Create sorted 2D array from 1D array

Question

I want to turn a 1d array into a sorted 2d array. The 1d array looks like this:

[1,5,8,9,9,1,4,6,7,8,41,4,5,31,6,11]

First, I want to split this array up into a 2d array with a width of 4.

    [[1,5,8,9]
     [9,1,4,6]
     [7,8,41,4]
     [5,31,6,11]
    ]

Then, I want to sort the 2d array from the 3rd value in the 2d array like this:

    [[9,1,4,6],
     [5,31,6,11],
     [1,5,8,9],
     [7,8,41,4]
    ]

I am anticipating that the 1d array will be much larger, so I do not want to manually create the 2d array. How do I approach this?

solarc · Accepted Answer · 2021-09-14 01:07:58Z

1

If you can't use numpy, you can do it like this:

a = [1,5,8,9,9,1,4,6,7,8,41,4,5,31,6,11]
result = []
l = len(a)
for i in range(0, l, 4):
    result.append(a[i:i+4])

result = sorted(result, key = lambda a: a[2])
# result is [[9, 1, 4, 6], [5, 31, 6, 11], [1, 5, 8, 9], [7, 8, 41, 4]]

answered Sep 14, 2021 at 1:07

solarc

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Mạnh Hùng Chu · Accepted Answer · 2021-09-14 01:02:21Z

0

You can try numpy array_split

import numpy as np
a=[1,5,8,9,9,1,4,6,7,8,41,4,5,31,6,11]
b=np.array_split(arr, len(a)/4)
for c in b:
   c.sort()

answered Sep 14, 2021 at 1:02

Mạnh Hùng Chu

613 bronze badges

1 Comment

Thierry Lathuille Over a year ago

Even after correcting the typo (arr/a), this doesn't provide the expected output.

Hanseo Park · Accepted Answer · 2021-09-14 01:09:19Z

0

If you use numpy.argsort, you can sort it easily.

import numpy as np
arr = np.array([1,5,8,9,9,1,4,6,7,8,41,4,5,31,6,11])
arr_2d = np.reshape(arr, (4,4))
sorted_arr = arr_2d[np.argsort(arr_2d[:, 2])]

answered Sep 14, 2021 at 1:09

Hanseo Park

112 bronze badges

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