Insertion Sort Algorithm in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the outer loop from the second element in the array.

DSA Go

for i := [1]; i < len(arr); i++ {
    key := arr[i]
    j := i - 1
    for j >= 0 && arr[j] > key {
        arr[j+1] = arr[j]
        j--
    }
    arr[j+1] = key
}

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

A0

B-1

C1

Dlen(arr)

Attempts:

3 left

💡 Hint

Common Mistakes

Starting the loop at 0 causes unnecessary comparisons.

Starting at len(arr) causes the loop to never run.

The insertion sort starts from the second element (index 1) because the first element is considered sorted.

2fill in blank

medium

Complete the code to move elements greater than the key one position ahead.

DSA Go

for j := i - 1; j >= 0 && arr[j] [1] key; j-- {
    arr[j+1] = arr[j]
}

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

A<

B<=

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' will move smaller elements incorrectly.

Using '==' will not move any elements.

We move elements that are greater than the key to the right to make space for the key.

3fill in blank

hard

Fix the error in placing the key after shifting elements.

DSA Go

arr[[1]] = key

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

Aj+1

Bi

Cj-1

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using j places the key in the wrong position.

Using i places the key at the current outer loop index without adjustment.

After the inner loop, j is decremented one extra time, so the correct position is j+1.

4fill in blank

hard

Fill both blanks to complete the inner loop condition and decrement.

DSA Go

for j := i - 1; j [1] 0 && arr[j] > key; j[2] {
    arr[j+1] = arr[j]
}

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

A>=

B<=

C--

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<=' causes the loop to run incorrectly.

Using '++' increments j, which is wrong here.

The loop continues while j is greater or equal to 0 and arr[j] is greater than key. We decrement j each time.

5fill in blank

hard

Fill all three blanks to create a function that sorts an array using insertion sort.

DSA Go

func [1](arr []int) {
    for i := [2]; i < len(arr); i++ {
        key := arr[i]
        j := i - 1
        for j >= 0 && arr[j] > key; j[3] {
            arr[j+1] = arr[j]
        }
        arr[j+1] = key
    }
}

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

AInsertionSort

B1

C--

DSortArray

Attempts:

3 left

💡 Hint

Common Mistakes

Starting outer loop at 0.

Using '++' instead of '--' for j.

Using unclear function names.

The function is named InsertionSort. The outer loop starts at 1. The inner loop decrements j with '--'.

Practice

1. What is the main idea behind the Insertion Sort algorithm?

easy

A. Divide the array into halves and sort each half recursively

B. Build the sorted array one element at a time by inserting elements into their correct position

C. Swap adjacent elements if they are in wrong order repeatedly

D. Select the smallest element and swap it with the first unsorted element

Solution

1. Step 1: Understand insertion sort process

   Insertion sort builds the sorted list by taking one element at a time and placing it in the correct position relative to the already sorted part.

2. Step 2: Compare with other sorting methods

   Other options describe merge sort (B), bubble sort (C), and selection sort (D), which are different algorithms.

3. Final Answer:

   Build the sorted array one element at a time by inserting elements into their correct position -> Option B

4. Quick Check:

   Insertion sort = Insert elements step-by-step [OK]

Hint: Insertion sort inserts elements one by one in order [OK]

Common Mistakes:

• Confusing insertion sort with bubble or selection sort
• Thinking insertion sort divides array recursively
• Assuming insertion sort swaps all elements repeatedly

2. Which of the following Go code snippets correctly starts the outer loop for insertion sort on a slice arr?

easy

A. for i := 1; i < len(arr); i++ {

B. for i := 0; i < len(arr); i++ {

C. for i := len(arr)-1; i >= 0; i-- {

D. for i := 1; i <= len(arr); i++ {

Solution

1. Step 1: Identify correct loop start for insertion sort

   Insertion sort starts from the second element (index 1) because the first element is considered sorted.

2. Step 2: Check loop conditions

   Loop runs while i is less than length of array. for i := 1; i < len(arr); i++ { matches this exactly.

3. Final Answer:

   for i := 1; i < len(arr); i++ { -> Option A

4. Quick Check:

   Start from index 1 to end [OK]

Hint: Start outer loop at 1, not 0, for insertion sort [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Using <= instead of < in loop condition
• Looping backwards incorrectly

3. What will be the output slice after running insertion sort on arr := []int{4, 3, 2, 1}?

medium

A. [1, 3, 2, 4]

B. [4, 3, 2, 1]

C. [1, 2, 3, 4]

D. [2, 3, 4, 1]

Solution

1. Step 1: Trace insertion sort on given array

   Start with 4 (sorted). Insert 3 before 4 -> [3,4,2,1]. Insert 2 before 3 -> [2,3,4,1]. Insert 1 before 2 -> [1,2,3,4].

2. Step 2: Confirm final sorted slice

   The array is sorted in ascending order: [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option C

4. Quick Check:

   Sorted ascending = [1, 2, 3, 4] [OK]

Hint: Insertion sort outputs sorted ascending array [OK]

Common Mistakes:

• Assuming no change in array
• Stopping sorting early
• Mixing up partial sorted states

4. Identify the error in this Go insertion sort snippet:

for i := 1; i < len(arr); i++ {
    key := arr[i]
    j := i - 1
    for j >= 0 && arr[j] > key {
        arr[j+1] = arr[j]
        j--
    }
    arr[j] = key
}

medium

A. The outer loop should start from i := 0

B. The inner loop condition should be arr[j] < key

C. The variable key should be declared outside the loop

D. The final assignment should be arr[j+1] = key, not arr[j] = key

Solution

1. Step 1: Analyze inner loop and final assignment

   After shifting elements greater than key, the correct position is j+1, but code assigns key to arr[j], which is incorrect.

2. Step 2: Correct the assignment

   It should be arr[j+1] = key to place key in the right spot after shifting.

3. Final Answer:

   The final assignment should be arr[j+1] = key, not arr[j] = key -> Option D

4. Quick Check:

   Insert key at arr[j+1] after shifting [OK]

Hint: Assign key to arr[j+1], not arr[j], after shifting [OK]

Common Mistakes:

• Assigning key to wrong index after inner loop
• Reversing comparison operator in inner loop
• Starting outer loop at 0

5. Given a slice arr := []int{5, 2, 9, 1, 5, 6}, how many total element shifts occur during insertion sort?

hard

A. 7

B. 6

C. 8

D. 5

Solution

1. Step 1: Trace shifts for each insertion

   Insert 2: shift 5 -> 1 shift
   Insert 9: no shift -> 0 shifts
   Insert 1: shift 9, 5, 2 -> 3 shifts
   Insert 5: shift 9, 5 -> 2 shifts
   Insert 6: shift 9 -> 1 shift

2. Step 2: Sum all shifts

   Total shifts = 1 + 0 + 3 + 2 + 1 = 7 shifts.

3. Step 3: Verify options

   7 is present as 7.

4. Final Answer:

   7 -> Option A

5. Quick Check:

   Count shifts carefully for each insertion [OK]

Hint: Count shifts by tracing each insertion step [OK]

Common Mistakes:

• Counting comparisons instead of shifts
• Missing shifts when multiple elements move
• Confusing shifts with swaps