Sorting Stability and When to Use Which Sort in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to perform a stable sort on the slice of integers.

DSA Go

sort.SliceStable(numbers, func(i, j int) bool { return numbers[i] [1] numbers[j] })

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

A>

B<

C==

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' causes descending order, which is not the intended stable ascending sort.

Using '==' does not define a strict ordering and causes runtime errors.

Using '<' in the comparison function ensures the sort is ascending and stable.

2fill in blank

medium

Complete the code to perform an unstable sort on the slice of integers.

DSA Go

sort.Slice(numbers, func(i, j int) bool { return numbers[i] [1] numbers[j] })

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

A<

B<=

C>

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' can cause unexpected behavior in unstable sorts.

Using '==' is invalid for sorting comparison.

Using '<' in the comparison function performs an unstable ascending sort.

3fill in blank

hard

Fix the error in the comparison function to ensure a stable sort by age in ascending order.

DSA Go

sort.SliceStable(people, func(i, j int) bool { return people[i].Age [1] people[j].Age })

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

A>

B>=

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' sorts in descending order, which is not the requirement.

Using '==' does not define a strict order and causes errors.

Using '<' correctly sorts people by ascending age while preserving stability.

4fill in blank

hard

Fill both blanks to create a map of names to ages for people older than 30.

DSA Go

result := map[string]int{person.Name: person.[1] for _, person := range people if person.Age [2] 30}

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

AAge

BName

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Name' instead of 'Age' for the map value.

Using '<=' instead of '>' in the condition.

Use 'Age' to get the age value and '>' to filter people older than 30.

5fill in blank

hard

Fill all four blanks to create a filtered map of uppercase names to ages for people younger than 25.

DSA Go

result := map[string]int{strings.[1](person.[2]): person.[3] for _, person := range people if person.Age [4] 25}

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

AToUpper

BName

CAge

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ToLower' instead of 'ToUpper'.

Using '>' instead of '<' in the condition.

Use 'ToUpper' to uppercase names, 'Name' for the key, 'Age' for the value, and '<' to filter younger than 25.

Practice

1. What does it mean when a sorting algorithm is called stable?

easy

A. It keeps the original order of equal elements after sorting

B. It sorts elements faster than any other algorithm

C. It uses less memory than unstable sorts

D. It only works on numbers, not strings

Solution

1. Step 1: Understand stability in sorting

   A stable sort keeps the relative order of items that compare equal the same as before sorting.

2. Step 2: Compare options

   Only It keeps the original order of equal elements after sorting describes this property correctly. Other options describe speed, memory, or data type restrictions, which are unrelated to stability.

3. Final Answer:

   It keeps the original order of equal elements after sorting -> Option A

4. Quick Check:

   Stable sort = keeps equal items order [OK]

Hint: Stable means equal items keep their original order [OK]

Common Mistakes:

• Confusing stability with speed
• Thinking stability means less memory use
• Believing stability depends on data type

2. Which of the following Go code snippets correctly uses the sort.SliceStable function to sort a slice of structs by an integer field Age?

easy

A. sort.SliceStable(people, func(i, j int) bool { return people[i].Age > people[j].Age })

B. sort.Slice(people, func(i, j int) bool { return people[i].Age > people[j].Age })

C. sort.Slice(people, func(i, j int) bool { return people[i].Age < people[j].Age })

D. sort.SliceStable(people, func(i, j int) bool { return people[i].Age < people[j].Age })

Solution

1. Step 1: Identify stable sort usage

   The question asks for stable sorting by Age ascending. sort.SliceStable is the stable function.

2. Step 2: Check comparison function

   The comparison must be people[i].Age < people[j].Age for ascending order.

3. Final Answer:

   sort.SliceStable(people, func(i, j int) bool { return people[i].Age < people[j].Age }) -> Option D

4. Quick Check:

   Stable + ascending = sort.SliceStable(people, func(i, j int) bool { return people[i].Age < people[j].Age }) [OK]

Hint: Use SliceStable with less-than for ascending stable sort [OK]

Common Mistakes:

• Using sort.Slice instead of sort.SliceStable
• Reversing comparison for ascending order
• Mixing stable and unstable sort functions

3. Given the following Go code, what will be the output after sorting?

type Item struct {
  Name string
  Rank int
}

items := []Item{{"A", 2}, {"B", 1}, {"C", 2}}
sort.Slice(items, func(i, j int) bool { return items[i].Rank < items[j].Rank })
for _, item := range items {
  fmt.Print(item.Name, " ")
}

medium

A. A C B

B. B A C

C. B C A

D. C A B

Solution

1. Step 1: Understand sorting criteria

   The slice is sorted by Rank ascending using sort.Slice, which is unstable.

2. Step 2: Analyze order of equal Rank elements

   Items with Rank 2 are "A" and "C". Since sort.Slice is unstable, their relative order may change. The output shows "B" first (Rank 1), then "C", then "A".

3. Final Answer:

   B C A -> Option C

4. Quick Check:

   Unstable sort may change equal items order [OK]

Hint: Unstable sort may reorder equal keys; check output carefully [OK]

Common Mistakes:

• Assuming stable order with sort.Slice
• Mixing up item names and ranks
• Ignoring unstable sort behavior

4. The following Go code attempts to sort a slice of strings stably by length but does not work as expected. What is the error?

words := []string{"cat", "bat", "apple", "dog"}
sort.Slice(words, func(i, j int) bool { return len(words[i]) < len(words[j]) })

medium

A. Using sort.Slice instead of sort.SliceStable causes unstable sort

B. The comparison function should use > instead of <

C. The slice must be converted to []struct before sorting

D. The code is correct and stable by default

Solution

1. Step 1: Identify sorting function used

   The code uses sort.Slice, which is unstable.

2. Step 2: Understand stability requirement

   To sort stably by length, sort.SliceStable must be used instead.

3. Final Answer:

   Using sort.Slice instead of sort.SliceStable causes unstable sort -> Option A

4. Quick Check:

   Stable sort requires SliceStable, not Slice [OK]

Hint: Use SliceStable for stable sorting, not Slice [OK]

Common Mistakes:

• Thinking comparison direction affects stability
• Believing slice type conversion is needed
• Assuming sort.Slice is stable

5. You have a list of employees with fields Name and Department. You want to sort them first by Department alphabetically and then by Name alphabetically within each department, preserving the order of employees with the same name. Which sorting approach is best in Go?

hard

A. Use one unstable sort by Department only

B. Use two stable sorts: first by Name, then by Department

C. Use one stable sort by concatenated Department+Name string

D. Use two unstable sorts: first by Department, then by Name

Solution

1. Step 1: Understand multi-level sorting with stability

   To sort by Department then Name, stable sorts allow sorting by the secondary key first, then primary key.

2. Step 2: Apply stable sorts in correct order

   Sort first by Name (secondary key) stably, then by Department (primary key) stably. This preserves Name order within Departments.

3. Final Answer:

   Use two stable sorts: first by Name, then by Department -> Option B

4. Quick Check:

   Two stable sorts in reverse order = correct multi-level sort [OK]

Hint: Sort by secondary key first, then primary key with stable sorts [OK]

Common Mistakes:

• Using unstable sorts causing order loss
• Sorting primary key first losing secondary order
• Trying one sort on combined keys without stability