StringBuilder for performance in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new StringBuilder instance.

Kotlin

val sb = [1]()

Drag options to blanks, or click blank then click option'

AString

BStringBuffer

CStringBuilder

DStringBuilderBuilder

Attempts:

3 left

💡 Hint

Common Mistakes

Using String instead of StringBuilder

Using a non-existent class like StringBuilderBuilder

Use StringBuilder() to create a new instance for efficient string concatenation.

2fill in blank

medium

Complete the code to append the word "Hello" to the StringBuilder.

Kotlin

sb.[1]("Hello")

Drag options to blanks, or click blank then click option'

Aappend

Badd

Cinsert

Dconcat

Attempts:

3 left

💡 Hint

Common Mistakes

Using add which does not exist

Using concat which is for Strings, not StringBuilder

The append method adds text to the end of the StringBuilder.

3fill in blank

hard

Fix the error in the code to convert the StringBuilder content to a String.

Kotlin

val result = sb.[1]()

Drag options to blanks, or click blank then click option'

AtoString

BasString

CtoStr

Dstringify

Attempts:

3 left

💡 Hint

Common Mistakes

Using toStr which does not exist

Using stringify which is not a Kotlin method

The correct method to get the string from StringBuilder is toString().

4fill in blank

hard

Fill both blanks to build a string with numbers 1 to 3 separated by commas.

Kotlin

val sb = StringBuilder()
for (i in 1..3) {
    sb.[1](i)
    if (i [2] 3) sb.append(", ")
}

Drag options to blanks, or click blank then click option'

Aappend

B<

C>

Dinsert

Attempts:

3 left

💡 Hint

Common Mistakes

Using insert instead of append

Using > instead of < in the condition

Use append to add numbers and commas. The condition i < 3 avoids adding a comma after the last number.

5fill in blank

hard

Fill all three blanks to create a StringBuilder, append "Kotlin", and convert to String.

Kotlin

val [1] = [2]()
[1].append("Kotlin")
val result = [1].toString()

Drag options to blanks, or click blank then click option'

Abuilder

BStringBuilder

Csb

DStringBuffer

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'builder' as variable but not consistently

Using StringBuffer which is different

We name the variable sb, create a StringBuilder(), and use sb to append and convert to string.

Practice

1. What is the main advantage of using StringBuilder in Kotlin when combining many strings?

easy

A. It automatically sorts the strings alphabetically.

B. It stores strings in a database for later use.

C. It converts strings to uppercase by default.

D. It improves performance by reducing the number of string copies.

Solution

1. Step 1: Understand string concatenation cost

   Regular string concatenation creates new strings each time, which is slow.

2. Step 2: Role of StringBuilder

   StringBuilder appends strings without creating new copies each time, improving speed.

3. Final Answer:

   It improves performance by reducing the number of string copies. -> Option D

4. Quick Check:

   StringBuilder reduces copies [OK]

Hint: StringBuilder avoids repeated copying for faster string joining [OK]

Common Mistakes:

• Thinking StringBuilder sorts strings
• Assuming it changes string case automatically
• Believing it stores strings externally

2. Which of the following is the correct way to create a StringBuilder and add text in Kotlin?

easy

A. val sb = StringBuilder(); sb.add("Hello")

B. val sb = StringBuilder(); sb.append("Hello")

C. val sb = StringBuilder("Hello"); sb.add("World")

D. val sb = StringBuilder(); sb.insert("Hello")

Solution

1. Step 1: Check StringBuilder creation

   Creating with StringBuilder() is correct syntax.

2. Step 2: Identify correct method to add text

   The method to add text is append(), not add() or insert() with a string argument.

