Writing Python arrays into txt file, one array per line

Question

I would like to write a number of Python Arrays into a txt file, with one array per line. After which I would like to read the Arrays line by line.

My work in progress code below. The problem I am working on involves about 100,000 arrays (length of L)

from __future__ import division
from array import array

M = array('I',[1,2,3])
N = array('I',[10,20,30])

L = [M,N]

with open('manyArrays.txt','w') as file:
    for a in L:
        sA = a.tostring()
        file.write(sA + '\n')

with open('manyArrays.txt','r') as file:
    for line in file:   
        lineRead = array('I', [])
        lineRead.fromstring(line)
        print MRead

The error message I get is

lineRead.fromstring(line)
ValueError: string length not a multiple of item size

array.array has a method called literally array.tostring. — ForceBru
– ForceBru, Commented Sep 14, 2018 at 16:19

Adrien Mau · Accepted Answer · 2018-09-14 16:25:36Z

1

You can either use numpy function for this, or code lines yourself:

You could concatenate your arrays in one 2D array and save it directly with np.savetxt, load it with np.genfromtext :

M = np.array([1,2,3],dtype='I')
N = np.array([10,20,30],dtype='I')
data= np.array([M,N])

file='test.txt'
np.savetxt(file,data)
M2,N2 = np.genfromtxt(file)

Or do :

file2='test2.txt'
form="%i %i %i \n"
with open(file2,'w') as f:
    for i in range(len(data)):
        f.write(form % (data[i,0],data[i,1],data[i,2]))

answered Sep 14, 2018 at 16:25

Adrien Mau

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2 Comments

ForceBru Over a year ago

array.array that OP use is not numpy.array.

Adrien Mau Over a year ago

Indeed @ForceBru. While this is not clear in my post, the numpy solution still works with array.array elements. np.savetxt('todel;txt',L) and M2,N2 = np.genfromtxt(file) will yield the wanted result. For the classical way you would just change the indexing with [i][k] :

file2='test2.txt' form="%i %i %i \n" with open(file2,'w') as f:     for i in range(len(data)):         f.write(form % (data[i][0],data[i][1],data[i][2]))

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