Sorting Stability and When to Use Which Sort in DSA Go - Build from Scratch

Sorting Stability and When to Use Which Sort

📖 Scenario: You work in a small bookstore. You have a list of books with their titles and prices. You want to sort the books by price but keep the original order of books with the same price.This helps you keep the order in which books arrived while sorting by price.

🎯 Goal: You will create a list of books, set a price threshold, sort the books by price using a stable sort, and then print the sorted list showing the order is kept for books with the same price.

📋 What You'll Learn

Create a list called books with these exact entries: {Title: "Book A", Price: 30}, {Title: "Book B", Price: 20}, {Title: "Book C", Price: 30}, {Title: "Book D", Price: 10}

Create an integer variable called priceThreshold and set it to 25

Use Go's sort.SliceStable to sort books by Price in ascending order

Print the sorted list in the format: Title: <Title>, Price: <Price> for each book

💡 Why This Matters

🌍 Real World

Sorting products or items by price or rating while keeping the order of items with the same value is common in e-commerce and inventory management.

💼 Career

Understanding stable sorting helps in data processing roles where order preservation is important, such as in databases, UI lists, and report generation.

1

Create the list of books

Create a slice of structs called books with these exact entries: {Title: "Book A", Price: 30}, {Title: "Book B", Price: 20}, {Title: "Book C", Price: 30}, {Title: "Book D", Price: 10}. Define a struct type Book with fields Title string and Price int.

DSA Go

type Book struct {
    Title string
    Price int
}

// Your code here

Hint

Define a struct Book with Title and Price. Then create a slice books with the given entries.

2

Set the price threshold

Create an integer variable called priceThreshold and set it to 25 inside the main function, after the books slice.

DSA Go

package main

import "fmt"

type Book struct {
    Title string
    Price int
}

func main() {
    books := []Book{
        {Title: "Book A", Price: 30},
        {Title: "Book B", Price: 20},
        {Title: "Book C", Price: 30},
        {Title: "Book D", Price: 10},
    }
    
    // Create priceThreshold variable
    // Your code here
}

Hint

Just create a variable priceThreshold and set it to 25.

3

Sort the books by price using stable sort

Use sort.SliceStable to sort the books slice by the Price field in ascending order inside the main function. Import the sort package.

DSA Go

package main

import "fmt"
import "sort"

type Book struct {
    Title string
    Price int
}

func main() {
    books := []Book{
        {Title: "Book A", Price: 30},
        {Title: "Book B", Price: 20},
        {Title: "Book C", Price: 30},
        {Title: "Book D", Price: 10},
    }
    priceThreshold := 25

    // Sort books by Price using sort.SliceStable
    // Your code here
}

Hint

Use sort.SliceStable(books, func(i, j int) bool { return books[i].Price < books[j].Price }) to sort by price.

4

Print the sorted list of books

Use a for loop to print each book in books in the format: Title: <Title>, Price: <Price> inside the main function.

DSA Go

package main

import (
    "fmt"
    "sort"
)

type Book struct {
    Title string
    Price int
}

func main() {
    books := []Book{
        {Title: "Book A", Price: 30},
        {Title: "Book B", Price: 20},
        {Title: "Book C", Price: 30},
        {Title: "Book D", Price: 10},
    }
    priceThreshold := 25

    sort.SliceStable(books, func(i, j int) bool {
        return books[i].Price , Price: 
    // Your code here
}

Hint

Use a for loop with range to print each book's title and price using fmt.Printf.

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