We Keep Coding

How to get first letter of each line in python?

here is what I got txt and open
txt file looks like
f = open('data.txt', 'r')
print(f.read())
the show['Cat\n','Dog\n','Cat\n','Dog\n'........]
output
But I would like to get this
['C\n','D\n','C\n','D\n'........]

First you'll want to open the file in read mode (r flag in open), then you can iterate through the file object with a for loop to read each line one at a time. Lastly, you want to access the first element of each line at index 0 to get the first letter.
first_letters = []
with open('data.txt', 'r') as f:
for line in f:
first_letters.append(line[0])
print(first_letters)
If you want to have the newline character still present in the string you can modify line 5 from above to:
first_letters.append(line[0] + '\n')

f = open("data.txt", "r")
for x in f:
print(x[0])
f.close()

How do append characters on either side of certain words on every line in a file in python?

Say I have a file my_file, and I want to search for a certain word x on every line of the file, and if the word exists, attach my variable y to the left and right side of the word. Then I want replace the old line with the new, modified line in my_new_file. How do I do this? So far I have:
output = open(omy_new_file, "w")
for line in open(my_file):
if (" " + x + "") in line:

You can try this:
y = "someword"
x = "target_string"
lines = [i.strip('\n') for i in open('filename.txt')]
final_lines = ["{}{}{}".format(y, i, y) if x in i else i for i in lines]
f = open(omy_new_file, "w")
for i in final_lines:
f.write("{}\n".format(i))
f.close()

with open('inputfile.txt', 'r') as infile:
with open('outfile.txt', 'w') as outfile:
for line in infile.readlines():
outfile.write(line.replace('string', y + 'string' + y)

Try This:
with open("my_file", "r") as my_file:
raw_data = my_file.read()
# READ YOUR FILE
new_data = raw_data.split("\n")
for line in new_data:
if "sd" in line:
my_new_line = "y" + line + "y"
raw_data = raw_data.replace(line, my_new_line)
print(raw_data)

It's tough to replace a line in a file while reading it, for the same reason that it's tough to safely modify a list as you iterate over it.
It's much better to read through the file, collect a list of lines, then overwrite the original. If the file is particularly large (such that it would be infeasible to hold it all in memory at once), you can write to disk twice.
import tempfile
y = "***"
your_word = "Whatever you're filtering by"
with tempfile.TemporaryFile(mode="w+") as tmpf:
with open(my_file, 'r') as f:
for line in f:
if your_word in line:
line = f"{y}{line.strip()}{y}\n"
tmpf.write(line) # write to the temp file
tmpf.seek(0) # move back to the beginning of the tempfile
with open(my_file, 'w') as f:
for line in tmpf: # reading from tempfile now
my_file.write(line)

how can i convert surname:name to name:surname? [duplicate]

In Python, calling e.g. temp = open(filename,'r').readlines() results in a list in which each element is a line from the file. However, these strings have a newline character at the end, which I don't want.
How can I get the data without the newlines?

You can read the whole file and split lines using str.splitlines:
temp = file.read().splitlines()
Or you can strip the newline by hand:
temp = [line[:-1] for line in file]
Note: this last solution only works if the file ends with a newline, otherwise the last line will lose a character.
This assumption is true in most cases (especially for files created by text editors, which often do add an ending newline anyway).
If you want to avoid this you can add a newline at the end of file:
with open(the_file, 'r+') as f:
f.seek(-1, 2) # go at the end of the file
if f.read(1) != '\n':
# add missing newline if not already present
f.write('\n')
f.flush()
f.seek(0)
lines = [line[:-1] for line in f]
Or a simpler alternative is to strip the newline instead:
[line.rstrip('\n') for line in file]
Or even, although pretty unreadable:
[line[:-(line[-1] == '\n') or len(line)+1] for line in file]
Which exploits the fact that the return value of or isn't a boolean, but the object that was evaluated true or false.
The readlines method is actually equivalent to:
def readlines(self):
lines = []
for line in iter(self.readline, ''):
lines.append(line)
return lines
# or equivalently
def readlines(self):
lines = []
while True:
line = self.readline()
if not line:
break
lines.append(line)
return lines
Since readline() keeps the newline also readlines() keeps it.
Note: for symmetry to readlines() the writelines() method does not add ending newlines, so f2.writelines(f.readlines()) produces an exact copy of f in f2.

temp = open(filename,'r').read().split('\n')

Reading file one row at the time. Removing unwanted chars from end of the string with str.rstrip(chars).
with open(filename, 'r') as fileobj:
for row in fileobj:
print(row.rstrip('\n'))
See also str.strip([chars]) and str.lstrip([chars]).

I think this is the best option.
temp = [line.strip() for line in file.readlines()]

temp = open(filename,'r').read().splitlines()

My preferred one-liner -- if you don't count from pathlib import Path :)
lines = Path(filename).read_text().splitlines()
This it auto-closes the file, no need for with open()...
Added in Python 3.5.
https://docs.python.org/3/library/pathlib.html#pathlib.Path.read_text

Try this:
u=open("url.txt","r")
url=u.read().replace('\n','')
print(url)

To get rid of trailing end-of-line (/n) characters and of empty list values (''), try:
f = open(path_sample, "r")
lines = [line.rstrip('\n') for line in f.readlines() if line.strip() != '']

You can read the file as a list easily using a list comprehension
with open("foo.txt", 'r') as f:
lst = [row.rstrip('\n') for row in f]

my_file = open("first_file.txt", "r")
for line in my_file.readlines():
if line[-1:] == "\n":
print(line[:-1])
else:
print(line)
my_file.close()

