How to add values to existing columns in a text file (using Python)

Question

I am currently generating 8 random values each time I run a program on Python. These 8 values are different each time I run the program, and I would like to be able to now save these 8 values each time I run the program to a text file in 8 separate columns. When saving these values for future runs, though, I would like to still be able to keep previous values. For example: after run 1, the text file will be 8x1, after run 2, the text file will be 8x2, and after run n, the text file will be 8xn.

I have been looking at solutions like this: save output values in txt file in columns python

And it seems using 'a' instead 'w' will append my new values instead of overwriting previous values. I've been trying to follow the documentation on the method .write but just don't quite see how I can write to a particular column using this method. I have been able to simply write each column in its own text file, but I'd rather be able to write the columns together in the same text file for future runs I do with this program.

Edit: my outputs will be 8 floating point numbers and to reiterate, they will be random each time.

So after 1 run, I will create 8 floating point values: Run11, Run12, Run13, Run14, Run15, Run16, Run17, Run18. After my second run, I will create another set of values (8 entries long): Run21, Run22, Run23, Run24, Run25, Run26, Run27, Run28.

In the text file, I would like these values to be placed in specific columns like this: https://i.sstatic.net/tqUQI.jpg (this is what it would look like after 2 runs). The "Value n:" titles are the headers for each column.

My output will be 8 floating point numbers. I will edit my main question to include an example. — Mathews24
– Mathews24, Commented Jul 20, 2015 at 14:19
@PadraicCunningham: My guess is that Mathews24 simply wants a new row with 8 columns each time. — PM 2Ring
– PM 2Ring, Commented Jul 20, 2015 at 14:24

Padraic Cunningham · Accepted Answer · 2015-07-20 14:57:38Z

1

import csv
from tempfile import NamedTemporaryFile
from shutil import move
from itertools import chain
with open("in.csv") as f, NamedTemporaryFile(dir=".", delete=False) as temp:
    r = csv.reader(f)
    new = [9, 10, 11, 12, 13, 14, 15, 16]
    wr = csv.writer(temp)
    wr.writerows(zip(chain.from_iterable(r), new))

move(temp.name, "in.csv")

Input:

Output:

1,9
2,10
3,11
4,12
5,13
6,14
7,15
8,16

To take the header into account:

with open("in.csv") as f, NamedTemporaryFile(dir=".", delete=False) as temp:
    r = csv.reader(f)
    header = next(r)
    new = [9, 10, 11, 12, 13, 14, 15, 16]
    wr = csv.writer(temp)
    wr.writerow(header+["Value {}:".format(len(header)+1)])
    wr.writerows(zip(chain.from_iterable(r), new))
move(temp.name, "in.csv")

Input:

Value 1:
1
2
3
4
5
6
7
8

Output:

Value 1:,Value 2:
1,9
2,10
3,11
4,12
5,13
6,14
7,15
8,16

If you are adding an actual row each tie and not a column then just append:

with open("in.csv","a") as f:
    new = [9, 10, 11, 12, 13, 14, 15, 16]
    wr = csv.writer(f)
    wr.writerow(new)

Input:

value 1:,value 2:,value 3:,value 4:,value 5:,value 6:,value 7:,value 8:
1,2,3,4,5,6,7,8

Output:

value 1:,value 2:,value 3:,value 4:,value 5:,value 6:,value 7:,value 8:
1,2,3,4,5,6,7,8
9,10,11,12,13,14,15,16

edited Jul 20, 2015 at 14:57

answered Jul 20, 2015 at 14:27

Padraic Cunningham

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2 Comments

PM 2Ring Over a year ago

Nice. And it conforms to the OP's last comment. But not to the layout in imgur.com/zxoxaKM

Padraic Cunningham Over a year ago

@PM2Ring, I was actually going to mention the header as I did not see any but I will edit to account for it.

Danny · Accepted Answer · 2015-07-20 15:10:18Z

-1

what about

a = [1, 2, 3, 4, 5, 6, 7, 8]

f = open('myFile.txt', 'a')
for n in a:
    f.write('%d\t'%n)
f.write('\n')
f.close()

and you get as file content after running it 4 times

1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8

======= EDIT =========

Try this, its ugly but works ;-)

import os.path

h = ['Header 1', 'Hea 2', 'Header 3', 'Header    4', 'H 5', 'Header', 'Header 7', 'Header 8']
a = [1, 2, 3, 4, 5, 6, 7, 8]

fileName = 'myFile.txt'

#write header
withHeader = not os.path.isfile(fileName)

f = open(fileName, 'a')
if withHeader:
    print 'Writing header'
    for s in h:
        f.write('%s\t'%s)
    f.write('\n')

#write numbers
for i in range(0, len(a)):
    space = len(h[i])/2;
    n = a[i]
    for c in range(0, space):
        f.write(' ')
    print 'Writing %d'%n
    f.write('%d'%n)
    for c in range(0, space):
        f.write(' ')
    f.write('\t')
f.write('\n')
f.close()

result:

Header 1    Hea 2   Header 3    Header    4 H 5 Header  Header 7    Header 8    
    1         2         3            4       5     6        7           8       
    1         2         3            4       5     6        7           8       
    1         2         3            4       5     6        7           8       
    1         2         3            4       5     6        7           8

edited Jul 20, 2015 at 15:10

answered Jul 20, 2015 at 14:25

Danny

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2 Comments

Mathews24 Over a year ago

Is there a good way to include headers for each column using this method, yet still being able to retrieve the values using np.loadtxt and not have the string headers interfere with this? @Danny

Mathews24 Over a year ago

I used your original example and for future use of these values I just did: Value1,Value2,Value3,Value4,Value5,Value6,Value7,Value8 = np.loadtxt("path", skiprows = 1, unpack = True). Thank you for the help!

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