3. Final Answer:

   val sb = StringBuilder(); sb.append("Hello") -> Option B

4. Quick Check:

   Use append() to add text [OK]

Hint: Use append() method to add text to StringBuilder [OK]

Common Mistakes:

• Using add() instead of append()
• Using insert() incorrectly
• Passing strings to insert() without index

3. What will be the output of the following Kotlin code?

val sb = StringBuilder()
sb.append("Hi")
sb.append(", ")
sb.append("there!")
println(sb.toString())

medium

A. Hi, there!

B. Hi there!

C. Hi,there!

D. Hi, there

Solution

1. Step 1: Trace each append call

   First append adds "Hi", second adds ", ", third adds "there!".

2. Step 2: Combine all appended strings

   Combined string is "Hi, there!" exactly as appended.

3. Final Answer:

   Hi, there! -> Option A

4. Quick Check:

   Appended strings combined [OK]

Hint: Append strings in order, then call toString() [OK]

Common Mistakes:

• Missing spaces in appended strings
• Forgetting to call toString()
• Assuming append adds spaces automatically

4. Identify the error in the following Kotlin code using StringBuilder:

val sb = StringBuilder()
sb.append("Hello")
sb.append(123)
println(sb)

medium

A. Cannot append an integer directly to StringBuilder.

B. append() method requires two arguments.

C. There is no error in the code.

D. StringBuilder must be initialized with a string.

Solution

1. Step 1: Check append usage

   append can accept integers and converts them to string automatically, so no error here.

2. Step 2: Check printing StringBuilder directly

   Printing StringBuilder object directly calls its toString() method, so it prints the content correctly; no error here either.

3. Step 3: Re-examine options

   Actually, printing sb directly works fine in Kotlin, so no error. The code is correct.

4. Final Answer:

   No error; code runs and prints Hello123 -> Option C

5. Quick Check:

   Printing StringBuilder directly works [OK]

Hint: StringBuilder prints content directly without toString() [OK]

Common Mistakes:

• Thinking append can't take integers
• Assuming toString() is mandatory for printing
• Believing append needs two arguments

5. You want to build a comma-separated list of numbers from 1 to 5 using StringBuilder efficiently. Which code snippet correctly avoids adding an extra comma at the end?

hard

A. val sb = StringBuilder()
for (i in 1..5) {
sb.append(i)
if (i != 5) sb.append(",")
}
sb.toString()

B. val sb = StringBuilder()
for (i in 1..5) {
if (i > 0) sb.append(i).append(",") else sb.append(i)
}
sb.toString()

C. val sb = StringBuilder()
for (i in 1..5) {
sb.append(i).append(",")
}
sb.toString()

D. val sb = StringBuilder()
for (i in 1..5) {
sb.append(i)
sb.append(",")
}
sb.toString().removePrefix(",")

Solution

1. Step 1: Analyze each option for trailing comma

   val sb = StringBuilder() for (i in 1..5) { sb.append(i).append(",") } sb.toString() adds comma after every number, including last, causing extra comma.

2. Step 2: Check conditional comma addition

   val sb = StringBuilder() for (i in 1..5) { if (i > 0) sb.append(i).append(",") else sb.append(i) } sb.toString() tries to avoid comma on last number by if-else, but condition i > 0 is always true, so always appends comma after each number, causing trailing comma.

3. Step 3: Verify correct conditional append

   val sb = StringBuilder() for (i in 1..5) { sb.append(i) if (i != 5) sb.append(",") } sb.toString() appends number first, then adds comma only if not last number, correctly avoiding trailing comma.

4. Step 4: Check the remove prefix method

   val sb = StringBuilder() for (i in 1..5) { sb.append(i) sb.append(",") } sb.toString().removePrefix(",") appends comma always, then removes prefix "," from start (which doesn't exist), leaving trailing comma.

5. Final Answer:

   val sb = StringBuilder() for (i in 1..5) { sb.append(i) if (i != 5) sb.append(",") } sb.toString() -> Option A

6. Quick Check:

   Append comma only if not last [OK]

Hint: Add comma only if not last item in loop [OK]

Common Mistakes:

• Adding comma after every item including last
• Appending number twice in conditional branches
• Removing suffix after building string instead of conditional append