Changing the middle of strings in a list--I know there is a betterway.

[Ben]

Hello All:

I am new to Python, and I love it!! I am running 2.6 on Windows. I
have a flat text file here is an example of 3 lines with numbers
changed for security:

999999999088869199999999990200810999999
999999999088869199999999990200810999999
999999999088869199999999990200810999999
I want to be able to replace specific slices with other values. My
code below reads a file into a list of strings. Since strings are
immutable I can't assign different values to a specific slice of the
string. How can I accomplish this? I read some posts on string
formatting but I am having trouble seeing how I can use those features
of the language to solve this problem.

The code below just puts an 'R' at the beginning of each line like
this:

R999999999088869199999999990200810999999
R999999999088869199999999990200810999999
R999999999088869199999999990200810999999
But what I want to do is change the middle of the string. Like this:

R999999999088869CHANGED99990200810999999
R999999999088869CHANGED99990200810999999
R999999999088869CHANGED99990200810999999

#My Current Code

# read the data file in as a list
F = open('C:\\path\\to\file', "r")
List = F.readlines()
F.close()

#Loop through the file and format each line
a=len(List)
while a 0:

List.insert(a,"2")
a=a-1

# write the changed data (list) to a file
FileOut = open("C:\\path\\to\\file", "w")
FileOut.writelines(List)
FileOut.close()

Thanks for any help and thanks for helping us newbies,

-Ben

[Duncan Booth]

Ben <bm*******@gmail.comwrote:

Since strings are
immutable I can't assign different values to a specific slice of the
string. How can I accomplish this?

....

#My Current Code

# read the data file in as a list
F = open('C:\\path\\to\file', "r")
List = F.readlines()
F.close()

#Loop through the file and format each line
a=len(List)
while a 0:

List.insert(a,"2")
a=a-1

# write the changed data (list) to a file
FileOut = open("C:\\path\\to\\file", "w")
FileOut.writelines(List)
FileOut.close()

Thanks for any help and thanks for helping us newbies,

You probably don't want to use readlines. It is almost always cleaner just
to iterate over the file itself.

You can't change an existing string, but creating a new string isn't
exactly rocket science.

lines = []
for line in F:
modified = line[:16] + "CHANGED" + line[23:]
# or maybe
modified = line.replace("1999999", "CHANGED")
lines.append(modified)
FileOut.writelines(lines)
To modify a file inplace have a look at the fileinput module.

[J. Cliff Dyer]

On Tue, 2008-10-21 at 10:28 -0700, Ben wrote:

Hello All:

I am new to Python, and I love it!! I am running 2.6 on Windows. I
have a flat text file here is an example of 3 lines with numbers
changed for security:

999999999088869199999999990200810999999
999999999088869199999999990200810999999
999999999088869199999999990200810999999
I want to be able to replace specific slices with other values. My
code below reads a file into a list of strings. Since strings are
immutable I can't assign different values to a specific slice of the
string. How can I accomplish this? I read some posts on string
formatting but I am having trouble seeing how I can use those features
of the language to solve this problem.

The code below just puts an 'R' at the beginning of each line like
this:

R999999999088869199999999990200810999999
R999999999088869199999999990200810999999
R999999999088869199999999990200810999999
But what I want to do is change the middle of the string. Like this:

R999999999088869CHANGED99990200810999999
R999999999088869CHANGED99990200810999999
R999999999088869CHANGED99990200810999999

Well, it depends on what you want. Do you want to replace by location
or by matched substring? One of the following functions might help.

lines = ['999999999088869199999999990200810999999'
'99999999088869199999999990200810999999'
'9999999088869199999999990200810999999']

def replace_by_location(string, replacement, start, end):
return string[:start] + replacement + string[end:]

def replace_by_match(string, substr, replacement):
return replacement.join(string.split(substr))

location_lines = [replace_by_location(x, 'CHANGED', 15, 22) for x in lines]
match_lines = [replace_by_match(x, '1999999', 'CHANGED') for x in lines]

print location_lines
print match_lines

Cheers,
Cliff

#My Current Code

# read the data file in as a list
F = open('C:\\path\\to\file', "r")
List = F.readlines()
F.close()

#Loop through the file and format each line
a=len(List)
while a 0:

