Select the K largest elements with QuickSelect - Python

Question

I am new in Python and I exercising in writing codes, but I am having some troubles.

I am trying to implement an algorithm based on QuickSelect in which it's possible to extract the K largest elements.

So, before I implement the QuickSelect algorithm and use it to find the K-th largest element in the array A. Then using the function k_largest_quickselect(A, K) I scan the array A to collect the K elements larger or equal to the K-th element found with QuickSelect Finally, sort the collected elements.

This is the code of my implementation:

def partition(A, left, right): 
    pivot = random.choice(A)  # pick a random number as pivot
    i = left - 1
    for j in range(left, right): 
        if A[j] <= pivot: 
            i = i+1 
            A[i], A[j] = A[j], A[i]
    A[i+1], A[right] = A[right], A[i+1] 
    return i+1

def QuickSelect(A, K, left, right): # Array, K-th element
    if left == right:
        return A[left]
    q = partition(A, left, right)
    i = q - left + 1
    if K == i:
        return A[i]
    if K < i:
        return QuickSelect(A, K, left, q - 1)
    else:
        return QuickSelect(A, K - i, q + 1, right)
    
def k_largest_quickselect(A, K):
    B = [i for i in A if i >= QuickSelect(A = A, K = K, left = 0, right = len(a)-1)] # elements >= the K-th element found with QuickSelect
    B.sort(reverse = True)
    return B

I tried to test the algorithm

a = get_random_array(10, 10)
print("Array a = " + str(a))
print(sorted(a)[-3:])
print(sorted(k_largest_quickselect(a, 3))) # from array a select the 3 highest number

getting this result

Array a = [0, 3, 0, 6, 2, 5, 1, 8, 1, 9]
[6, 8, 9]
[2, 5, 6, 6, 9, 9]

As you can see, by using the function k_largest_quickselect(A = a, K = 3) I did not get the 3 largest elements of the array.

Please, could you help me to solve this problem? Thank you very much!

score 0 · Answer 1 · answered Jun 13 '21 at 11:09

Here is a different approach to your algorithm:

def kSelect(a, k):
    largestElements = []
    
    for interation in range(0,k):
        largestElementInArray = max(a)
        largestElements.append(largestElementInArray)
        a.remove(largestElementInArray)

    return largestElements

Not sure if this is what you wanted to achieve..?

Select the K largest elements with QuickSelect - Python

1 Answers1