How to append an array to a text file in python

Question

I am trying to create a file with several lines as header and append a column of numbers. I am having some issue appending a numpy.matrix after a header. I wrote a code as below

import numpy as np
import pandas as pd
import xlrd

df = pd.read_csv('Region_SGeMS.csv', header=None)

matrix = np.asmatrix(df)

#print(matrix)

s = np.shape(matrix)
print(s)
row = s[0]
col = s[1]

a = np.flip(matrix, 0)

b = np.reshape(a, (400, 1))

print(b)

f = open('Region.txt', 'w')

f.write(str(s[0]))
f.write(' ')
f.write(str(s[1]))
f.write(' 1 \n')
f.write('1 \n')
f.write('facies \n')

with open('Region.txt', 'a+') as outfile:
    np.savetxt(outfile,b)

However, the highlighted number should be 2, not 0. I also attached a screenshot of my original excel file. Here is a screenshot of my result

You really shouldn't use numpy.matrix objects, stick to normal multi-dimensional arrays. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Feb 21, 2019 at 21:33

jyalim · Accepted Answer · 2019-02-21 21:41:20Z

1

Note that numpy.savetxt will append a header string to the file for you if you let it. For instance:

import numpy as np
# Recreate the original csv data
one_block = np.ones((10,10))
A   = np.block([ [0*one_block,one_block],[2*one_block,3*one_block] ])
M,N = A.shape
# Recreate b
a   = np.flip(A,0)
b   = a.reshape(400,1)
hdr_str = """{M:d} {N:d} 1
1
facies""".format(M=M,N=N)
outfile = 'Region.txt'
np.savetxt(outfile,b,header=hdr_str,comments='')

Below is a screenshot of my attempt at recreating your problem. There is no spurious 0.

edited Feb 21, 2019 at 21:41

answered Feb 21, 2019 at 6:33

jyalim

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2 Comments

Z.Cao Over a year ago

My b vector should start with a 2. I attached a screenshot of my original excel file. Please have a look.

Z.Cao Over a year ago

I guess the issue may be related to the 'with open' command. Nevertheless, thank you for your help.

orli · Accepted Answer · 2019-02-21 04:24:37Z

0

f = open('Region.txt', 'w')
f.write('first line \n')
f.write('second line \n')
f.close()

with open('Region.txt', 'a+') as outfile:
    np.savetxt(outfile,np.random.rand(10,3))

answered Feb 21, 2019 at 4:24

orli

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2 Comments

orli Over a year ago

i assumed since the question is about appending, that you first want a txt file, and then later append a np array to it...

Z.Cao Over a year ago

The issue is that there is a weird 0 at the beginning of my array but it should be a 2. I also noticed that if I put np.savetxt(outfile, b, fmt='%0.4f'), I will lose some of my data.

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