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Replace space in column with letter for several rows

 
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# 1  
Old 04-09-2016
Replace space in column with letter for several rows
I have a pbd file, which has the following format:

Code:
TITLE     Protein X
MODEL        1
ATOM      1  N   PRO    24      45.220  71.410  43.810  1.00  0.00            
ATOM      2  H1  PRO    24      45.800  71.310  42.000  1.00  0.00            
TER
ENDMDL

Column 22 is the chain identifier for lines that begin with ATOM. I would like to replace the space in column 22 with the letter A. The actual file has thousands of rows labeled ATOM. An example output is given below:

Code:
TITLE     Protein X
MODEL        1
ATOM      1  N   PRO A  24      45.220  71.410  43.810  1.00  0.00            
ATOM      2  H1  PRO A  24      45.800  71.310  42.000  1.00  0.00            
TER
ENDMDL

Any help is much appreciated.
# 2  
Old 04-09-2016
Try:
Code:
sed '/^ATOM /s/./A/22' file

This User Gave Thanks to Scrutinizer For This Post:
Egy
# 3  
Old 04-09-2016
That worked perfectly. Very much appreciated!
# 4  
Old 04-10-2016
To make sure you replace nothing but a space, do it like
Code:
 sed '/^ATOM /s/ /A/22' file

. Scrutinizer's proposal will replace ANY character.
# 5  
Old 04-10-2016
Quote:
Originally Posted by RudiC
To make sure you replace nothing but a space, do it like
Code:
 sed '/^ATOM /s/ /A/22' file

. Scrutinizer's proposal will replace ANY character.
The above code will change the 22nd <space> character on a line; not a space that is the 22nd character on a line. If the requirement is to change the 22nd character on a line to an <A> if and only if that character was a <space>, try something more like (assuming you have a POSIX-conforming version of sed:
Code:
sed '/^\(ATOM ................\) /s//\1A/' file

This User Gave Thanks to Don Cragun For This Post:
RudiC
# 6  
Old 04-10-2016
Which could be reduced to:
Code:
sed '/^ATOM /s/\(.\{21\}\) /\1A/' file

or
Code:
sed 's/^\(ATOM .\{16\}\) /\1A/' file

 
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