Print a column in one file while processing the other file using awk

Question

So I was trying to understand this answer for merging two files using awk and I was coming up with my solution for a requirement of mine.

awk 'FNR==NR{a[$1]=$2 FS $3;next} {a[$1]=$2 FS $3}{ print a[$1]}' file2 file1

My files are as follows:-

file1 and file2 contents are as follows:-

1 xyz pqr F -
1 abc def A -
1 abc mno G -


1 abc def A
1 xyz pqr T

I am expecting an output as below:-

1 xyz pqr F - T
1 abc def A - A

Basically to match columns 1,2,3 from file2 on file1 and print append the content of the last column on file2 over the result.

So my understanding of the solution I did as follows,

FNR==NR{a[$1]=$2 FS $3;next} will process on file2 storing the entries of the array a as column2 space column3 till the end of file2.
Now on file1, I can match those rows from file2 by doing {a[$1]=$2 FS $3} which will give me all those rows in file1 whose column $1's value a[$1] is same as column2 value $2 space column3 value $3. Now here comes the problem.
After having matched them in file1, I don't know how to print the values as expected. I tried printing $0 and a[$1] and they are giving me

outputs as sequentially,

1 xyz pqr F -
1 abc def A -

xyz pqr
abc def

respectively. My biggest concern was since I did not capture the last column from file2 during the FNR==NR pass, I may not have the value stored in my array? Or do I have it stored?

Wouldn't it be best to say awk 'FNR==NR{a[$1 FS $2 FS $3]=$4;next} (($1 FS $2 FS $3) in a) {print $0, a[$1 FS $2 FS $3]}' f2 f1? — fedorqui
– fedorqui, Commented Jun 14, 2016 at 11:39
@fedorqui: Missed the logic of a[...]=$4, that would have helped me! Please provide it as an answer, so that it will useful for reference! — Inian
– Inian, Commented Jun 14, 2016 at 11:48

sat · Accepted Answer · 2016-06-14 12:18:05Z

6

Use this awk:

awk 'NR==FNR{a[$2 FS $3]=$4; next} $2 FS $3 in a{print $0, a[$2 FS $3]}' file2 file1

There are some issues in your awk.

Your main concern is $4 from file2. But, you haven't stored it.
While accessing file1, you are reassigning an array a with values of file1. (this: a[$1]=$2 FS $3)

As suggested by @EdMorton, a more readable form :

awk '{k=$2 FS $3} NR==FNR{a[k]=$4; next} k in a{print $0, a[k]}' file2 file1

edited Jun 14, 2016 at 12:18

answered Jun 14, 2016 at 11:39

sat

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2 Comments

Inian Over a year ago

That is an excellent answer! I will of-course accept it, just waiting for an even more efficient logic than mine

Inian Over a year ago

@EdMorton: Please post it as an answer for a future reference!

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