extract, modify, and write column of data

Could you please give me some help with this? I'm new to awk. Thanks.

I would like to extract the 2nd column of a data file, multiply a constant to the extracted data, and then replace the original data with the modified data.

For example:
multiplicative constant = 2

Orignal data (testdata.dat)
#test data
#
1 2
2 4
3 6
#
#end of test data

Resulting data
#test data
#
1 4
2 8
3 12
#
#end of test data


I've gotten so far as to extract the 2nd column of data and doing the multiplication. However, I'm unable to reinsert it back. I can only put it back in a row at the end of the file. Here's what I have right now:

set fluence = `cat testdata.dat | grep -v "#" | awk '{print $2 * 1}'`

echo $fluence >> testdata.dat

result:
#test data
#
1 2
2 4
3 6
#
#end of test data
4 8 12

try something like this

yes linm, as you say you are new to awk, also to shell.
please discover: grep, sed, awk, more, less, lp, lpr and a lot other
cmds are able to open a file.

Thanks Jim....The data is now in what I need it to be.

FWIW- IMO - These are one time scripts:

I'm not sure what stupid means in programming scripts. 'stupid' = Unnecessary process creation I suppose.

If these scripts were running day and night in production on a big multi-user system, then anything you can do to optimize a script is probably good. Up to the point where it becomes un-maintainable, I would guess.

If this is important to you, run your script(s) under tusc or strace and count the number of exec calls.

Jim,

An additional question...what is the correct syntax to make the multiplier a variable?
What I have below does not seem to work. Thanks.

set multfactor = 2
cat testdata.dat | grep -v "#" | awk '{print $1 " " $2 * $multfactor" " $3 " " $4}' > testdata2.dat

There are two ways - it's simpler to read:

http://www.unix.com/showthread.php?s=&threadid=13955

so jim
cat testdata.dat | grep -v "#" | awk '{print $1 " " $2 * $multfactor" " $3 " " $4}' >output
is sure better maintainable as:
awk '$0 !~ /#/{ $2 *= $multfactor; print;}' >output
one should know awk to mantain this, that's the problem...

sorry, i forgot the inputfile, shoud be:
awk '$0 !~ /#/{ $2 *= $multfactor; print;}' input >output
and you still ignore
cat testdata.dat | grep -v "#" | awk ....
and
grep -v "#" testdata.dat | awk ....
are the same
this is what i call, scripting-stupidity
i also work on big multi-user system >x000 server && x0000 of clients
these scripts ARE running day and night in production AND cause a big.
very big stupid power wasting...

I have the extract, multiply, and write part working. Something new...I would like to copy the commented parts as well in the output.

This is what I have so far.
cat testfile.tmp | grep -v "#" | awk '{print $1 " " $2*2}' > newtestfile.tmp

Some info about the comments:
1)there are always only 4 comment lines before the data though their contents may be different.
2)the last set of comments after the data always starts with "Attenuation History" but the comments after that have no set amount (there can be 1 or 20 comment lines afterwards).

I'm not sure how to go about copying only the first 4 lines of the file and copying all lines for the Attenuation History comments. Any help would be appreciated.


Input "testfile.tmp"
# spect_gen: 74 15.000 1 50.00
# 50 1.0000 72.0000
# 1 2 (unit flags)
# Energy #/mAs-cm^2-kev
8.000 1.0000E+00
9.000 2.0000E+00
10.000 3.0000E+00
# Attenuation History:
# 3
# glass_pyrex 1.4800E-01
# oil 3.0000E-01
# al_1100 2.8000E-01

Output file "newtestfile.tmp":
# spect_gen: 74 15.000 1 50.00
# 50 1.0000 72.0000
# 1 2 (unit flags)
# Energy #/mAs-cm^2-keV
8.000 2.0000E+00
9.000 4.0000E+00
10.000 6.0000E+00
# Attenuation History:
# 3
# glass_pyrex 1.4800E-01
# oil 3.0000E-01
# al_1100 2.8000E-01