Repeat-while loop in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple repeat-while loop

What is the output of this Swift code using a repeat-while loop?

Swift

var count = 1
repeat {
    print(count)
    count += 1
} while count <= 3

A

1
2
3

B

1
2
3
4

C

2
3
4

D

0
1
2
3

💡 Hint

Remember that repeat-while runs the loop body first, then checks the condition.

The loop starts with count = 1, prints it, then increments. It repeats while count <= 3, so it prints 1, 2, and 3.

❓ Predict Output

intermediate

Repeat-while loop with condition false initially

What will this Swift code print?

Swift

var number = 5
repeat {
    print(number)
    number += 1
} while number < 5

A Error: infinite loop

B 5

C

5
6
7
8...

D No output

💡 Hint

The repeat-while loop runs the body once before checking the condition.

Even though the condition is false at the start, the loop body runs once and prints 5.

🔧 Debug

advanced

Identify the error in this repeat-while loop

What error does this Swift code produce?

Swift

var i = 0
repeat {
    print(i)
    i += 1
} while i = 5

A SyntaxError: Cannot assign to value in condition

B RuntimeError: Infinite loop

C No error, prints 0 to 4

D TypeError: i is not a Boolean

💡 Hint

Check the condition syntax in the while statement.

The condition uses '=' which is assignment, not comparison. Swift requires '==' for comparison, so this causes a syntax error.

🧠 Conceptual

advanced

Repeat-while loop vs while loop behavior

Which statement correctly describes the difference between repeat-while and while loops in Swift?

A repeat-while loops check the condition before executing the body

B Both loops execute the body at least once regardless of condition

C repeat-while executes the loop body at least once; while may not execute if condition is false initially

D while executes the loop body at least once; repeat-while may not execute if condition is false initially

💡 Hint

Think about when the condition is checked in each loop type.

repeat-while runs the body first, then checks the condition, so it always runs at least once. while checks the condition first and may skip the body.

❓ Predict Output

expert

Output of nested repeat-while loops

What is the output of this Swift code with nested repeat-while loops?

Swift

var x = 1
repeat {
    var y = 1
    repeat {
        print("(\(x),\(y))")
        y += 1
    } while y <= 2
    x += 1
} while x <= 2

A

(1,1)
(2,1)
(2,2)

B

(1,1)
(2,1)
(1,2)
(2,2)

C

(1,1)
(1,2)
(1,3)
(2,1)
(2,2)

D

(1,1)
(1,2)
(2,1)
(2,2)

💡 Hint

The inner loop runs fully for each iteration of the outer loop.

For x=1, y runs 1 to 2 printing (1,1) and (1,2). Then x=2, y runs again 1 to 2 printing (2,1) and (2,2).

Practice

1. What is the main difference between a repeat-while loop and a while loop in Swift?

easy

A. A repeat-while loop runs only once and never repeats.

B. A repeat-while loop checks the condition before running the code block.

C. A repeat-while loop cannot be used with Boolean conditions.

D. A repeat-while loop runs the code block at least once before checking the condition.

Solution

1. Step 1: Understand loop behavior

   A while loop checks the condition before running the code block, so it may not run at all if the condition is false initially.

2. Step 2: Compare with repeat-while

   A repeat-while loop runs the code block first, then checks the condition to decide if it should repeat, guaranteeing at least one execution.

3. Final Answer:

   A repeat-while loop runs the code block at least once before checking the condition. -> Option D

4. Quick Check:

   Repeat-while runs first, then checks condition [OK]

Hint: Repeat-while always runs code once before checking condition [OK]

Common Mistakes:

• Thinking repeat-while checks condition first
• Confusing repeat-while with while loop
• Believing repeat-while can't repeat

2. Which of the following is the correct syntax for a repeat-while loop in Swift?

easy

A. repeat (condition) { /* code */ }

B. repeat { /* code */ } while condition

C. while condition { /* code */ } repeat

D. do { /* code */ } while condition

Solution

1. Step 1: Recall Swift syntax for repeat-while

   The correct syntax starts with the keyword repeat, followed by a code block in braces, then the while keyword with the condition.

