; i < 4; i++ { }

Solution

1. Step 1: Identify the correct range syntax

   0..<4 is a half-open range including 0,1,2,3 but excluding 4.

2. Step 2: Check Swift for-in loop syntax

   Swift uses 'for variable in range { }' syntax; 'for i in 0..<4 { }' is correct.

3. Final Answer:

   for i in 0..<4 { } -> Option C

4. Quick Check:

   Half-open range excludes last number = for i in 0..<4 { } [OK]

Hint: Use '..<' for half-open range excluding last number [OK]

Common Mistakes:

• Using '...' when last number should be excluded
• Using 'to' keyword which is invalid in Swift
• Using C-style for loop syntax which is not valid in Swift

3. What is the output of this Swift code?

var sum = 0
for number in 1..<5 {
  sum += number
}
print(sum)

medium

A. 10

B. 15

C. 14

D. Error

Solution

1. Step 1: Identify the range and loop values

   The range 1..<5 includes 1, 2, 3, 4 (excluding 5).

2. Step 2: Calculate the sum of these numbers

   Sum = 1 + 2 + 3 + 4 = 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   Sum of 1 to 4 = 10 [OK]

Hint: Half-open range excludes last number; sum 1+2+3+4 = 10 [OK]

Common Mistakes:

• Including 5 in the sum
• Adding numbers incorrectly
• Confusing closed and half-open ranges

4. The following Swift code has an error. What is it?

for i in 1..< { print(i) }

medium

A. No error, code runs fine

B. Using wrong loop variable

C. Using closed range instead of half-open

D. Missing end value in range

Solution

1. Step 1: Check the range syntax

   The range 1..< is incomplete; it misses the end value after '..<' operator.

2. Step 2: Identify the error type

   This causes a syntax error because Swift expects a number after '..<' to define the range.

3. Final Answer:

   Missing end value in range -> Option D

4. Quick Check:

   Range must have start and end values [OK]

Hint: Range must have start and end values, '..<' alone is invalid [OK]

Common Mistakes:

• Leaving out the end value in range
• Confusing '..<' with '...' syntax
• Assuming loop variable causes error

5. You want to print all even numbers from 2 to 10 inclusive using a for-in loop with a range. Which code correctly does this?

hard

A. for i in 2...10 where i % 2 == 0 { print(i) }

B. for i in 2..<10 { if i % 2 == 0 { print(i) } }

C. for i in 2...10 { if i % 2 == 1 { print(i) } }

D. for i in 2...9 where i % 2 == 0 { print(i) }

Solution

1. Step 1: Understand the range and condition

   We want even numbers from 2 to 10 inclusive, so use closed range 2...10.

2. Step 2: Use 'where' clause to filter even numbers

   The 'where i % 2 == 0' filters only even numbers in the loop.

3. Final Answer:

   for i in 2...10 where i % 2 == 0 { print(i) } -> Option A

4. Quick Check:

   Closed range + where filter = for i in 2...10 where i % 2 == 0 { print(i) } [OK]

Hint: Use closed range and 'where' to filter even numbers [OK]

Common Mistakes:

• Using half-open range excluding 10
• Printing odd numbers by mistake
• Forgetting to filter even numbers