How do I rearrange my Numpy array by a specific sequence?

Question

I'd like to rearrange the following Numpy array:

X = [ 1.  5.  2.  4.  2.  4.  1.  5.  2.  1.  2.  1.  5.  6.  2.  6.  5.  4.
  3.  1.  4.  6.  5.  3.  1.  5.  4.  5.  3.  3.  1.  4.  4.  5.  4.  4.
  3.  6.  1.  5.  4.  1.  4.  4.  1.  5.  1.  2.  1.  4.  6.  1.  3.  4.
  1.  6.  3.  1.  1.  5.  6.  4.  5.  2.  6.  3.  1.  3.  4.  6.  3.  2.
  1.  4.  2.  4.  2.  1.  2.  2.  1.  1.  6.  4.  3.  6.  1.  1.  4.  1.
  4.  4. nan nan nan nan]

by the following sequence 3, 2, 6, 5, 4, 1.

Essentially, the entire array is arranged such that all the 3's come first, 2's, 6's, 5's, 4's and finally 1's. What is the best way to do this, while retaining the Numpy array instead of turning it into a list?

Possible duplicate of sort numpy array with custom predicate — Derte Trdelnik
– Derte Trdelnik, Commented May 24, 2019 at 7:11

Chris · Accepted Answer · 2019-05-24 07:11:21Z

1

Using list.index:

ind = [3, 2, 6, 5, 4, 1]
sorted(X, key=lambda x : ind.index(x) if x in ind else -1)

Output:

[nan,
 nan,
 nan,
 nan,
 3,
 ...
 1,
 1,
 1]

If you want nan to come at last:

last = len(ind)
sorted(X, key=lambda x : ind.index(x) if x in ind else last)

Output:

[3,
 3,
 ...
 1,
 1,
 nan,
 nan,
 nan,
 nan]

answered May 24, 2019 at 7:11

Chris

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6 Comments

Michiko C Over a year ago

Hi Chris, thank you! Is there a way of retaining the object type such that it is a numpy array rather than a list?

Chris Over a year ago

@MichikoC You can always wrap sorted(...) with np.array: np.array(sorted(X, key = ...))

Michiko C Over a year ago

Thanks, I tried that, but it seems that key can't be something like ind = [3, 2, 6, 5, 4, 1] though?

Chris Over a year ago

You mean without using lambda? No you need to use ind inside of lambda just like in the answer ;)

Michiko C Over a year ago

Oh! I apologize - I meant, I tried using np.array(sorted(X, key = ...)) but it seems key can't be something like key = ind.

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