This script here will take lines from file and save every line without newline with ,0 at the end in file2.
file = open("temp.txt", "+r")
file2 = open("res.txt", "+w")
for line in file:
file2.writelines(f"{line.splitlines()[0]},0\n")
file2.close()
if you looked at line, this value is data\n, so we put splitlines()
to make it as an array and [0] to choose the only word data

import csv
with open(filename) as f:
csvreader = csv.reader(f)
for line in csvreader:
print(line[0])

Read text file to list in python

I want to create a text file which contains positive/negative numbers separated by ','.
i want to read this file and put it in data = []. i have written the code below and i think that it works well.
I want to ask if you guys know a better way to do it or if is it well written
thanks all
#!/usr/bin/python
if __name__ == "__main__":
#create new file
fo = open("foo.txt", "w")
fo.write( "111,-222,-333");
fo.close()
#read the file
fo = open("foo.txt", "r")
tmp= []
data = []
count = 0
tmp = fo.read() #read all the file
for i in range(len(tmp)): #len is 11 in this case
if (tmp[i] != ','):
count+=1
else:
data.append(tmp[i-count : i])
count = 0
data.append(tmp[i+1-count : i+1])#append the last -333
print data
fo.close()

You can use split method with a comma as a separator:
fin = open('foo.txt')
for line in fin:
data.extend(line.split(','))
fin.close()

Instead of looping through, you can just use split:
#!/usr/bin/python
if __name__ == "__main__":
#create new file
fo = open("foo.txt", "w")
fo.write( "111,-222,-333");
fo.close()
#read the file
with open('foo.txt', 'r') as file:
data = [line.split(',') for line in file.readlines()]
print(data)
Note that this gives back a list of lists, with each list being from a separate line. In your example you only have one line. If your files will always only have a single line, you can just take the first element, data[0]

To get the whole file content(numbers positive and negative) into list you can use split and splitlines
file_obj = fo.read()#read your content into string
list_numbers = file_obj.replace('\n',',').split(',')#split on ',' and newline
print list_numbers

Read lines from a text file, reverse and save in a new text file

So far I have this code:
f = open("text.txt", "rb")
s = f.read()
f.close()
f = open("newtext.txt", "wb")
f.write(s[::-1])
f.close()
The text in the original file is:
This is Line 1
This is Line 2
This is Line 3
This is Line 4
And when it reverses it and saves it the new file looks like this:
4 eniL si sihT 3 eniL si sihT 2 eniL si sihT 1 eniL si sihT
When I want it to look like this:
This is line 4
This is line 3
This is line 2
This is line 1
How can I do this?

You can do something like:
with open('test.txt') as f, open('output.txt', 'w') as fout:
fout.writelines(reversed(f.readlines()))

read() returns the whole file in a single string. That's why when you reverse it, it reverses the lines themselves too, not just their order. You want to reverse only the order of lines, you need to use readlines() to get a list of them (as a first approximation, it is equivalent to s = f.read().split('\n')):
s = f.readlines()
...
f.writelines(s[::-1])
# or f.writelines(reversed(s))

f = open("text.txt", "rb")
s = f.readlines()
f.close()
f = open("newtext.txt", "wb")
s.reverse()
for item in s:
print>>f, item
f.close()

The method file.read() returns a string of the whole file, not the lines.
And since s is a string of the whole file, you're reversing the letters, not the lines!
First, you'll have to split it to lines:
s = f.read()
lines = s.split('\n')
Or:
lines = f.readlines()
And your method, it is already correct:
f.write(lines[::-1])
Hope this helps!

There are a couple of steps here. First we want to get all the lines from the first file, and then we want to write them in reversed order to the new file. The code for doing this is as follows
lines = []
with open('text.txt') as f:
lines = f.readlines()
with open('newtext.txt', 'w') as f:
for line in reversed(lines):
f.write(line)
Firstly, we initialize a variable to hold our lines. Then we read all the lines from the 'test.txt' file.
Secondly, we open our output file. Here we loop through the lines in reversed order, writing them to the output file as we go.

A sample using list so it will be much easier:
I'm sure there answer that are more elegant but this way is clear to understand.
f = open(r"c:\test.txt", "rb")
s = f.read()
f.close()
rowList = []
for value in s:
rowList.append(value + "\n")
rowList.reverse()
f = open(r"c:\test.txt", "wb")
for value in rowList:
f.write(value)
f.close()

You have to work line by line.
f = open("text.txt", "rb")
s = f.read()
f.close()
f = open("newtext.txt", "wb")
lines = s.split('\n')
f.write('\n'.join(lines[::-1]))
f.close()

Use it like this if your OS uses \n to break lines
f = open("text.txt", "rb")
s = f.read()
f.close()
f = open("newtext.txt", "wb")
f.write(reversed(s.split("\n")).join("\n"))
f.close()
Main thing here is reversed(s.split("\n")).join("\n").
It does the following:
Split your string by line breaks - \n,
resulting an array
reverses the array
merges the array back with linebreaks \n to a string
Here the states:
string: line1 \n line2 \n line3
array: ["line1", "line2", "line3"]
array: ["line3", "line2", "line1"]
string: line3 \n line2 \n line1 \n

If your input file is too big to fit in memory, here is an efficient way to reverse it:
Split input file into partial files (still in original order).
Read each partial file from last to first, reverse it and append to output file.
Implementation:
import os
from itertools import islice
input_path = "mylog.txt"
output_path = input_path + ".rev"
with open(input_path) as fi:
for i, sli in enumerate(iter(lambda: list(islice(fi, 100000)), []), 1):
with open(f"{output_path}.{i:05}", "w") as fo:
fo.writelines(sli)
with open(output_path, "w") as fo:
for file_index in range(i, 0, -1):
path = f"{output_path}.{file_index:05}"
with open(path) as fi:
lines = fi.readlines()
os.remove(path)
for line in reversed(lines):
fo.write(line)