Inserting Line at Specified Position of a Text File

Inserting Line at Specified Position of a Text File

The best way to make "pseudo-inplace" changes to a file in Python is with the fileinput module from the standard library:

import fileinput

processing_foo1s = False

for line in fileinput.input('1.txt', inplace=1):
  if line.startswith('foo1'):
    processing_foo1s = True
  else:
    if processing_foo1s:
      print 'foo bar'
    processing_foo1s = False
  print line,

You can also specify a backup extension if you want to keep the old version around, but this works in the same vein as your code -- uses .bak as the backup extension but also removes it once the change has successfully completed.

Besides using the right standard library module, this code uses simpler logic: to insert a "foo bar" line after every run of lines starting with foo1, a boolean is all you need (am I inside such a run or not?) and the bool in question can be set unconditionally just based on whether the current line starts that way or not. If the precise logic you desire is slightly different from this one (which is what I deduced from your code), it shouldn't be hard to tweak this code accordingly.

How to insert text at line and column position in a file?

This is possibly a duplicate of below SO thread

Fastest Way to Delete a Line from Large File in Python

In above it's a talk about delete, which is just a manipulation, and yours is more of a modification. So the code would get updated like below

def update(filename, lineno, column, text):
    fro = open(filename, "rb")

    current_line = 0
    while current_line < lineno - 1:
        fro.readline()
        current_line += 1

    seekpoint = fro.tell()
    frw = open(filename, "r+b")
    frw.seek(seekpoint, 0)

    # read the line we want to update
    line = fro.readline()
    chars = line[0: column-1] + text + line[column-1:]

    while chars:
        frw.writelines(chars)
        chars = fro.readline()

    fro.close()
    frw.truncate()
    frw.close()

if __name__ == "__main__":
    update("file.txt", 4, 13, "History ")

In a large file it make sense to not make modification till the lineno where the update needs to happen, Imagine you have file with 10K lines and update needs to happen at 9K, your code will load all 9K lines of data in memory unnecessarily. The code you have would work still but is not the optimal way of doing it

Add a new line at a specific position in a text file.

This will add the line where you want it. (Make sure you have using System.IO; and using System.Linq; added)

public void CreateEntry(string npcName) //npcName = "item1"
{
    var fileName = "test.txt";
    var endTag = String.Format("[/{0}]", npcName);
    var lineToAdd = "//Add a line here in between the specific boundaries";

    var txtLines = File.ReadAllLines(fileName).ToList();   //Fill a list with the lines from the txt file.
    txtLines.Insert(txtLines.IndexOf(endTag), lineToAdd);  //Insert the line you want to add last under the tag 'item1'.
    File.WriteAllLines(fileName, txtLines);                //Add the lines including the new one.
}

Insert text at a fixed position on each line in a text file

To fix the problem, need few changes.

You might call continue instead of break. Modify

if (s == null) break;

to

if (s == null) continue;

Final code snippet should be...

while (reader.Peek() >= 0)
{
    string s = reader.ReadLine();
    if(string.IsNullOrEmpty(s)) continue;

    s = s.Insert(0, "-814590");
    writer.WriteLine(s);
}

If you are interested, we can also do this using few Linq statements.

 var formattedLines = File.ReadAllLines("C:/Temp/BOM.txt")
                .Where(line => !string.IsNullOrEmpty(line))
                .Select(s => s.Insert(12, "-814590")).ToArray();

File.WriteAllLines(@"C:/Temp/xBOM.txt", formattedLines);

Insert line at middle of file with Python?

This is a way of doing the trick.

with open("path_to_file", "r") as f:
    contents = f.readlines()

contents.insert(index, value)

with open("path_to_file", "w") as f:
    contents = "".join(contents)
    f.write(contents)

index and value are the line and value of your choice, lines starting from 0.

Simple way to insert line to particular position in file

awk 'f==1{print last}{last=$0;f=1}END{print "NEW WORD\n"$0}' file 

Edit specific line at specific position in Text file in Python

Simply open the existing file and read the lines. Assuming that you want to replace the values that always follow 'addmoney' and 'netmoney', you can locate those lines and use re.sub() to substitute the values into those lines. Keep in mind that you can't simply overwrite text files in-place, so you store the modified lines and then recreate a new file at the end, like so:

x1 = 123.12
y1 = 123.45

import re

with open('mytextfile.txt', 'r') as f:
    lines = f.readlines()

    for i, l in enumerate(lines):

        if l.startswith('addmoney'):
            lines[i] = re.sub(r'[0-9.]+', str(x1), lines[i])
        elif l.startswith('netmoney'):
            lines[i] = re.sub(r'[0-9.]+', str(y1), lines[i])

out = open('modifiedtextfile.txt', 'w'); out.writelines(lines); out.close()

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