Sorting a "1D" ndarray in Python

Question

As following my previous question. I now have a ndarray with the form:

 [[  0.        ]
 [ 18.21545873]
 [ 14.93752607]
 [ 15.55222205]
 [ 14.93752607]
 [ 10.68406032]
 [ 11.54598659]
 [ 15.55222205]
 [ 11.54598659]
 [ 12.30302814]]

How do I sort this array? Since it is not a normal list. It is a ndarray, in other words, it is a list of lists.
Is normal that people sort this list of lists directly, or people usually change it to a normal list

[ 0. , 18.21545873, 14.93752607, 15.55222205, 14.93752607, 10.68406032, 11.54598659, 15.55222205, 11.54598659, 12.30302814]

and then sort this list?

Many thanks!

You can sort a 2D numpy array by a specific column. In your case, this will simply be column 0. See here stackoverflow.com/a/2828371/2296458 — Cory Kramer
– Cory Kramer, Commented Apr 4, 2014 at 20:00
Thanks @Cyber, I just found out this, and applied to my case. But still, about my second question: usually do people treat this kind of 1d array as a "1d ndarray", or they would change it to a list? And how can we change this 1d array as a list? Thanks! — Murphy
– Murphy, Commented Apr 4, 2014 at 20:18
it depends on your intentions. If your 2D array will only ever have one element in each array, then of course you only need a 1D array. To answer your other question, the way you turn a numpy array to a list is: say your numpy array is numpyArray, you just say newList = list(numpyArray) — Cory Kramer
– Cory Kramer, Commented Apr 4, 2014 at 20:21
Hi @Cyber, what I got from newList = list(numpyArray), is array([0.]), array([18.21545873]), .... But what I meant is [0., 18.21545873, ...]. Do I need to open another question for this? — Murphy
– Murphy, Commented Apr 4, 2014 at 20:32
sorry my mistake, need some more coffee ;) say your numpy array was named x, try this: [i for i in x[:,0]] — Cory Kramer
– Cory Kramer, Commented Apr 4, 2014 at 20:38

Octopus · Accepted Answer · 2014-04-04 20:03:03Z

1

Nothing complicated.

l = [[  0.        ],
     [ 18.21545873],
     [ 14.93752607],
     [ 15.55222205],
     [ 14.93752607],
     [ 10.68406032],
     [ 11.54598659],
     [ 15.55222205],
     [ 11.54598659],
     [ 12.30302814]]
l.sort()

produces what you want. Maybe I'm missing what the problem is.

answered Apr 4, 2014 at 20:03

Octopus

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3 Comments

Murphy Over a year ago

thank you! But when I tried this, I got "None". Is this related to the fact that my 1d array does not have "," as a separator? And as my second question in my original post, do you know that usually people use "ndarrays" or "list of lists"? And is there any way to transfer a ndarray to a list? Thanks!

Cory Kramer Over a year ago

@Murphy sort() does not return anything, so you cannot say newArray = l.sort(). It will be sorting the array l in place, so if you print l after you sort() it, you will notice it has been sorted.

Saullo G. P. Castro Over a year ago

@Murphy, if you want to get the returned value, you can perhaps use new = sorted(l)

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