Repeat-while loop in Swift - Time & Space Complexity

Time Complexity: Repeat-while loop

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a repeat-while loop changes as the input grows.

Specifically, how many times the loop runs depending on the input size.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

var count = 0
let limit = n
repeat {
    print(count)
    count += 1
} while count < limit

This code prints numbers from 0 up to n-1 using a repeat-while loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The repeat-while loop runs the print and increment steps.
• How many times: It runs once for each number from 0 up to n-1, so n times.

How Execution Grows With Input

As n grows, the loop runs more times, directly matching n.

Input Size (n)	Approx. Operations
10	10
100	100
1000	1000

Pattern observation: The number of operations grows in a straight line with input size.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the loop grows directly with the size of n.

Common Mistake

[X] Wrong: "The repeat-while loop runs only once because it always runs at least once."

[OK] Correct: The loop runs once at minimum, but then keeps running until the condition is false, so it can run many times depending on n.

Interview Connect

Understanding how loops grow with input size helps you explain your code clearly and shows you can think about efficiency.

Self-Check

"What if we changed the loop to stop when count <= limit instead of count < limit? How would the time complexity change?"

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