GroupBy for categorization in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Kotlin code using groupBy?

Consider this Kotlin code that groups a list of words by their first letter. What will be printed?

Kotlin

val words = listOf("apple", "banana", "apricot", "blueberry", "cherry")
val grouped = words.groupBy { it.first() }
println(grouped)

A [[apple, apricot], [banana, blueberry], [cherry]]

B {apple=[a, p, p, l, e], banana=[b, a, n, a, n, a], apricot=[a, p, r, i, c, o, t], blueberry=[b, l, u, e, b, e, r, r, y], cherry=[c, h, e, r, r, y]}

C {a=[apple, apricot], b=[banana, blueberry], c=[cherry]}

D {a=[apple], b=[banana], a=[apricot], b=[blueberry], c=[cherry]}

💡 Hint

Remember that groupBy creates a map where keys are the grouping criteria and values are lists of items matching that key.

The groupBy function groups words by their first character. So all words starting with 'a' are grouped together, and so on.

❓ Predict Output

intermediate

What is the output of grouping numbers by even or odd?

Given this Kotlin code, what will be printed?

Kotlin

val numbers = listOf(1, 2, 3, 4, 5, 6)
val grouped = numbers.groupBy { if (it % 2 == 0) "even" else "odd" }
println(grouped)

A {true=[2, 4, 6], false=[1, 3, 5]}

B [[1, 3, 5], [2, 4, 6]]

C {0=[2, 4, 6], 1=[1, 3, 5]}

D {even=[2, 4, 6], odd=[1, 3, 5]}

💡 Hint

Look at the string keys used in the groupBy lambda.

The lambda returns "even" or "odd" strings, so the map keys are those strings with lists of numbers accordingly.

🔧 Debug

advanced

Why does this Kotlin code cause a compilation error?

Look at this Kotlin code that tries to group a list of strings by length but causes a compilation error. What is the cause?

Kotlin

val words = listOf("cat", "dog", "bird")
val grouped = words.groupBy { length }
println(grouped)

A The lambda uses 'length' without 'it.', so 'length' is undefined.

B The list 'words' is immutable, so groupBy cannot be called.

C The groupBy function requires a return type of Int, but 'length' is a String.

D The println statement is missing parentheses.

💡 Hint

Check how to access the length property of each string inside the lambda.

Inside the lambda, 'it' refers to each string. You must write 'it.length' to access the length property. Writing just 'length' causes an error because 'length' is not defined in that scope.

🧠 Conceptual

advanced

How many groups are created by this Kotlin groupBy call?

Given this code, how many keys will the resulting map have?

Kotlin

val items = listOf("red", "green", "blue", "yellow", "black")
val grouped = items.groupBy { it.length }
println(grouped.keys.size)

A 5

B 4

C 3

D 2

💡 Hint

Count the distinct lengths of the strings in the list.

The lengths are: red(3), green(5), blue(4), yellow(6), black(5). Distinct lengths are 3, 4, 5, 6, so 4 groups.

❓ Predict Output

expert

What is the output of this Kotlin code grouping by first letter and counting?

What will this Kotlin code print?

Kotlin

val words = listOf("ant", "ape", "bat", "bear", "cat", "car")
val grouped = words.groupBy { it.first() }.mapValues { it.value.size }
println(grouped)

A {a=2, b=2, c=2}

B {a=2, b=3, c=2}

C {a=3, b=2, c=2}

D {a=[ant, ape], b=[bat, bear], c=[cat, car]}

💡 Hint

Look at how mapValues transforms the grouped map.

The groupBy groups words by first letter, then mapValues replaces each list with its size. Each letter has exactly 2 words, so the counts are 2.