For-in loop with ranges in Swift - Time & Space Complexity

Time Complexity: For-in loop with ranges

O(n)

Understanding Time Complexity

When we use a for-in loop with ranges in Swift, we want to know how the time it takes to run changes as the range gets bigger.

We ask: How does the number of steps grow when the range increases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

This code prints numbers from 1 up to n using a for-in loop with a range.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loop runs once for each number from 1 to n.
• How many times: Exactly n times, where n is the size of the range.

How Execution Grows With Input

As n gets bigger, the number of times the loop runs grows the same way.

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

Pattern observation: The number of steps grows directly with n; if n doubles, steps double too.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line with the size of the range.

Common Mistake

[X] Wrong: "The loop runs in constant time because it just prints numbers."

[OK] Correct: Even though printing is simple, the loop runs once for each number, so time grows with n.

Interview Connect

Understanding how loops grow with input size helps you explain your code clearly and shows you know how programs scale.

Self-Check

"What if we changed the range to stride(from: 1, through: n, by: 2)? How would the time complexity change?"

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