String methods (substring, split, trim) in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of substring and trim

What is the output of this Kotlin code?

Kotlin

val text = "  Kotlin Programming  "
val result = text.trim().substring(0, 6)
println(result)

A " Kot"

B " Kotlin"

C "Kotlin "

D "Kotlin"

💡 Hint

Remember trim() removes spaces at the start and end before substring is applied.

The trim() removes spaces at both ends, so the string becomes "Kotlin Programming". Then substring(0,6) takes characters from index 0 up to 6 (not including 6), which is "Kotlin".

❓ Predict Output

intermediate

Splitting a string and accessing elements

What will be printed by this Kotlin code?

Kotlin

val data = "apple,banana, cherry, date"
val fruits = data.split(",")
println(fruits[2].trim())

A "banana"

B " cherry"

C "cherry"

D "date"

💡 Hint

Check how split works and remember to trim spaces after splitting.

Splitting by comma creates a list: ["apple", "banana", " cherry", " date"]. The element at index 2 is " cherry" with a space. After trim(), it becomes "cherry".

🔧 Debug

advanced

Identify the error in substring usage

What error will this Kotlin code produce?

Kotlin

val s = "Hello"
val sub = s.substring(3, 2)
println(sub)

A StringIndexOutOfBoundsException

B Prints "lo"

C Compilation error: wrong substring parameters

D No output, prints empty string

💡 Hint

Check the order of start and end indices in substring.

substring(startIndex, endIndex) requires startIndex <= endIndex. Here startIndex=3 and endIndex=2, which is invalid and throws StringIndexOutOfBoundsException.

❓ Predict Output

advanced

Count words after splitting and trimming

What is the output of this Kotlin code?

Kotlin

val sentence = "  Kotlin is fun  "
val words = sentence.trim().split(" ").filter { it.isNotEmpty() }
println(words.size)

A 2

B 3

C 5

D 4

💡 Hint

Trim removes spaces at ends, split by space, then filter removes empty strings.

After trim(), string is "Kotlin is fun". Splitting by space gives ["Kotlin", "is", "fun"]. None are empty, so size is 3.

🧠 Conceptual

expert

Understanding substring and split behavior

Given the Kotlin code below, what will be the output?

Kotlin

val input = "one,two,,four,"
val parts = input.split(",")
val sub = parts[2].substring(0, 0)
println("${parts.size} ${sub.isEmpty()}")

A "5 true"

B "4 false"

C "5 false"

D "4 true"

💡 Hint

Check how split handles consecutive commas and trailing commas, and substring with same start and end index.

Splitting "one,two,,four," by comma results in ["one", "two", "", "four", ""], so size is 5. substring(0,0) returns empty string, so isEmpty() is true.

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