Python disregarding zeros when sorting numbers in a list

Question

I'm trying to sort a list of dollar amounts from lowest to highest using python's built in sort ability, but when I call on it, it sorts the numbers super screwy. It starts at $10,000 then goes up to $19,0000 (which is the highest) then jumps down to $2,000 and counts up from there ostensibly because 2 is bigger than 1. I don't know how to correct for this. The code I've used is below.

numbers=[['$10014.710000000001'], ['$10014.83'],['$11853.300000000001'],
['$19060.010000000006'],['$2159.1099999999997'],['$3411.1400000000003']]

print(sorted(numbers))

its because your "numbers" are not numbers and poor python has to Lexicographical sort the strings. want numerical sort, you'd need to temporarily convert to numbers for sorting. — Paritosh Singh, Dec 15 '18 at 22:31
Possible duplicate of https://stackoverflow.com/questions/3426108/how-to-sort-a-list-numerically. — miike3459, Dec 15 '18 at 22:31
Possible duplicate of [How to sort a list numerically?](https://stackoverflow.com/questions/3426108/how-to-sort-a-list-numerically) — snakecharmerb, Dec 15 '18 at 22:37

user8408080 · Answer 1 · 2018-12-15T22:41:17.627

2

You are not sorting numbers but strings, which explains the "weird" result. Instead, change your type to float and sort the resulting list:

In [3]: sorted([[float(el[0][1:])] for el in numbers])
Out[3]: 
[[2159.1099999999997],
 [3411.1400000000003],
 [10014.710000000001],
 [10014.83],
 [11853.300000000001],
 [19060.010000000006]]

I need the el[0] because every number is inside its own list, which is not a good style, but I guess you have your reasons for this. The [1:] strips away the $ sign.

EDIT really good point made in the comments. More robust solution:

from decimal import Decimal

import decimal

decimal.getcontext().prec = 4

sorted([Decimal(el[0][1:]) for el in numbers])
Out[8]: 
[Decimal('2159.1099999999997'),
 Decimal('3411.1400000000003'),
 Decimal('10014.710000000001'),
 Decimal('10014.83'),
 Decimal('11853.300000000001'),
 Decimal('19060.010000000006')]

edited Dec 15 '18 at 22:41

answered Dec 15 '18 at 22:33

user8408080

2,428
1
10
19

Yes, the list within a list formatting is due to some other functions I have to perform with the data. Thanks for your inputs! – Ace Savage Dec 15 '18 at 22:37
If you don't want this kind of encapsulation in the sorted list, just change `sorted([[float(el[0][1:])] for el in numbers])` to `sorted([float(el[0][1:]) for el in numbers])` :) – user8408080 Dec 15 '18 at 22:38
1

Don't use `float` for currency values! – Daniel Pryden Dec 15 '18 at 22:38

finefoot · Answer 2 · 2018-12-15T22:46:12.007

Your numbers are currency values. So as pointed out in the comments below, it might make sense to use Python's decimal module which offers several advantages over the float datatype. (See link for further information.)

If, however, this is only an exercise for better getting to know Python, as I suspect. You might look for a simpler solution:

The reason, why your sorting doesn't work, is because your numbers are stored in the list inside another list as a string. You have to convert them to integers or floats before sorting has the effect you're looking for:

numbers=[
    ['$10014.710000000001'],
    ['$10014.83'],
    ['$11853.300000000001'],
    ['$19060.010000000006'],
    ['$2159.1099999999997'],
    ['$3411.1400000000003']
]

numbers_float = [float(number[0][1:]) for number in numbers]
numbers_float.sort()

print(numbers_float)

Which prints:

[2159.1099999999997, 3411.1400000000003, 10014.710000000001, 10014.83, 11853.300000000001, 19060.010000000006]

When you look at float(number[0][1:]), then [0] takes the first (and only) number of your (inner) number list, [1:] strips the $ sign and finally float does the conversion to floating point number.

If you want the $ sign back:

for number in numbers_float:
    print("${}".format(number))

Which prints:

$2159.1099999999997
$3411.1400000000003
$10014.710000000001
$10014.83
$11853.300000000001
$19060.010000000006

Okay, that makes sense. I'll give that a shot. Thanks very much! — Ace Savage, Dec 15 '18 at 22:36
Don't use `float` for currency values, since it doesn't round the same way decimal numbers round. You probably want to use the `decimal` module instead. — Daniel Pryden, Dec 15 '18 at 22:37
@DanielPryden I highly suspect that this is not necessary here but you're definitely right to mention it. Doesn't hurt, so I added it to the answer as a disclaimer. — finefoot, Dec 15 '18 at 22:46

Daniel Pryden · Accepted Answer · 2018-12-15T22:51:18.300

The key insight here is that the values in your list are actually strings, and strings are compared lexically: each character in the string is compared one at a time until the first non-matching character. So "aa" sorts before "ab", but that also means that "a1000" sorts before "a2". If you want to sort in a different way, you need to tell the sort method (or the sorted function) what it is you want to sort by.

In this case, you probably should use the decimal module. And you want the key attribute of the sort method. This will sort the existing list you have, only using the converted values during the sorting process.

import decimal

def extract_sortable_value(value):
    # value is a list, so take the first element
    first_value = value[0]
    return decimal.Decimal(first_value.lstrip('$'))

numbers.sort(key=extract_sortable_value)

Equivalently, you could do:

print(sorted(numbers, key=extract_sortable_value))

Demo: https://repl.it/repls/MiserableDarkPatches

Python disregarding zeros when sorting numbers in a list

3 Answers3