python: Detecting a negative number within a string

Question

So I have a number of text files with a line like this:

STRT .M                 -9.0:  START DEPTH

I wish to detect the negative number and replace it with 0.1.

I can detect the negative number, simply by looking for the '-'

text.count('-')

if text.count('-') > 0, there is a negative number.

My question is: How do I replace '-9.0' in the string that with the number 0.1? Ultimately, I want to output:

STRT .M                  0.1:  START DEPTH

Note: the negative number varies between files. It's not always -9.0. — Flux Capacitor
– Flux Capacitor, Commented May 26, 2011 at 0:59
Is this fixed-width? (i.e., does it matter if the replacement doesn't have as many characters as the original?) — Tim Yates
– Tim Yates, Commented May 26, 2011 at 1:00
No, it could be -9.0, -10.1, -11.123, etc. so I need a general solution for replacing any negative decimal number with 0.1. — Flux Capacitor
– Flux Capacitor, Commented May 26, 2011 at 1:04
Let me clarify: if I replace a number like "-100000" with "0.1" and don't add any extra spaces, does it matter that the rest of the line won't be in the same place as before? — Tim Yates
– Tim Yates, Commented May 26, 2011 at 1:06
@Tim Yates I don't think it would matter, but I am not 100% sure. — Flux Capacitor
– Flux Capacitor, Commented May 26, 2011 at 1:42

Tadeck · Accepted Answer · 2011-05-27 19:15:02Z

6

The simple solution is to user .replace('-9.0','0.1') (see documentation for .replace()), but I think you need more flexible solution based on regular expressions:

import re
new_string = re.sub(r'-\d+\.\d+', '0.1', your_string)

edited May 27, 2011 at 19:15

answered May 26, 2011 at 0:56

Tadeck

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5 Comments

Flux Capacitor Over a year ago

Agreed. I should have been more clear. The negative number could be any negative number.

Tadeck Over a year ago

@Flux Can it be also integer?

Flux Capacitor Over a year ago

Good question. I doubt it would be an integer, but I cannot be 100% sure since I have not looked through each of the ~200 files.

Tadeck Over a year ago

@bradley.ayers You are right - I posted it before I tested it. Now it replaces every negative float with decimal part consisting at least of 1 digit (does not match eg. .5).

ThomasH Over a year ago

Shouldn't the regex include a '-' in front of the decimals?!

Wang Dingwei · Accepted Answer · 2011-05-26 01:14:50Z

4

Looks like you are working with LAS files. You can check out libLAS to see if it works for you. And here is a tutorial.

answered May 26, 2011 at 1:14

Wang Dingwei

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Comments

bradley.ayers · Accepted Answer · 2011-05-26 01:03:08Z

1

Regular expressions can do this:

>>> import re
>>> regex = re.compile(r' -\d+(\.\d+)?:')
>>> regex.sub(' 0.1:', 'STRT .M                 -9.0:  START DEPTH')
'STRT .M                 0.1:  START DEPTH'
>>> regex.sub(' 0.1:', 'STRT .M                 -19.01:  START DEPTH')
'STRT .M               0.1:  START DEPTH'
>>> regex.sub(' 0.1:', 'STRT .M                 -9:  START DEPTH')
'STRT .M                 0.1:  START DEPTH'

re.sub documentation

edited May 26, 2011 at 1:03

answered May 26, 2011 at 0:55

bradley.ayers

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3 Comments

Flux Capacitor Over a year ago

Will this work if the negative number is ANY negative number and not just -9.0?

Tim Yates Over a year ago

Note that this wouldn't work for, e.g., -10.0, much less -9 (no decimal place). The fix is simple if it isn't fixed-width, but the OP hasn't specified...

joar Over a year ago

It has been revised and now seems to be able to do just that.

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