Array Reversal Techniques in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse the array using a simple loop.

DSA Go

for i := 0; i < len(arr)/2; i++ {
    arr[i], arr[len(arr)-1-[1]] = arr[len(arr)-1-[1]], arr[i]
}

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

Ai

B1

C0

Dlen(arr)

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(arr) instead of len(arr)-1 for the last index.

Not using the loop variable to calculate the opposite index.

The index from the end is calculated as len(arr)-1-i to swap elements correctly.

2fill in blank

medium

Complete the code to reverse the array using two pointers.

DSA Go

left, right := 0, len(arr)-1
for left < [1] {
    arr[left], arr[right] = arr[right], arr[left]
    left++
    right--
}

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

A0

Bleft

Clen(arr)

Dright

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(arr) instead of right pointer.

Using <= instead of < causing extra swaps.

The loop continues while the left pointer is less than the right pointer to swap elements until they meet.

3fill in blank

hard

Fix the error in the recursive array reversal function.

DSA Go

func reverse(arr []int, start, end int) {
    if start >= [1] {
        return
    }
    arr[start], arr[end] = arr[end], arr[start]
    reverse(arr, start+1, end-1)
}

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

Alen(arr)

Bstart

Cend

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(arr) instead of end for base case.

Incorrect base case causing infinite recursion.

The base case should stop when start is greater than or equal to end to avoid overlapping swaps.

4fill in blank

hard

Fill both blanks to create a function that reverses an array in place using a for loop.

DSA Go

func reverseArray(arr []int) {
    for i, j := 0, len(arr)-1; i [1] j; i, j = i+1, j[2]1 {
        arr[i], arr[j] = arr[j], arr[i]
    }
}

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

A<

B>

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of < in loop condition.

Using + instead of - to decrement j.

The loop runs while i is less than j, and j decreases by 1 each iteration.

5fill in blank

hard

Fill all three blanks to create a map that stores reversed elements with their indices as keys.

DSA Go

func reversedMap(arr []int) map[int]int {
    result := make(map[int]int)
    for i, v := range arr {
        result[[1]] = arr[[2]]
    }
    return result
}

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

Ai

Blen(arr)-1-i

Cv

Dlen(arr)-i

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(arr)-i instead of len(arr)-1-i causing index out of range.

Using v as key instead of value.

The map key is the original index i, the value is the element from the reversed position len(arr)-1-i, which is v in the loop.

Practice

1. What does reversing an array mean in simple terms?

easy

A. Changing the order of elements so the last becomes first and first becomes last

B. Sorting the array in ascending order

C. Removing duplicate elements from the array

D. Adding new elements at the end of the array

Solution

1. Step 1: Understand array order

   An array has elements arranged in a sequence from first to last.

2. Step 2: Define reversal

   Reversing means flipping this order so the last element comes first and the first goes last.

3. Final Answer:

   Changing the order of elements so the last becomes first and first becomes last -> Option A

4. Quick Check:

   Reverse = Flip order [OK]

Hint: Reverse means flip the order of elements [OK]

Common Mistakes:

• Confusing reversal with sorting
• Thinking reversal removes elements
• Believing reversal adds elements

2. Which of the following Go code snippets correctly reverses an array named arr of integers?

easy

A. for i := len(arr)-1; i >= 0; i-- { arr[i] = arr[i] }

B. for i := 0; i < len(arr); i++ { arr[i] = arr[len(arr)-1-i] }

C. for i := 0; i < len(arr)/2; i++ { arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i] }

D. for i := 0; i < len(arr); i++ { arr[i] = arr[i+1] }

Solution

1. Step 1: Identify correct swapping logic

   for i := 0; i < len(arr)/2; i++ { arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i] } swaps elements from start and end moving towards the center, which is the correct reversal method.

2. Step 2: Check other options for errors

   for i := 0; i < len(arr); i++ { arr[i] = arr[len(arr)-1-i] } overwrites elements without swapping, losing data. for i := 0; i < len(arr); i++ { arr[i] = arr[i+1] } shifts elements incorrectly. for i := len(arr)-1; i >= 0; i-- { arr[i] = arr[i] } does nothing effective.