List.insert(a,"2")
a=a-1

# write the changed data (list) to a file
FileOut = open("C:\\path\\to\\file", "w")
FileOut.writelines(List)
FileOut.close()

Thanks for any help and thanks for helping us newbies,

-Ben
--
http://mail.python.org/mailman/listinfo/python-list

[Ben]

On Oct 21, 1:53*pm, "J. Cliff Dyer" <[e-mail]:

On Tue, 2008-10-21 at 10:28 -0700, Ben wrote:

Hello All:

I am new to Python, and I love it!! I am running 2.6 on Windows. I
have a flat text file here is an example of 3 lines with numbers
changed for security:

999999999088869199999999990200810999999
999999999088869199999999990200810999999
999999999088869199999999990200810999999

I want to be able to replace specific slices with other values. My
code below reads a file into a list of strings. Since strings are
immutable I can't assign different values to a specific slice of the
string. How can I accomplish this? I read some posts on string
formatting but I am having trouble seeing how I can use those features
of the language to solve this problem.

The code below just puts an 'R' at the beginning of each line like
this:

R999999999088869199999999990200810999999
R999999999088869199999999990200810999999
R999999999088869199999999990200810999999

But what I want to do is change the middle of the string. Like this:

R999999999088869CHANGED99990200810999999
R999999999088869CHANGED99990200810999999
R999999999088869CHANGED99990200810999999

Well, it depends on what you want. *Do you want to replace by location
or by matched substring? *One of the following functions might help.

lines = ['999999999088869199999999990200810999999'
* * * * *'99999999088869199999999990200810999999'
* * * * *'9999999088869199999999990200810999999']

def replace_by_location(string, replacement, start, end):
* * return string[:start] + replacement + string[end:]

def replace_by_match(string, substr, replacement):
* * return replacement.join(string.split(substr))

location_lines = [replace_by_location(x, 'CHANGED', 15, 22) for x in lines]
match_lines = [replace_by_match(x, '1999999', 'CHANGED') for x in lines]

print location_lines
print match_lines

Cheers,
Cliff

#My Current Code

# read the data file in as a list
F = open('C:\\path\\to\file', "r")
List = F.readlines()
F.close()

#Loop through the file and format each line
a=len(List)
while a 0:

* * List.insert(a,"2")
* * a=a-1

# write the changed data (list) to a file
FileOut = open("C:\\path\\to\\file", "w")
FileOut.writelines(List)
FileOut.close()

Thanks for any help and thanks for helping us newbies,

-Ben
--
http://mail.python.org/mailman/listinfo/python-list

Thanks for your help, this explains it I needed a little mental jump
start.

-Ben

[Terry Reedy]

Duncan Booth wrote:

Ben <bm*******@gmail.comwrote:

>Since strings are
immutable I can't assign different values to a specific slice of the
string. How can I accomplish this?

You probably don't want to use readlines. It is almost always cleaner just
to iterate over the file itself.

You can't change an existing string, but creating a new string isn't
exactly rocket science.

lines = []
for line in F:
modified = line[:16] + "CHANGED" + line[23:]
# or maybe
modified = line.replace("1999999", "CHANGED")
lines.append(modified)
FileOut.writelines(lines)

If you want your program to scale to indefinitely large files, and you
edit each line independently, without using date from other lines, read
a line *and* write the new line immediately

for line in F:
FileOut.write(<modified>)

Where <modifiedis one of the suggested expressions, or a temporary
variable calculated on multiple lines.

To edit a line 'in-place', one can use the array module for mutable
array of bytes (or the 3.0 bytearray), but I would only both with
extensive editing.