Python: Adding a column of the same value

Question

I have a tab-delimeted file that contains 3 columns. I would like to add a new first column of just same number 1.

inputfile is

This is what I would like to have for my outputfile:

This is what I have so far:

#!/usr/bin/env python
import sys
import csv  
f=open('inputfile.txt', 'r+')
t=[]
for line in f.readlines():
    t.append('\n')
    t.append(1)
    f.writelines(t)

However, I am getting an error: Traceback (most recent call last): File "./py.py", line 6, in sys.stdout('inputfile.txt', 'w') TypeError: 'file' object is not callable

sys.stdout is used for output stream. It's not a function. — Kasravnd
– Kasravnd, Commented Apr 1, 2016 at 18:33
You'd want to open your output file the same way you opened your input. fout = open('outputfile.txt', 'w') and fin=open('inputfile.txt', 'r+') Also, why are you trying to read and write from your input file? — Aaron
– Aaron, Commented Apr 1, 2016 at 18:37
File objects in python used by the interpreter for representing the open files. And sys.stdout object initialized to file objects corresponding to the interpreter’s standard output. It's just a regular text file like those returned by the open() function. Read more docs.python.org/3.4/library/sys.html#sys.stdout — Kasravnd
– Kasravnd, Commented Apr 1, 2016 at 18:41

Parfait · Accepted Answer · 2016-04-01 19:43:36Z

2

Simply open both files and write to it, concatenating a 1 and tab \t to each line in a running list. Then output list to new file:

f1 = "Input.txt"
f2 = "Output.txt"

t = []
with open(f1, 'r') as txt1:
    for rline in txt1:       
        t.append("1\t" + rline)

with open(f2, 'w') as txt2:
    for i in t:
        txt2.write(i)

#1  a   3   6
#1  b   3   5
#1  c   3   5
#1  d   8   4

Alternatively, to avoid use of a list (but requires appending to file with 'a'):

with open(f1, 'r') as txt1:
    for rline in txt1:       
        rline = "1\t" + rline

        with open(f2, 'a') as txt2:
            txt2.write(rline)

And even further suggested by @JonClements that avoids the overhead of opening/closing file with each line:

with open(f1) as txt1, open(f2, 'w') as txt2:
        txt2.writelines('1\t' + line for line in txt1)

edited Apr 1, 2016 at 19:43

answered Apr 1, 2016 at 18:42

Parfait

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2 Comments

Jon Clements Over a year ago

You can change your later example to be: with open(f1) as txt1, open(f2, 'w') as txt2 then make your entire statement block txt2.writelines('1\t' + line for line in txt1) - this'll avoid the overhead of opening/closing the file each line of the input...

Jon Clements Over a year ago

Just to note - it's not a list-comp - it's a generator expression :)

mushfiq · Accepted Answer · 2016-04-01 18:45:58Z

1

You don't need sys module for the solution:

#!/usr/bin/env python
output_file = open('output.txt', 'w')
input_file = open('input.txt', 'r+')
for line in input_file.readlines():
    line = line.split(" ")
    line.insert(1, str(1))
    line = (" ").join(line)
    output_file.write(line)

input_file.close()    
output_file.clsoe()

cat output.txt gives output:

answered Apr 1, 2016 at 18:45

mushfiq

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Comments

Aaron · Accepted Answer · 2016-04-01 18:43:22Z

0

This will give you exactly what your desired output shows:

fin = open('inputfile.txt', 'r+')
fout = open('outputfile.txt', 'w')
t=[]

for line in fin.readlines():
    t.append('1 ' + line)

fout.writelines(t)

answered Apr 1, 2016 at 18:43

Aaron

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1 Comment

Jon Clements Over a year ago

OP states I have a tab-delimeted file (so your '1 ' + line' will be prepending a value to the first column - not creating a new one) - also - readlines will read the entire file into a list first, you're then append then to another completely new list - you can just loop over fin directly and write line-by-line.

scope · Accepted Answer · 2016-04-01 18:54:00Z

0

If you do not want nothing else you can skip csv module and just prepend each line in a file, like this:

open('out.txt', 'w').writelines(["1\t" + line for line in open("in.txt")])

edited Apr 1, 2016 at 18:54

answered Apr 1, 2016 at 18:41

scope

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