How to make 2 deep copy of a list?-CodePudding

I am making 2 deep copies of a list. The problem is that first deep copy behaves fine, but second deep copy of the list is being altered based on changes on my original list.

import copy

def findThreeLargestNumbers(array):
    # Write your code here.
    org_array2=copy.deepcopy(array)

    org_array1=copy.deepcopy(array)
    
    largest1=0
    largest2=0
    largest3=0
    for i in range(0,len(array)-1):
        if(array[i] >= array[i 1]):
            array[i 1]=array[i]
    largest1=array[-1]
    print(largest1)
    print(array)
    org_array1.remove(largest1)
    
    for i in range(0,len(org_array1)-1):
        if(org_array1[i] >= org_array1[i 1]):
            org_array1[i 1]=org_array1[i]
    largest2=org_array1[-1]
    print(largest2)
    print(org_array1)
    org_array2.remove(largest2)
    
    for i in range(0,len(org_array2)-1):
        if(org_array2[i] >= org_array2[i 1]):
            org_array2[i 1]=org_array2[i]
    largest3=org_array2[-1]
    print(largest3)
    print(org_array2)
array= [10,5,9,10,12,3,517,69,81,74]
findThreeLargestNumbers(array)

517
[10, 10, 10, 10, 12, 12, 517, 517, 517, 517]
81
[10, 10, 10, 10, 12, 12, 69, 81, 81]
517
[10, 10, 10, 10, 12, 12, 517, 517, 517]

I want my both the newly copied lists to be independent of each other, as well as independent of original list.

CodePudding user response：

Your copies are actually fine. The issue is that you have only removed the second largest value from org_array2. So when your logic looks for the largest value, it finds 514 again. Here's your code with that logic corrected:

import copy

def findThreeLargestNumbers(array):
    # Write your code here.
    org_array2=copy.deepcopy(array)

    org_array1=copy.deepcopy(array)
    
    largest1=0
    largest2=0
    largest3=0
    for i in range(0,len(array)-1):
        if(array[i] >= array[i 1]):
            array[i 1]=array[i]
    largest1=array[-1]
    print(largest1)
    print(array)
    org_array1.remove(largest1)
    
    for i in range(0,len(org_array1)-1):
        if(org_array1[i] >= org_array1[i 1]):
            org_array1[i 1]=org_array1[i]
    largest2=org_array1[-1]
    print(largest2)
    print(org_array1)
    org_array2.remove(largest2) # you are removing the 2nd largest only - leaving the largest
    org_array2.remove(largest1) # here you need to remove the largest one as well
    
    for i in range(0,len(org_array2)-1):
        if(org_array2[i] >= org_array2[i 1]):
            org_array2[i 1]=org_array2[i]
    largest3=org_array2[-1]
    print(largest3)
    print(org_array2)
array= [10,5,9,10,12,3,517,69,81,74]
findThreeLargestNumbers(array)

Here are the results:

$ python findlargest
517
[10, 10, 10, 10, 12, 12, 517, 517, 517, 517]
81
[10, 10, 10, 10, 12, 12, 69, 81, 81]
74
[10, 10, 10, 10, 12, 12, 69, 74]

CodePudding user response：

If the goal is to find the largest n values (in this case largest three values), consider sorting list in reverse order and then, extracting the first n non repeating values from the sorted list.

from copy import copy, deepcopy
from typing import List

array= [10,5,9,10,12,3,517,69,81,74]

def find_nth_largest(array: 'List', n: int) -> 'List':
    # copy1 = deepcopy(array)
    # Here deepcopy is not necessary since you don't have nested lists shallow copy will  do. 
    copy1 = array[:] # this gets you a copy of one dimensional array. 
    copy1.sort(reverse=True)
    largest_n, count = [copy1[0]], 0
    for item in copy1[1:]:
        if count < n - 1:
            if item != largest_n[-1]:
                largest_n.append(item)
                count  = 1
        else:
            return largest_n
    return largest_n

print(find_nth_largest(array, 3))

CodePudding user response：

I wanted to play with the heapq.largest method suggested by ShadowRanger and I came up with this bit of code (doesn't answer the OC, but it's good to know):

import heapq
array= [10,5,517,9,10,81,12,3,517,69,81,74]

# Repeats are not ok
print( sorted(list(set(array)))[-3:] )

# repeats allowed
print( sorted(array)[-3:] )

# repeats not allowed
print(heapq.nlargest( 3, list(set(array)) ) )

# repeats allowed
print(heapq.nlargest( 3, array ) )

Here are the results:

[74, 81, 517]
[81, 517, 517]
[517, 81, 74]
[517, 517, 81]

Fortunately, the results are the same (more or less). I had to modify the input a little bit to give extra large values.

The list(set(array)) was used to eliminate "repeats".