substitue string by index without using regular expressions

Question

It should be very easy, but I am looking for an efficient way to perform it.

I know that I could split the string into two parts and insert the new value, but I have tried to substitute each line between the indexes 22-26 as follows:

line.replace(line[22:26],new_value)

The Problem

However, that function substitutes everything in the line that is similar to the pattern in line[22:26].

In the example below, I want to replace the marked number 1 with number 17:

Here are the results. Note the replacement of 1 with 17 in several places:

Thus I don't understand the behavior of replace command. Is there a simple explanation of what I'm doing wrong?

Why I don't want RE

The values between index 22-26 are not unified in form.

Note: I am using python 3.5 on Unix/Linux machines.

line[22:26] gives you a string with 1 and some spaces. You use this pattern to replace every occurences in this line by new_value. — Eric Duminil
– Eric Duminil, Commented Feb 16, 2017 at 14:21

Haleemur Ali · Accepted Answer · 2017-02-16 15:10:21Z

1

str.replace replaces 1 sub-string pattern with another everywhere in the string.

e.g.

'ab cd ab ab'.replace('ab', 'xy')
# produces output 'xy cd xy xy'

similarly,

mystr = 'ab cd ab ab'
mystr.replace(mystr[0:2], 'xy')
# also produces output 'xy cd xy xy'

what you could do instead, to replace just the characters in position 22-26

line = line[0:22] + new_value + line[26:]

Also, looking at your data, it seems to me to be a fixed-width text file. While my suggestion will work, a more robust way to process this data would be to read it & separate the different fields in the record first, before processing the data.

If you have access to the pandas library, it provides a useful function just for reading fixed-width files

answered Feb 16, 2017 at 15:10

Haleemur Ali

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