How to sort values and place it in a array in descending order?

Question

How can I sort the values and place it in the array in descending order directly in the for loop itself rather than using the sorted function after the for loop?

final_lst = []
for row in data:
    score = function_returns_a_score()
    final_lst.append({"score": score})

print(final_lst)
# returns 
# [{"score": 10}, {"score": 40}, {"score": 90}, {"score": 15}]

print(sorted(final_lst, key=lambda k: k['score'], reverse=True))
# returns
# [{'score': 90}, {'score': 40}, {'score': 15}, {'score': 10}]

Answer 1

You could use a heap queue:

import random
import heapq

final_list = []
for score in random.sample(range(100), 20): # in random order
    # negative because we're sorting in reverse order
    heapq.heappush(final_list, -score)

final_list = [{'score': -heapq.heappop(final_list)} for _ in range(len(final_list))]

Sample result:

[{'score': 95}, {'score': 94}, {'score': 89}, {'score': 72}, {'score': 71}, {'score': 65}, {'score': 60}, {'score': 58}, {'score': 51}, {'score': 50}, {'score': 45}, {'score': 44}, {'score': 36}, {'score': 35}, {'score': 33}, {'score': 26}, {'score': 25}, {'score': 18}, {'score': 6}, {'score': 3}]

I'm not sure that this would have better complexity than sorting, but it lets you extract the data in sorted order whenever you want: you can call heapq.heappop(final_list) when you need the next value - sorting, on the contrary, is done here and now.

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Also, iff your scores are fixed-width integers (say, integers from 0 to 100), you could use the radix sort which would be O(3n) in this case.

Answer 2

You could create a reverse bisect function to get the correct index for insertion.

Borrowing the code from this answer

def reverse_bisect(a, x, lo=0, hi=None):
    """Return the index at which x could be inserted in a assuming a
    is reverse-sorted.

    Optional args lo (default 0) and hi (default len(a)) bound the
    slice of a to be searched.
    """
    if lo < 0:
        raise ValueError('lo must be non-negative')
    if hi is None:
        hi = len(a)
    while lo < hi:
        mid = (lo+hi)//2
        if x > a[mid]: hi = mid
        else: lo = mid+1
    return lo

scores = []
for row in data:
    score = function_returns_a_score()
    idx = reverse_bisect(scores, score) # this is the correct index to insert the value
    scores.insert(idx, score)
final_lst = [{"score": score} for score in scores]

Complexity Analysis: (taking N = number of elements in array)

Sorting takes O(NlogN) time. Binary search has a time complexity of O(logN). Calling it N times for each element makes it run O(NlogN) times again. Over that, N is smaller in the beginning.

Space Complexity: O(N) You are taking extra space here to store the values, so you may have to take that into account as well.

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The answer below is for someone who wants to insert a value in "normal" (not reversed) order.

To insert the score at the right index, you need to know the correct index first. If the list is sorted (an empty lit is sorted), we can use Binary Search to find the correct index to insert an element.

You could use the bisect module to find the index at which you need to insert your score.

import bisect

final_lst = []
scores = []
for row in data:
    score = function_returns_a_score()
    idx = bisect.bisect(scores, score) # this is the correct index to insert the value
    final_lst.insert(idx, {"score": score})
    scores.insert(idx, score)

Answer 3

You can just sort the array in place after the loop:

final_lst = []
for row in data:
    score = function_returns_a_score()
    final_lst.append({"score": row})

final_list.sort(key=lambda k: k["score"], reverse=True)

print(final_lst)
# returns
# [{'score': 90}, {'score': 40}, {'score': 15}, {'score': 10}]

If you really want to maintain a sorted list for whatever reason, then look into using a PriorityQueue with a class wrapping your objects for a custom comparison function.