Split Numpy Array into Arrays of each column

Question

I have a Numpy array as a list of lists with dimension of n by 4 (row, column). I am trying to separate the data from each individual list instance into four separate arrays each containing all the information from a single column so I can add it to a pandas data frame. From this:

[[126 188 166   1]
 [111 173 149   1]
 [ 81 119 123   2]
 [ 83 122 124   2]
 [ 84 122 124   2]
 [255 255 255   3]
 [255 255 255   3]
 [255 255 255   3]]

To this:

bBand = [126,111,81,...,255]
gBand = [188,173,119,...,255]
rBand = [166,149,123,...,255]
class = [1,1,2,...,3]

Current Code:

   MasterList = np.arrray([[126, 188, 166,   1],[111, 173, 149,   1],[ 81, 119, 123,   2],[ 83, 122, 124,   2],[ 84, 122, 124,   2],[255, 255, 255,   3],[255, 255, 255,   3],[255, 255, 255,   3]])
   print(MasterList)
   columns = ["bBand","gBand","rBand","class"]
   df = pd.DataFrame(MasterList.reshape(-1, len(MasterList)),columns=columns)

You say you want to create four separate arrays "so I can add it to a pandas data frame", but if all you want is df, pd.DataFrame(MasterList, columns=columns) would have worked. Why do you need the separation, and why the reshape? — DSM
– DSM, Commented Aug 30, 2018 at 16:50

Dani Mesejo · Accepted Answer · 2018-08-30 17:08:08Z

As @DSM mentioned, you can do it like this:

import numpy as np
import pandas as pd

data = np.array([[126, 188, 166, 1],
                 [111, 173, 149, 1],
                 [81, 119, 123, 2],
                 [83, 122, 124, 2],
                 [84, 122, 124, 2],
                 [255, 255, 255, 3],
                 [255, 255, 255, 3],
                 [255, 255, 255, 3]])

frame = pd.DataFrame(data=data, columns=["bBand","gBand","rBand","class"])
print(frame)

Output

   bBand  gBand  rBand  class
0    126    188    166      1
1    111    173    149      1
2     81    119    123      2
3     83    122    124      2
4     84    122    124      2
5    255    255    255      3
6    255    255    255      3
7    255    255    255      3

There is no need to reshape the array. If you want separate lists, you can try this:

data = np.array([[126, 188, 166, 1],
                 [111, 173, 149, 1],
                 [81, 119, 123, 2],
                 [83, 122, 124, 2],
                 [84, 122, 124, 2],
                 [255, 255, 255, 3],
                 [255, 255, 255, 3],
                 [255, 255, 255, 3]])


for name, column in zip(["bBand","gBand","rBand","class"], data.T):
    print(name, column)

Output

bBand [126 111  81  83  84 255 255 255]
gBand [188 173 119 122 122 255 255 255]
rBand [166 149 123 124 124 255 255 255]
class [1 1 2 2 2 3 3 3]

Finally you can set the values directly:

bBand = list(data[:, 0])
gBand = list(data[:, 1])
rBand = list(data[:, 2])
_class = list(data[:, 3])

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