String methods (substring, split, trim) in Kotlin - Time & Space Complexity

Time Complexity: String methods (substring, split, trim)

O(n)

Understanding Time Complexity

When working with strings, it is important to know how long operations like cutting parts, splitting, or cleaning spaces take.

We want to understand how the time needed grows as the string gets longer.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

val text = "  apple, banana, cherry, date  "
val trimmed = text.trim()
val parts = trimmed.split(",")
val firstFruit = parts[0].substring(1, 4)
    

This code trims spaces, splits the string by commas, and extracts a substring from the first part.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The split method scans the entire string once to find commas.
• How many times: It looks at each character once, so it runs about n times, where n is the string length.

How Execution Grows With Input

As the string gets longer, the time to trim and split grows roughly in direct proportion to its length.

Input Size (n)	Approx. Operations
10	About 10 checks for trim and split
100	About 100 checks for trim and split
1000	About 1000 checks for trim and split

Pattern observation: The work grows steadily as the string length increases.

Final Time Complexity

Time Complexity: O(n)

This means the time needed grows in a straight line with the string length.

Common Mistake

[X] Wrong: "Splitting a string is instant and does not depend on its length."

[OK] Correct: The split method must check each character to find separators, so it takes longer for longer strings.

Interview Connect

Understanding how string methods work under the hood helps you write efficient code and answer questions about performance clearly.

Self-Check

"What if we used a regular expression with split instead of a simple character? How would the time complexity change?"

Practice

1. What does the Kotlin trim() method do when called on a string?

easy

A. Converts the string to uppercase

B. Splits the string into parts based on spaces

C. Extracts a part of the string by index

D. Removes spaces from the start and end of the string

Solution

1. Step 1: Understand the purpose of trim()

   The trim() method removes whitespace characters from the beginning and end of a string.

2. Step 2: Compare with other string methods

   Unlike split() or substring(), trim() does not split or extract parts but cleans spaces around the string.

3. Final Answer:

   Removes spaces from the start and end of the string -> Option D

4. Quick Check:

   trim() removes outer spaces [OK]

Hint: trim() cleans spaces only at string edges [OK]

Common Mistakes:

• Thinking trim() splits the string
• Confusing trim() with substring()
• Assuming trim() changes case

2. Which of the following is the correct Kotlin syntax to get a substring from index 2 to 5 (exclusive) of a string text?

easy

A. text.substring(2..5)

B. text.subString(2, 5)

C. text.substring(2, 5)

D. text.substring(2, 6)

Solution

1. Step 1: Recall Kotlin substring syntax

   The correct method call is substring(startIndex, endIndex) where endIndex is exclusive.

2. Step 2: Check each option

   text.substring(2..5) uses a range which is invalid for substring. text.subString(2, 5) has wrong capitalization. text.substring(2, 6) uses 6 as endIndex which extracts more characters than asked.

3. Final Answer:

   text.substring(2, 5) -> Option C

4. Quick Check:

   substring(start, end) uses exclusive end [OK]

Hint: Use substring(start, end) with end exclusive [OK]

Common Mistakes:

• Using range syntax inside substring
• Wrong method name capitalization
• Confusing inclusive vs exclusive end index

3. What is the output of the following Kotlin code?

val data = " apple, banana , cherry "
val fruits = data.trim().split(",")
println(fruits[1].trim())

medium

A. ananab

B. banana

C. anana

D. banan

Solution

1. Step 1: Apply trim() on the original string

   Calling trim() removes spaces at start and end, so string becomes "apple, banana , cherry".

2. Step 2: Split by comma and trim the second element

   Splitting by "," gives ["apple", " banana ", " cherry"]. Then trimming the second element " banana " removes spaces, resulting in "banana".

3. Final Answer:

   banana -> Option B

4. Quick Check:

   trim() cleans spaces after split [OK]

Hint: Trim after split to remove spaces around parts [OK]

Common Mistakes:

• Not trimming after split, leaving spaces
• Assuming split trims automatically
• Confusing index positions in list

4. Find the error in this Kotlin code snippet:

val sentence = "Hello World"
val part = sentence.substring(6, 5)
println(part)

medium

A. substring end index is less than start index causing error

B. substring method requires only one argument

C. substring indexes are inclusive, so 6 to 5 is valid

D. No error, output is "World"

Solution

1. Step 1: Check substring parameters

   substring(startIndex, endIndex) requires startIndex ≤ endIndex. Here, startIndex=6 and endIndex=5, which is invalid.

2. Step 2: Understand Kotlin substring behavior

   This causes an exception because the end index is less than the start index.

3. Final Answer:

   substring end index is less than start index causing error -> Option A

4. Quick Check:

   substring end must be ≥ start [OK]

Hint: End index must be >= start index in substring [OK]

Common Mistakes:

• Assuming substring indexes are inclusive
• Using end index less than start index
• Thinking substring accepts one argument only

5. Given a string input = " red, green , blue , yellow ", write Kotlin code to create a list of trimmed color names without spaces. Which code snippet correctly achieves this?

hard

A. input.split(",").map { it.trim() }

B. input.trim().split(",")

C. input.split(",").trim()

D. input.trim().map { it.split(",") }

Solution

1. Step 1: Split the string by commas

   Using split(",") breaks the string into parts including spaces.

2. Step 2: Trim spaces from each part

   Using map { it.trim() } removes spaces from each color name, producing a clean list.

3. Step 3: Evaluate other options

   input.trim().split(",") trims only the ends before splitting, so inner spaces remain. input.split(",").trim() tries to trim a list, which is invalid. input.trim().map { it.split(",") } trims the whole string then maps incorrectly.

4. Final Answer:

   input.split(",").map { it.trim() } -> Option A

5. Quick Check:

   Split then trim each part for clean list [OK]

Hint: Split first, then trim each piece with map [OK]

Common Mistakes:

• Trimming before split only removes outer spaces
• Trying to trim a list directly
• Incorrect chaining of methods