How do I sort by first item of array element?

Question

let's assume I have an array of form

a = [ 
  [ [12, 4, 2, 5] , [10, 12, 4, 2], [2, 2, 10, 2], [7, 10, 3 ,2] ],
  [ [13, 23, 4, 5] , [10, 12, 4, 2], [2, 2, 12, 1], [7, 10, 3 ,2] ],
  [ [4, 4, 2, 5] , [10, 12, 4, 2], [2, 2, 3, 2], [7, 10, 3 ,2] ], 
]

I want to sort by the first column of each element, so the first entry of a would read:

[ [2, 2, 10, 2], [7, 10, 3 ,2], [10, 12, 4, 2], [12, 4, 2, 5] ]

I've found a solution that looks like a[a[:,1].argsort()] and tried a for loop over the first index like:

for i in range(0,4):
  a[i,...] = a[a[i,0,:].argsort()]

or

for i in range(0,4):
  a[i,...] = a[i, a[i,0,:].argsort()]

This doesn't work tho. I really can't get my head around this problem.

yatu · Accepted Answer · 2020-07-09 13:59:31Z

2

You can use argsort of the array indexed along the first columns, and expanding dims appropriately, and then np.take_along_axis:

ixs = a[...,0].argsort(1)[...,None]
np.take_along_axis(a, ixs, 1)

array([[[ 2,  2, 10,  2],
        [ 7, 10,  3,  2],
        [10, 12,  4,  2],
        [12,  4,  2,  5]],

       [[ 2,  2, 12,  1],
        [ 7, 10,  3,  2],
        [10, 12,  4,  2],
        [13, 23,  4,  5]],

       [[ 2,  2,  3,  2],
        [ 4,  4,  2,  5],
        [ 7, 10,  3,  2],
        [10, 12,  4,  2]]])

answered Jul 9, 2020 at 13:59

yatu

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user3482098 · Accepted Answer · 2020-07-09 13:53:08Z

1

Try:

sorted_array = [sorted(x, key=lambda y: y[0]) for x in a]

How does this work?

first we use list comprehension to map the array a into a new one, performing an operation on each element (e.g. [x + 1 for x in my_numbers])
The operation we perform on each element of a is to use sorted. This returns a copy of an array, sorted by comparing whatever value is returned by key.
key must be a function that returns a 'sortable' value. Here, we're using a lambda to create a one-line function that returns the first item from the array passed to it. This ensures that we sort based on the first column of each element of a.

answered Jul 9, 2020 at 13:53

user3482098

2 Comments

Axel Stoll Over a year ago

This solution didn't let me plot the array with matplot anymore.

user3482098 Over a year ago

Well, in fairness, you didn't mention you were using matplot ¯\_(ツ)_/¯

ehsan khodadadi · Accepted Answer · 2020-07-09 15:49:20Z

1

If you don’t want to use lambda method try this one:

def select(item):
  return item[0]


s_array=[]
for i in a:
  s_array.append(sorted(i, key=select))

edited Jul 9, 2020 at 15:49

answered Jul 9, 2020 at 15:22

ehsan khodadadi

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