[ask awk] remove certain row in a column

dhodho · 06-08-2010, 01:27 AM

Hi, everyone.

I am newbie in awk programming.
If I have input file like below

2006 2 3
2007 1 2
2008 2 4
2009 1 3
2010 2 1

I wish get output file like:

2006 2
2007 1
2008 2
2009 1 3
2010 2 1

Is it possible to get such output file using awk?
Thanks very much.

druuna · 06-08-2010, 01:49 AM

Hi,

Your example isn't very clear: Do you only want to remove the third column if the year (first column) is 2008 or less?

dhodho · 06-08-2010, 02:21 AM

Sorry for unclear explanation from my previous email. Ok, I will explain what I mean.

Below my real input file :

20080901 0.000024
20080902 0.000020
20080903 0.000021
20080904 0.000032
20080905 0.000000
20080906 0.000000
20080907 0.000013
20080908 0.000009
20080909 0.000000
20080910 0.000015
20080911 0.000050
20080912 0.000028
20080913 0.000020
...

I want to remove the fourth first row of the second column, so I wish get output file like:

20080901
20080902
20080903
20080904
20080905 0.000000
20080906 0.000000
20080907 0.000013
20080908 0.000009
20080909 0.000000
20080910 0.000015
20080911 0.000050
20080912 0.000028
20080913 0.000020

Is it possible getting such above output file using awk?
Thanks ..

grail · 06-08-2010, 02:29 AM

Check out the NR builtin variable. Should be dead easy.

syg00 · 06-08-2010, 02:31 AM

Very simple - check the record number is less than 5 and set field 2 to null.
Any decent tutorial should give you the leads you need.

dhodho · 06-08-2010, 06:07 AM

Thanks for your clue. I used awk '{for(i=NR;i<=4;i++)$2="";print $0}' input.dat > output dat and I got what I needed.

druuna · 06-08-2010, 06:10 AM

Hi,

Great to see you got what you wanted.

Would you please tag this thread as Solved (See Thread Tools).

syg00 · 06-08-2010, 06:33 AM

Whilst anyone with a C background can see what you've done, I tend toward the KISS principle. How about something like

Code:

awk 'NR<5 {$2=""}1' input.dat

(keep an eye an awk threads - these people have taught me plenty).

colucix · 06-08-2010, 07:09 AM

A bit more cryptic... he he..

Code:

awk '$0 = $(NR < 5)' input.dat

syg00 · 06-08-2010, 07:14 AM

That being one of the guilty parties I was referring to ...
(what happens if more than 2 fields .... ????)

grail · 06-08-2010, 07:18 AM

hehe ... nice one

David the H. · 06-08-2010, 07:20 AM

Quote:

Originally Posted by syg00

Code:

awk 'NR<5 {$2=""}1' input.dat

Side question here. I've seen this final "1" used on the end of an awk expression a few times now, and ran a few tests that tell me it makes awk print out every line, modified or not. But I've yet to track down any documentation that explains what it's doing. Could you fill me in a bit here?

colucix · 06-08-2010, 07:36 AM

Quote:

Originally Posted by syg00

That being one of the guilty parties I was referring to ...

Quote:

(what happens if more than 2 fields .... ????)

Well... nothing special. All the fields but the first one are removed from the first 4 lines. Trying to remove only the last field brings to more complicate expressions (and bad programming, too).

grail · 06-08-2010, 07:41 AM

Any true statement, ie equal to not 0, is considered to take on the default action which is to print.

colucix · 06-08-2010, 07:45 AM

Quote:

Originally Posted by David the H.

Side question here. I've seen this final "1" used on the end of an awk expression a few times now, and ran a few tests that tell me it makes awk print out every line, modified or not. But I've yet to track down any documentation that explains what it's doing. Could you fill me in a bit here?

You have to take in mind two things:
1) In awk TRUE is represented by a non-null string or a numeric value other than 0, FALSE is the null-string or 0.
2) An awk rule is made of

Code:

pattern { action }

when the action is not specified, the default one is applied (that is print $0).
Said that the following are very simple programs that evaluates the pattern as TRUE and apply the default action:

Code:

awk 1 input.dat
awk 2 input.dat
awk 3.14159265358979 input.dat

where you see that the numeric value is not necessarily 1.

Edit: ...as grail said using much less words!