Radix Sort Algorithm in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the digit at a given place value in Radix Sort.

DSA C

int getDigit(int number, int place) {
    return (number / [1]) % 10;
}

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

A10

Bplace

C1

Dplace * 10

Attempts:

3 left

💡 Hint

Common Mistakes

Using 10 instead of the place value causes wrong digit extraction.

Multiplying place by 10 instead of using it as a divisor.

We divide the number by the place value to shift the digit to the units place, then take modulo 10 to get that digit.

2fill in blank

medium

Complete the code to find the maximum number in the array for Radix Sort.

DSA C

int getMax(int arr[], int n) {
    int max = arr[0];
    for (int i = 1; i < n; i++) {
        if (arr[i] [1] max) {
            max = arr[i];
        }
    }
    return max;
}

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

A>

B<

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than operator causes finding minimum instead of maximum.

Using equality operator causes no updates.

We update max when the current element is greater than max.

3fill in blank

hard

Fix the error in the counting sort step of Radix Sort by completing the code to initialize count array.

DSA C

void countSort(int arr[], int n, int exp) {
    int output[n];
    int count[10] = {0};

    for (int i = 0; i < n; i++) {
        count[(arr[i] / [1]) % 10]++;
    }
    // rest of the code
}

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

A1

B10

Cexp

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using 10 instead of exp causes wrong digit extraction.

Using n or 1 causes incorrect indexing.

We use 'exp' to get the digit at the current place value by dividing the number.

4fill in blank

hard

Fill both blanks to correctly build the output array in counting sort step of Radix Sort.

DSA C

for (int i = n - 1; i >= 0; i--) {
    output[count[(arr[i] / [1]) % 10] [2] 1] = arr[i];
    count[(arr[i] / [1]) % 10]--;
}

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

Aexp

B+

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Adding 1 instead of subtracting causes index errors.

Using wrong variable instead of exp.

We use 'exp' to get the digit place. We subtract 1 from count to get the correct index for output array.

5fill in blank

hard

Fill all three blanks to copy the output array back to the original array after counting sort step.

DSA C

for (int i = 0; i < n; i++) {
    arr[i] = [1][[2]];
}

int max = [3](arr, n);

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

Aoutput

Bi

CgetMax

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong array name causes compilation errors.

Using wrong index variable causes wrong copying.

Calling wrong function to find max.

We copy from 'output' array at index 'i' back to 'arr'. Then we find the maximum value using 'getMax' function.

Practice

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

easy

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

B. Compare each element with every other element and swap if needed

C. Sort numbers digit by digit starting from the least significant digit

D. Use a pivot to partition the array and sort partitions separately

Solution

1. Step 1: Understand Radix Sort process

   Radix Sort sorts numbers by processing each digit from least significant to most significant.

2. Step 2: Compare with other sorting methods

   Unlike comparison-based sorts, Radix Sort uses digit positions to sort without direct comparisons.

3. Final Answer:

   Sort numbers digit by digit starting from the least significant digit -> Option C

4. Quick Check:

   Radix Sort = Digit by digit sorting [OK]

Hint: Remember: Radix Sort sorts digits from right to left [OK]

Common Mistakes:

• Confusing Radix Sort with comparison-based sorts
• Thinking it sorts from most significant digit first
• Assuming it uses pivot or recursion

2. Which of the following is the correct way to initialize the counting array for Radix Sort in C?

easy

A. int count[10] = {0};

B. int count[10]; // no initialization

C. int count[10] = {1};

D. int count[10] = NULL;

Solution

1. Step 1: Understand counting array role

   The counting array stores frequency of digits 0-9 and must start at zero.

2. Step 2: Check initialization options

   Only int count[10] = {0}; sets all elements to zero correctly in C.

3. Final Answer:

   int count[10] = {0}; -> Option A

4. Quick Check:

   Counting array must start with zeros [OK]

Hint: Initialize counting array with zeros using {0} [OK]

Common Mistakes:

• Leaving counting array uninitialized
• Initializing with all ones
• Assigning NULL to an int array

3. What is the output of the following C code snippet using Radix Sort on array arr = {170, 45, 75, 90, 802, 24, 2, 66}?

void printArray(int arr[], int n) {
    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);
    printf("\n");
}

int main() {
    int arr[] = {170, 45, 75, 90, 802, 24, 2, 66};
    int n = sizeof(arr)/sizeof(arr[0]);
    radixSort(arr, n); // Assume radixSort is correctly implemented
    printArray(arr, n);
    return 0;
}

medium

A. 24 2 45 66 75 90 170 802

B. 170 45 75 90 802 24 2 66

C. 802 170 90 75 66 45 24 2

D. 2 24 45 66 75 90 170 802

Solution

1. Step 1: Understand Radix Sort output

   Radix Sort sorts the array fully in ascending order after processing all digits.

2. Step 2: Sort given array

   Sorted array is [2, 24, 45, 66, 75, 90, 170, 802].

3. Final Answer:

   2 24 45 66 75 90 170 802 -> Option D

4. Quick Check:

   Sorted array matches 2 24 45 66 75 90 170 802 [OK]

Hint: Radix Sort outputs fully sorted array ascending [OK]

Common Mistakes:

• Assuming output is unchanged
• Thinking output is reversed
• Confusing partial digit sorting with final output

4. Identify the error in this Radix Sort counting step code snippet:

for (int i = 0; i < n; i++) {
    count[(arr[i] / exp) % 10]++;
}
for (int i = 1; i < 10; i++) {
    count[i] += count[i - 1];
}
for (int i = n - 1; i >= 0; i--) {
    output[count[(arr[i] / exp) % 10]] = arr[i];
    count[(arr[i] / exp) % 10]--;
}

medium

A. Counting array size should be 100, not 10

B. Indexing output array should be count[...] - 1

C. Loop for count prefix sum should start from 0

D. Division by exp should be replaced by modulo exp

Solution

1. Step 1: Analyze output array indexing

   Indexing output array must use count[...] - 1 because count stores cumulative counts.

2. Step 2: Check other parts

   Counting array size 10 is correct for digits 0-9; prefix sum loop starts at 1; division by exp is correct.

3. Final Answer:

   Indexing output array should be count[...] - 1 -> Option B

4. Quick Check:

   Output index = count - 1 [OK]

Hint: Subtract 1 from count for output indexing [OK]

Common Mistakes:

• Using count directly as index without subtracting 1
• Wrong counting array size
• Incorrect loop start for prefix sums
• Misusing division and modulo operators

5. Given an array of strings representing numbers with leading zeros, e.g., ["002", "010", "001", "100"], how can Radix Sort be adapted to sort them correctly as numbers?

hard

A. Pad all strings to equal length and sort digit by digit from right to left

B. Sort strings lexicographically without changes

C. Convert strings to integers first, then apply Radix Sort

D. Use Radix Sort only on the first character of each string

Solution

1. Step 1: Understand string number sorting

   Strings with leading zeros must be equal length to compare digits correctly.

2. Step 2: Adapt Radix Sort

   Pad strings to same length, then sort digit by digit from right to left to preserve numeric order.

3. Final Answer:

   Pad all strings to equal length and sort digit by digit from right to left -> Option A

4. Quick Check:

   Equal length padding + digit-wise sort = correct numeric order [OK]

Hint: Pad strings to same length before digit-wise sorting [OK]

Common Mistakes:

• Sorting strings lexicographically without padding
• Converting to int loses leading zeros importance
• Sorting only first character causes wrong order