search and replace for certain lines

[mors123456]

hi there,

i need a help with replacing, let's say, all 'L' to 'C' in a large file (project.dat), but only for certain line numbers. let's say we have an index file with the line numbers we are interested to be changed (project.idx):
2
18
...

then the project.dat file looks like follows:
L 1 2 3 4 5 6
L 1 2 3 4 5 6
L 1 2 3 4 5 6
....

i need to replace 'L' to 'C' in lines 2, 18, etc. what i can do by myself:

while read line
do
    sed "$line{s/L/C/}" project.dat > project.tmp
    mv project.tmp project.dat
done < project.idx

rm -f project.tmp

but the problem is that this script is too slow for large files *.idx and *.dat
is there an more optimal way to do it? thank you.

[drl]

Hi.

See post # 7 at sed in a for loop - The UNIX and Linux Forums for a procedure that creates a script that will then perform a single pass over the data file ... cheers, drl

[mors123456]

drl, thank you so much. it worked!

[drl]

Hi, mors123456.

You're welcome, good to hear that you got it working ... cheers, drl