Python Forum - sorting 2D lists by column

Hello everyone,

I would like to sort a multidimensional list by the second column.

Example:

x = [
    [1,18,2],
    [2,9,4],
    [3,1,1]
    ]

x = sorted(x, key=lambda x: x[2])
print(x)

The output I am looking for should be:

[
[3,1,1],
[2,9,4],
[1,18,2]
]

Unfortunately lamdba sorts like this:

[
[3,1,1],
[1,18,2],
[2,9,4]
]

So lamda does put 18 below 9, because of the '1' in the front.
I did try numpy arrays too, but I just can't find the right way.
Can anyone help me with this problem?

thank you very much!
/ToffieFaye

You are not sorting by second column. As Python uses 0-based indexing you are sorting by third column.

Well, that is embarrasing.
Thank you! If I change 2 to 1 it works in the simple example.

Originally I wanted to sort a list out of a file and there it still doesn't work.
I'll show you the whole code. Maybe you can help?
I'd honestly really appreciate that!

So this is the code:

with open('example.txt','r') as textFile2:
    data_list = [line.split() for line in textFile2]
textFile2.close()

data_list = sorted(data_list, key=lambda data_list: data_list[1])

print(data_list)

and this is how the example.txt file looks like:

16	43	23.5	85.8
17	61	21.1	86.4
18	40	22.5	90.0
19	37	28.5	86.1
20	22	19.4	78.9
21	156	29.2	89.2
22	63	35.4	86.3
23	105	31.3	86.7

This is the output:

Output:
[['23', '105', '31.3', '86.7'], ['21', '156', '29.2', '89.2'], ['20', '22', '19.4', '78.9'], ['19', '37', '28.5', '86.1'], ['18', '40', '22.5', '90.0'], ['16', '43', '23.5', '85.8'], ['17', '61', '21.1', '86.4'], ['22', '63', '35.4', '86.3']]

It works for the numbers with 2 letters, but it still puts 105 and 156 in the front and I don't know why.
Do you understand that?

what you show are strings, not numbers. you need to convert them to number (e.g. float) in order to compare them as you expect

Some observations:

- you don't need to close file if using 'with' (Python does it for you)
- as buran pointed out you sorting strings not ints

Just convert into int and you are good to go: int(data_list[1])

(Sep-25-2019, 06:42 AM)buran Wrote: [ -> ]what you show are strings, not numbers. you need to convert them to number (e.g. float) in order to compare them as you expect

I already tried that.. doesn't make a difference..
I put

map(float, data_list)

as a line before the sorting command

(Sep-25-2019, 06:55 AM)perfringo Wrote: [ -> ]Some observations: - you don't need to close file if using 'with' (Python does it for you) - as buran pointed out you sorting strings not ints Just convert into int and you are good to go: int(data_list[1])

thank you! It still doesn't work though..

Output:
TypeError: int() argument must be a string, a bytes-like object or a number, not 'list'

If you need all data to be floats then you can do it while reading the file. Just add conversion into list comprehension (row #2):

[[float(x) for x in line.split()] for line in textFile2]

(Sep-25-2019, 06:55 AM)ToffieFaye Wrote: [ -> ]It still doesn't work though..

based on 'data' you provided:

>>> sorted(data, key=lambda x: int(x[1]))                 
[['20', '22', '19.4', '78.9'],
 ['19', '37', '28.5', '86.1'],
 ['18', '40', '22.5', '90.0'],
 ['16', '43', '23.5', '85.8'],
 ['17', '61', '21.1', '86.4'],
 ['22', '63', '35.4', '86.3'],
 ['23', '105', '31.3', '86.7'],
 ['21', '156', '29.2', '89.2']]

No errors and sorted by second column.

although I prefer list comprehension as shown by @perfringo, if you prefer map

data_list = [list(map(float, line.split())) for line in textFile2]

(Sep-25-2019, 07:23 AM)buran Wrote: [ -> ]although I prefer list comprehension as shown by @perfringo, if you prefer map
data_list = [list(map(float, line.split())) for line in textFile2]

thank you so much @perfringo and @buran!!