2. Step 2: Check each option

   repeat { /* code */ } while condition matches the correct syntax exactly. repeat (condition) { /* code */ } incorrectly places the condition in parentheses after repeat. while condition { /* code */ } repeat mixes while and repeat incorrectly. do { /* code */ } while condition uses do-while which is not valid Swift syntax.

3. Final Answer:

   repeat { /* code */ } while condition -> Option B

4. Quick Check:

   repeat { } while condition syntax [OK]

Hint: Remember: repeat { } while condition is the correct form [OK]

Common Mistakes:

• Using do-while instead of repeat-while
• Placing condition inside parentheses after repeat
• Mixing while and repeat keywords

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

var count = 3
repeat {
    print(count)
    count -= 1
} while count > 0

medium

A. 3 2 1

B. 2 1 0

C. 3 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initial count is 3. The loop prints count, then decreases it by 1. It repeats while count > 0.

2. Step 2: List printed values

   First iteration: print 3, count becomes 2.
   Second iteration: print 2, count becomes 1.
   Third iteration: print 1, count becomes 0.
   Loop stops because count is not > 0.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Prints 3,2,1 then stops [OK]

Hint: Count down prints before condition check, stops at zero [OK]

Common Mistakes:

• Including 0 in output
• Printing after decrement instead of before
• Confusing order of printed numbers

4. Identify the error in this Swift repeat-while loop code:

var x = 5
repeat {
    print(x)
    x += 1
} while x < 5

medium

A. No error; the loop runs exactly once.

B. The loop will never run because condition is false initially.

C. Syntax error: missing parentheses around condition.

D. The loop will run infinitely because x keeps increasing.

Solution

1. Step 1: Analyze loop condition and variable change

   Initial x is 5. The loop runs once before checking condition. Inside loop, x increases by 1 each time.

2. Step 2: Check condition after first iteration

   After first iteration, x becomes 6. Condition is x < 5, which is false, so loop should stop. But since x increases, if condition was true, it would never stop. Here, it stops after one iteration.

3. Step 3: Re-examine the condition logic

   Actually, since x starts at 5, condition x < 5 is false, so loop runs once then stops. No infinite loop.

4. Final Answer:

   No error; the loop runs exactly once. -> Option A

5. Quick Check:

   Loop runs once then stops [OK]

Hint: Check variable change and condition carefully to avoid infinite loops [OK]

Common Mistakes:

• Assuming infinite loop without checking condition
• Thinking condition needs parentheses
• Believing loop won't run at all

5. You want to use a repeat-while loop to ask a user for a password until they enter "swift123". Which code snippet correctly implements this?

hard

A. repeat {
print("Enter password:")
input = readLine() ?? ""
} while input == "swift123"

B. var input = ""
while input != "swift123" {
print("Enter password:")
input = readLine() ?? ""
}

C. var input = ""
repeat {
print("Enter password:")
input = readLine() ?? ""
} while input != "swift123"

D. var input = ""
repeat {
input = readLine() ?? ""
} while input != "swift123"
print("Enter password:")

Solution

1. Step 1: Understand the requirement

   The loop must ask for password at least once and repeat until the user enters "swift123".

2. Step 2: Check each option

   var input = "" repeat { print("Enter password:") input = readLine() ?? "" } while input != "swift123" uses repeat-while, asks inside loop, and repeats while input is not "swift123". This matches the requirement.
   var input = "" while input != "swift123" { print("Enter password:") input = readLine() ?? "" } uses while loop, which may skip asking if input is already "swift123".
   repeat { print("Enter password:") input = readLine() ?? "" } while input == "swift123" repeats while input equals "swift123", which is opposite of requirement.
   var input = "" repeat { input = readLine() ?? "" } while input != "swift123" print("Enter password:") prints prompt after loop ends, so user is not prompted before input.

3. Final Answer:

   var input = "" repeat { print("Enter password:") input = readLine() ?? "" } while input != "swift123" -> Option C

4. Quick Check:

   Repeat-while asks first, repeats until correct [OK]

Hint: Use repeat-while to ask first, then check condition [OK]

Common Mistakes:

• Using while loop that may skip first prompt
• Reversing condition logic
• Prompting outside the loop