Array Reversal Techniques in DSA Go - Build from Scratch

Array Reversal Techniques

📖 Scenario: You are working on a simple program that helps reverse the order of numbers in a list. This is useful when you want to see the list backwards, like reading a list of tasks from last to first.

🎯 Goal: Build a Go program that creates an array of numbers, sets up a helper variable for the array length, reverses the array in place, and then prints the reversed array.

📋 What You'll Learn

Create an array called numbers with these exact values: 1, 2, 3, 4, 5

Create a variable called length that stores the length of the numbers array

Use a for loop with a variable i to reverse the numbers array in place

Print the reversed numbers array

💡 Why This Matters

🌍 Real World

Reversing arrays is useful in many real-world tasks like undoing actions, reversing playlists, or processing data backwards.

💼 Career

Understanding array reversal helps in software development roles where data manipulation and algorithmic thinking are important.

1

Create the initial array

Create an array called numbers with these exact values: 1, 2, 3, 4, 5

DSA Go

# Create the array called numbers with values 1, 2, 3, 4, 5
# Your code here

Hint

Use numbers := []int{1, 2, 3, 4, 5} to create the array.

2

Set up the length variable

Create a variable called length that stores the length of the numbers array using len(numbers)

DSA Go

package main

import "fmt"

func main() {
    numbers := []int{1, 2, 3, 4, 5}
    // Create length variable here
    // Your code here
}

Hint

Use length := len(numbers) to get the array length.

3

Reverse the array in place

Use a for loop with variable i from 0 to length/2 to swap elements and reverse the numbers array in place

DSA Go

package main

import "fmt"

func main() {
    numbers := []int{1, 2, 3, 4, 5}
    length := len(numbers)
    // Reverse the array using a for loop
    // Your code here
}

Hint

Use a for loop to swap elements at positions i and length-1-i.

4

Print the reversed array

Print the numbers array using fmt.Println(numbers)

DSA Go

package main

import "fmt"

func main() {
    numbers := []int{1, 2, 3, 4, 5}
    length := len(numbers)
    for i := 0; i < length/2; i++ {
        numbers[i], numbers[length-1-i] = numbers[length-1-i], numbers[i]
    }
    // Print the reversed array
    // Your code here
}

Hint

Use fmt.Println(numbers) to print the reversed array.

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