Add a list to a numpy array

Question

Right now I'm writing a function that reads data from a file, with the goal being to add that data to a numpy array and return said array.

I would like to return the array as a 2D array, however I'm not sure what the complete shape of the array will be (I know the amount of columns, but not rows). What I have right now is:

columns = _____
for line in currentFile:
     currentLine = line.split()
     data = np.zeros(shape=(columns),dtype=float)
     tempData = []
     for i in range(columns):
          tempData.append(currentLine[i])
      data = np.concatenate((data,tempdata),axis=0)

However, this makes a 1D array.

Essentially what I'm asking is:

Is there any way to have add a python list as a row to a numpy array with a variable amount of rows?

Don't even bother reading it by hand like this, you can simply use numpy.loadtxt to read write into a numpy array. Similarly you can use numpy.genfromtxt — Cory Kramer
– Cory Kramer, Commented Jun 7, 2016 at 22:33

Abstracted · Accepted Answer · 2016-06-07 22:40:37Z

1

If your file data.txt is

All you need to do is

>>> import numpy as n
>>> data_array = n.loadtxt("data.txt")
>>> data_array

array([[1., 2., 3., 4.],
       [1., 2., 3., 4.],
       [1., 2., 3., 4.],
       [1., 2., 3., 4.],
       [1., 2., 3., 4.],
       [1., 2., 3., 4.]])

answered Jun 7, 2016 at 22:40

Abstracted

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Comments

hpaulj · Accepted Answer · 2016-06-08 00:14:08Z

1

Look at the common way of constructing an array:

np.array([[1,2,3],[4,5,6],[7,8,9]])
np.array([[1,2,3],
          [4,5,6],
          [7,8,9]])

The input is a list of lists. Your reader could imitate that

Roughly:

data = []
for line in f.readline():
    values = line.strip().split(',')
    values = [int(v) for v in values]
    data.append(values)
data = np.array(data)

np.loadtxt and np.genfromtxt do essentially that, just with more bells and whistles.

edited Jun 8, 2016 at 0:14

answered Jun 7, 2016 at 22:43

hpaulj

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Comments

Souvik Mukherjee · Accepted Answer · 2020-07-25 02:06:58Z

0

If you modify @Abstracted 's solution as:

data_array = np.loadtxt("data.txt", dtype =int)

you will get the array in integer form if you want it that way.

edited Jul 25, 2020 at 2:06

answered Jul 23, 2020 at 14:45

Souvik Mukherjee

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