Why Repeat-while loop in Swift? - Purpose & Use Cases

The Big Idea

What if you could ask a question once and let the computer handle all the repeating for you?

The Scenario

Imagine you want to keep asking a friend if they want to play a game until they say yes. You have to ask at least once, then keep asking again and again if they say no.

The Problem

If you try to do this by writing the question many times manually, it becomes boring and you might forget to ask again or stop too early. It's slow and easy to make mistakes.

The Solution

The repeat-while loop lets you ask the question once, then automatically keep asking as long as the answer is no. It saves time and avoids errors by repeating the steps for you.

Before vs After

✗ Before

print("Do you want to play? (yes/no)")
var answer = readLine() ?? ""
if answer != "yes" {
  print("Do you want to play? (yes/no)")
  answer = readLine() ?? ""
  // ... repeated many times
}

✓ After

var answer = ""
repeat {
  print("Do you want to play? (yes/no)")
  answer = readLine() ?? ""
} while answer != "yes"

What It Enables

You can run a set of actions at least once and then repeat them easily until a condition is met, making your code cleaner and smarter.

Real Life Example

When filling out a form, you want to check the input at least once and keep asking the user to correct it until it's valid. Repeat-while loop handles this smoothly.

Key Takeaways

Repeat-while loop runs code at least once before checking a condition.

It helps avoid repeating code manually and reduces mistakes.

Great for tasks that need to happen first, then repeat based on a condition.

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