Jodo1992 Jodo1992 - 1 month ago 21
Python Question

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.

Answer

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)