3. Final Answer:

   for i := 0; i < len(arr)/2; i++ { arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i] } -> Option C

4. Quick Check:

   Swap pairs from ends to center = for i := 0; i < len(arr)/2; i++ { arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i] } [OK]

Hint: Swap pairs from start and end until middle [OK]

Common Mistakes:

• Not swapping but overwriting elements
• Looping over full array instead of half
• Ignoring index bounds causing errors

3. What is the output of the following Go code?

arr := []int{1, 2, 3, 4, 5}
for i := 0; i < len(arr)/2; i++ {
    arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i]
}
fmt.Println(arr)

medium

A. [5 4 3 2 1]

B. [1 2 3 4 5]

C. [5 2 3 4 1]

D. [1 5 3 2 4]

Solution

1. Step 1: Trace swaps in loop

   Loop runs i=0 and i=1 (since len=5, half=2). - i=0: swap arr[0] and arr[4] -> arr becomes [5 2 3 4 1] - i=1: swap arr[1] and arr[3] -> arr becomes [5 4 3 2 1]

2. Step 2: Check final array after loop

   After both swaps, array is [5 4 3 2 1]. But the code prints after loop, so output is [5 4 3 2 1].

3. Final Answer:

   [5 4 3 2 1] -> Option A

4. Quick Check:

   Swapped ends inward = reversed array [OK]

Hint: Swap pairs from ends, middle stays same if odd length [OK]

Common Mistakes:

• Only swapping first and last once
• Not swapping pairs correctly
• Confusing output with original array

4. Find the error in this Go code that tries to reverse an array arr:

for i := 0; i <= len(arr)/2; i++ {
    arr[i] = arr[len(arr)-1-i]
}

medium

A. The loop should run only till i < len(arr)/2, not <=

B. It overwrites elements without swapping, losing data

C. The code swaps elements correctly, no error

D. The index len(arr)-1-i is out of range

Solution

1. Step 1: Analyze loop and assignment

   The code assigns arr[i] = arr[len(arr)-1-i] without swapping, so original arr[i] is lost.

2. Step 2: Understand consequence

   Without swapping, the array is partially overwritten and not reversed correctly.

3. Final Answer:

   It overwrites elements without swapping, losing data -> Option B

4. Quick Check:

   Overwrite without swap loses data [OK]

Hint: Always swap pairs, don't overwrite [OK]

Common Mistakes:

• Using <= instead of < in loop
• Not swapping both elements
• Ignoring data loss from overwrite

5. Given a slice arr := []int{1, 2, 3, 4, 5, 6}, write a Go function that reverses the slice in place and returns it. Which of the following is the correct implementation?

hard

A. func reverse(arr []int) []int {
for i := 0; i < len(arr); i++ {
arr[i] = arr[len(arr)-1-i]
}
return arr
}

B. func reverse(arr []int) []int {
for i := 0; i <= len(arr)/2; i++ {
arr[i] = arr[len(arr)-1-i]
}
return arr
}

C. func reverse(arr []int) []int {
for i := len(arr)-1; i >= 0; i-- {
arr[i] = arr[i]
}
return arr
}

D. func reverse(arr []int) []int {
for i := 0; i < len(arr)/2; i++ {
arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i]
}
return arr
}

Solution

1. Step 1: Check correct swapping logic

   func reverse(arr []int) []int { for i := 0; i < len(arr)/2; i++ { arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i] } return arr } swaps elements from start and end moving inward, correctly reversing the slice in place.

2. Step 2: Verify other options

   func reverse(arr []int) []int { for i := 0; i < len(arr); i++ { arr[i] = arr[len(arr)-1-i] } return arr } overwrites without swapping, losing data. func reverse(arr []int) []int { for i := len(arr)-1; i >= 0; i-- { arr[i] = arr[i] } return arr } does nothing effective. func reverse(arr []int) []int { for i := 0; i <= len(arr)/2; i++ { arr[i] = arr[len(arr)-1-i] } return arr } overwrites without swapping and uses <= causing index issues.

3. Final Answer:

   func reverse(arr []int) []int { for i := 0; i < len(arr)/2; i++ { arr[i], arr[len(arr)-1-i] = arr[len(arr)-1-i], arr[i] } return arr } -> Option D

4. Quick Check:

   Swap pairs from ends to center and return [OK]

Hint: Swap pairs up to half length, then return [OK]

Common Mistakes:

• Overwriting elements without swapping
• Looping beyond half length
• Not returning the modified slice