CODEWITHSUNDEEP
pythonpython-3.xsortingtuples
Jodo1992
Jodo1992

Reputation: 733

list.sort() not sorting values correctly by second tuple parameter

I am trying to sort a list of tuples by the second parameter in the tuple in Python 3.5.2 to find which algorithms take the least -> most time in ascending order, however for some reason the looks to be sorting by random. My code:

import math

def speeds(n):

    new_dictionary = {}

    six_n_log_n = 6 * n * math.log(n, 2)
    thr_n_exp05 = 3 * (n ** 0.5)
    four_expn = 4 ** n
    ceil_sqrt_n = math.ceil(math.sqrt(n))
    five_n = 5 * n
    n_cubed = n ** 3
    log_log_n = math.log(math.log(n, 2))
    n_exp_01 = n ** 0.01
    floor_2_n_log_exp2_n = math.floor(2 * n * (math.log(n, 2)**2))
    n_exp2_log_n = (n ** 2) * math.log(n, 2)
    log_exp2_n = math.log(n, 2) ** 2
    one_div_n = 1 / n
    two_exp_n = 2 ** n
    four_exp_logn = 4 ** (math.log(n, 2))
    two_exp_logn = 2 ** (math.log(n, 2))
    four_n_exp_threehalves = 4 * (n ** (3/2))
    n_exp2 = n ** 2
    sqrt_log_n = math.sqrt(math.log(n, 2))

    new_dictionary[0] = six_n_log_n
    new_dictionary[1] = thr_n_exp05
    new_dictionary[2] = four_expn
    new_dictionary[3] = ceil_sqrt_n
    new_dictionary[4] = five_n
    new_dictionary[5] = n_cubed
    new_dictionary[6] = log_log_n
    new_dictionary[7] = n_exp_01
    new_dictionary[8] = floor_2_n_log_exp2_n
    new_dictionary[9] = n_exp2_log_n
    new_dictionary[10] = log_exp2_n
    new_dictionary[11] = one_div_n
    new_dictionary[12] = two_exp_n
    new_dictionary[13] = four_exp_logn
    new_dictionary[14] = two_exp_logn
    new_dictionary[15] = four_n_exp_threehalves
    new_dictionary[16] = n_exp2
    new_dictionary[17] = sqrt_log_n

    sorted_list = []
    for key in new_dictionary:
        sorted_list.append((key, new_dictionary[key]))

    sorted_list.sort(key=lambda x: x[1])

    for i, x in sorted_list:
        print(sorted_list[i])

    return sorted_list

n = 15
speeds(n)

The expected output should be tuples in ascending order by the second parameter, but instead, I receive this:

(15, 232.379000772445)
(10, 15.263794126054286)
(14, 15.000000000000002)
(2, 1073741824)
(17, 1.9765855902562173)
(7, 1.027450511266727)
(9, 879.0503840119167)
(13, 225.00000000000006)
(3, 4)
(12, 32768)
(8, 457)
(5, 3375)
(11, 0.06666666666666667)
(4, 75)
(16, 225)
(1, 11.618950038622252)
(0, 351.6201536047667)
(6, 1.3627418135330593)

Can anyone tell me why I'm getting a seemingly random order from this? Can't seem to find where my problem is.

Upvotes: 0

Views: 190

Answers (2)

Erik Godard
Erik Godard

Reputation: 6030

If you examine sorted_list following the sort, you will see that it has been sorted correctly.

[(11, 0), (7, 1.027450511266727), (6, 1.3627418135330593), (17, 1.9765855902562173), (3, 4.0), (1, 11.618950038622252), (14, 15.000000000000002), (10, 15.263794126054286), (15, 60), (4, 75), (16, 225), (13, 225.00000000000006), (0, 351.6201536047667), (8, 457.0), (9, 879.0503840119167), (5, 3375), (12, 32768), (2, 1073741824)]

The error occurs in the following line:

for i, x in sorted_list:

You are not iterating over the keys and values as you think. Rather, this is unpacking each tuple in the list and assigning its first component to i and its second component to x. You are then accessing the element at the ith position in the list, which leads to what appears to be a random ordering. You can instead write:

for i, x in enumerate(sorted_list):

Or more simply, you can print the tuple you are trying to display

for item in sorted_list:
    print(item)

Upvotes: 4

enderland
enderland

Reputation: 14135

When you iterate over your tuples, you want to print the tuple itself:

for tup in sorted_list:
    print(tup)

otherwise, you are printing the values at the index based on the first value of the index. For example, the first value in the sorted list is:

(11, 0)

is actually looking for:

sorted_list[11]

which is why you see the improper first value.

Upvotes: 1

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