Quick Sort Algorithm in DSA C - Build from Scratch

Quick Sort Algorithm

📖 Scenario: You are working on a program that needs to sort a list of numbers quickly. Sorting helps organize data so it is easier to search and analyze.

🎯 Goal: Build a C program that uses the Quick Sort algorithm to sort an array of integers in ascending order.

📋 What You'll Learn

Create an integer array with exact values

Define a helper variable for array size

Implement the Quick Sort function with partition logic

Print the sorted array in the specified format

💡 Why This Matters

🌍 Real World

Sorting is used in databases, search engines, and organizing data for reports.

💼 Career

Understanding Quick Sort helps in optimizing software performance and solving coding interviews.

1

Create the array to sort

Create an integer array called arr with these exact values: 29, 10, 14, 37, 13.

DSA C

# Your code here

Hint

Use curly braces to list the numbers inside the array.

2

Define the array size

Create an integer variable called n and set it to the size of the array arr using sizeof(arr) / sizeof(arr[0]).

DSA C

int arr[] = {29, 10, 14, 37, 13};
# Your code here

Hint

Use sizeof operator to calculate the number of elements.

3

Implement Quick Sort and partition functions

Write the functions partition and quickSort to sort the array arr between indexes low and high. Use the last element as pivot in partition. Swap elements to place smaller values before pivot and larger after. Recursively sort the subarrays.

DSA C

int arr[] = {29, 10, 14, 37, 13};
int n = sizeof(arr) / sizeof(arr[0]);

// Your code here

Hint

Use the last element as pivot and swap smaller elements before it. Then recursively sort left and right parts.

4

Sort and print the array

Call quickSort(arr, 0, n - 1) to sort the array. Then print the sorted array elements separated by spaces on one line.

DSA C

int arr[] = {29, 10, 14, 37, 13};
int n = sizeof(arr) / sizeof(arr[0]);

int partition(int arr[], int low, int high) {
    int pivot = arr[high];
    int i = low - 1;
    for (int j = low; j < high; j++) {
        if (arr[j] < pivot) {
            i++;
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        }
    }
    int temp = arr[i + 1];
    arr[i + 1] = arr[high];
    arr[high] = temp;
    return i + 1;
}

void quickSort(int arr[], int low, int high) {
    if (low < high) {
        int pi = partition(arr, low, high);
        quickSort(arr, low, pi - 1);
        quickSort(arr, pi + 1, high);
    }
}

// Your code here

Hint

Call quickSort with low = 0 and high = n - 1. Then print each element separated by space.

Practice

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

easy

A. Divide the array into smaller parts using a pivot and sort them recursively

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

C. Insert elements one by one into a sorted list

D. Merge two sorted arrays into one sorted array

Solution

1. Step 1: Understand Quick Sort's approach

   Quick Sort picks a pivot element and divides the array into parts smaller and larger than the pivot.

2. Step 2: Recursive sorting of parts

   It then sorts these parts recursively, combining them to get the sorted array.

3. Final Answer:

   Divide the array into smaller parts using a pivot and sort them recursively -> Option A

4. Quick Check:

   Quick Sort = Divide and conquer [OK]

Hint: Remember: Quick Sort uses a pivot to split and sort recursively [OK]

Common Mistakes:

• Confusing Quick Sort with Bubble Sort
• Thinking Quick Sort merges arrays like Merge Sort
• Believing Quick Sort inserts elements one by one

2. Which of the following is the correct way to declare the partition function header in C for Quick Sort?

easy

A. int partition(int arr, int low, int high);

B. int partition(int arr[], int low, int high);

C. void partition(int arr[], int low, int high);

D. int partition(int arr[], int start, int end, int pivot);

Solution

1. Step 1: Check parameter types for partition

   The partition function takes an array and two indices low and high to work on a subarray.

2. Step 2: Confirm return type

   It returns an integer index of the pivot after partitioning.

3. Final Answer:

   int partition(int arr[], int low, int high); -> Option B

4. Quick Check:

   Partition returns pivot index [OK]

Hint: Partition returns pivot index; use array and low, high indices [OK]

Common Mistakes:

• Using void return type for partition
• Passing array as int instead of int[]
• Adding extra parameters like pivot

3. What will be the output array after applying Quick Sort on arr = {3, 6, 8, 10, 1, 2, 1}?

medium

A. [3, 6, 8, 10, 1, 2, 1]

B. [10, 8, 6, 3, 2, 1, 1]

C. [1, 1, 2, 3, 6, 8, 10]

D. [1, 2, 1, 3, 6, 8, 10]

Solution

1. Step 1: Apply Quick Sort logic on the array

   Quick Sort rearranges elements around pivots recursively, sorting the array in ascending order.

2. Step 2: Sort the given array

   Sorting {3,6,8,10,1,2,1} results in {1,1,2,3,6,8,10}.

3. Final Answer:

   [1, 1, 2, 3, 6, 8, 10] -> Option C

4. Quick Check:

   Sorted array ascending = [1,1,2,3,6,8,10] [OK]

Hint: Quick Sort outputs sorted array ascending by default [OK]

Common Mistakes:

• Assuming Quick Sort keeps original order
• Confusing ascending with descending order
• Not sorting duplicates correctly

4. Identify the error in this Quick Sort partition function snippet:

int partition(int arr[], int low, int high) {
  int pivot = arr[high];
  int i = low;
  for (int j = low; j < high; j++) {
    if (arr[j] < pivot) {
      i++;
      swap(&arr[i], &arr[j]);
    }
  }
  swap(&arr[i + 1], &arr[high]);
  return i + 1;
}

medium

A. Variable i should start at low - 1, not low

B. Pivot should be arr[low], not arr[high]

C. Loop should run till j <= high, not j < high

D. Swap function is called with wrong arguments

Solution

1. Step 1: Check initialization of i

   In partition, i should start at low - 1 to correctly track smaller elements.

2. Step 2: Analyze loop and swaps

   Starting i at low causes skipping the first element comparison and wrong swaps.

3. Final Answer:

   Variable i should start at low - 1, not low -> Option A

4. Quick Check:

   i = low - 1 fixes partition indexing [OK]

Hint: Initialize i to low - 1 before partition loop [OK]

Common Mistakes:

• Starting i at low causes incorrect partitioning
• Changing pivot position unnecessarily
• Extending loop beyond high index

5. Given an array with many duplicate elements, which modification to Quick Sort helps improve performance?

hard

A. Use Bubble Sort instead for duplicates

B. Always choose the first element as pivot

C. Skip sorting duplicates by checking equality before partition

D. Use 3-way partitioning to separate less than, equal to, and greater than pivot

Solution

1. Step 1: Understand problem with duplicates in Quick Sort

   Standard Quick Sort can degrade with many duplicates due to repeated swaps and partitions.

2. Step 2: Apply 3-way partitioning

   3-way partitioning divides array into three parts: less than, equal to, and greater than pivot, reducing unnecessary work.

3. Final Answer:

   Use 3-way partitioning to separate less than, equal to, and greater than pivot -> Option D

4. Quick Check:

   3-way partitioning optimizes duplicates [OK]

Hint: 3-way partitioning handles duplicates efficiently [OK]

Common Mistakes:

• Choosing fixed pivot without randomization
• Skipping duplicates without partitioning
• Switching to slower sorts unnecessarily