Counting Sort Algorithm in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the count array with zeros.

DSA Go

count := make([]int, [1])

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

A0

Blen(arr)

Cmax+1

Dlen(arr)+1

Attempts:

3 left

💡 Hint

Common Mistakes

Using length of input array instead of max+1 for count array size.

Setting count array size to zero.

The count array size must be max+1 to cover all values from 0 to max.

2fill in blank

medium

Complete the code to count the occurrences of each element in arr.

DSA Go

for _, num := range arr {
    count[[1]]++
}

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

Ai

Bnum

Ccount

Darr

Attempts:

3 left

💡 Hint

Common Mistakes

Using loop index i instead of element num.

Using count or arr as index.

We increment count at index num for each element num in arr.

3fill in blank

hard

Fix the error in updating the count array to cumulative counts.

DSA Go

for i := 1; i < len(count); i++ {
    count[i] = count[i] [1] count[i-1]
}

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

A+

B-

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition.

Using multiplication or division.

We add previous count to current to get cumulative counts.

4fill in blank

hard

Fill both blanks to place elements in output array in correct order.

DSA Go

for i := len(arr) - 1; i >= 0; i-- {
    output[count[arr[i]][1]] = arr[i]
    count[arr[i]][2]
}

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

A-1

B+1

C--

D++

Attempts:

3 left

💡 Hint

Common Mistakes

Using +1 instead of -1 for index.

Incrementing count instead of decrementing.

We place element at count[arr[i]]-1 index and then decrement count[arr[i]].

5fill in blank

hard

Fill all three blanks to copy sorted elements back to original array.

DSA Go

for i := 0; i < len(arr); i++ {
    arr[i] = output[[1]]
    [2] := i
    [3]++
}

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

Ai

Bindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variable names.

Not incrementing index.

We copy output[i] to arr[i], declare index = i, then increment index.

Practice

1. What is the main idea behind the Counting Sort Algorithm?

easy

A. Swapping adjacent elements if they are in wrong order

B. Comparing each element with every other element

C. Counting the number of occurrences of each element to sort

D. Dividing the array into halves and sorting recursively

Solution

1. Step 1: Understand Counting Sort principle

   Counting sort counts how many times each value appears in the input.

2. Step 2: Use counts to place elements in order

   Using these counts, it places elements directly in sorted order without comparisons.

3. Final Answer:

   Counting the number of occurrences of each element to sort -> Option C

4. Quick Check:

   Counting occurrences = D [OK]

Hint: Counting sort counts elements, no comparisons needed [OK]

Common Mistakes:

• Thinking it compares elements like bubble sort
• Confusing with divide and conquer methods
• Assuming it swaps elements repeatedly

2. Which of the following is the correct way to declare the count array in Go for counting sort when input values range from 0 to 9?

easy

A. count := make([]int, 10)

B. count := make([]int, 9)

C. count := make([]int, 11)

D. count := make([]int, 0)

Solution

1. Step 1: Determine size of count array

   For values 0 to 9, we need 10 slots to count each number.

2. Step 2: Use make function with correct length

   Using make([]int, 10) creates an integer slice of length 10, perfect for counting 0-9.

3. Final Answer:

   count := make([]int, 10) -> Option A

4. Quick Check:

   Length 10 for 0-9 range = A [OK]

Hint: Count array length = max value + 1 [OK]

Common Mistakes:

• Using length 9 misses counting number 9
• Using length 0 creates empty slice
• Using length 11 wastes space unnecessarily

3. What will be the output of the following Go code snippet using counting sort logic?

arr := []int{4, 2, 2, 8, 3, 3, 1}
count := make([]int, 9)
for _, v := range arr {
    count[v]++
}
var sorted []int
for i, c := range count {
    for j := 0; j < c; j++ {
        sorted = append(sorted, i)
    }
}
fmt.Println(sorted)

medium

A. [0 1 2 2 3 3 4 8]

B. [4 2 2 8 3 3 1]

C. [1 2 3 4 8]

D. [1 2 2 3 3 4 8]

Solution

1. Step 1: Count occurrences of each number

   Count array after loop: count[1]=1, count[2]=2, count[3]=2, count[4]=1, count[8]=1, others 0.

2. Step 2: Build sorted slice using counts

   Append each number i count[i] times: 1 once, 2 twice, 3 twice, 4 once, 8 once.

3. Final Answer:

   [1 2 2 3 3 4 8] -> Option D

4. Quick Check:

   Sorted array = [1 2 2 3 3 4 8] [OK]

Hint: Count then expand counts in order to sort [OK]

Common Mistakes:

• Printing original array instead of sorted
• Including zero counts as elements
• Miscounting duplicates

4. Identify the bug in this Go code snippet implementing counting sort:

arr := []int{3, 1, 4, 1, 5}
count := make([]int, 6)
for _, v := range arr {
    count[v]++
}
var sorted []int
for i, c := range count {
    for j := 0; j <= c; j++ {
        sorted = append(sorted, i)
    }
}
fmt.Println(sorted)

medium

A. Inner loop uses <= instead of < causing extra elements

B. Count array size is too small for max value 5

C. Appending index instead of value

D. Missing initialization of sorted slice

Solution

1. Step 1: Check count array size

   Count array size 6 covers indices 0-5, correct for max value 5.

2. Step 2: Analyze inner loop condition

   Inner loop uses j <= c, which runs one extra time than needed, adding one extra element per non-zero count.

3. Final Answer:

   Inner loop uses <= instead of < causing extra elements -> Option A

4. Quick Check:

   Loop condition off by one = A [OK]

Hint: Use < not <= in loops counting occurrences [OK]

Common Mistakes:

• Ignoring off-by-one errors in loops
• Assuming count array size error causes runtime panic
• Confusing index and value in append

5. Given an array with negative and positive integers: arr := []int{-2, -1, 0, 1, 2}, how can you adapt counting sort to sort this array correctly?

hard

A. Use counting sort directly without changes

B. Shift all values by adding 2 to make them non-negative, then apply counting sort

C. Sort only positive numbers and append negatives at the end

D. Counting sort cannot be used with negative numbers

Solution

1. Step 1: Identify problem with negatives in counting sort

   Counting sort uses array indices as counts, so negative values cause invalid indices.

2. Step 2: Shift values to non-negative range

   Add the absolute value of the minimum (2) to all elements, making them 0 to 4, then apply counting sort.

3. Final Answer:

   Shift all values by adding 2 to make them non-negative, then apply counting sort -> Option B

4. Quick Check:

   Shift negatives to zero-based index = B [OK]

Hint: Shift negatives to zero or positive before counting [OK]

Common Mistakes:

• Trying to use negative indices directly
• Ignoring negative values in sorting
• Assuming counting sort works on all integers without adjustment