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Explanation of FNR in this awk script

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# 1  
Old 05-20-2015
Explanation of FNR in this awk script

To merge mutiple *.tab files as:
rs1 A A
rs2 A A
rs3 C C
rs4 C C

rs1 T T
rs2 T T
rs3 G G
rs4 G G

rs1 B B
rs2 B B
rs3 L L
rs4 L L

AA       TT       BB
AA       TT       BB
CC       GG       LL
CC       GG       LL

which could be done with:
awk 'BEGIN{for(i=1;i<ARGC;i++)printf ARGV[i]" ";print ""}{a[FNR]=length(a[FNR])?a[FNR]$2$3FS:$2$3FS}END{for(i=0;i++<FNR;)print a[i]}' *.tab

I have hard time to understand a[FNR] and a[FNR]=length(a[FNR]) in the part:

Can anybody please explain it for me?
Thanks a lot!

Last edited by yifangt; 05-20-2015 at 08:00 PM..
# 2  
Old 05-20-2015
That expression is a mouthful; the ? is a short-form if/else/equals which allows for brief but sometimes impenetrable expressions. To spell it out:

a[FNR]=length(a[FNR])? # if length(a[FNR]) != 0
        a[FNR]$2$3FS # a[FNR] = a[FNR] $2 $3 FS
        :                   # else
        $2$3FS          # a[FNR]=$2 $3 FS

This User Gave Thanks to Corona688 For This Post:
# 3  
Old 05-20-2015
I think I get the ? part, but not the a[FNR]. When I tried {print a[FNR], length(a[FNR])}, all are "0". It seems to me a[FNR] is dynamically accumulating one after another file, so that files are appending at the end one by one.
I tried paste *.tab, but here I am trying to understand a[FNR]. More clue please!
# 4  
Old 05-20-2015
In awk, NR is the number of records read so far from all of the input files and FNR is the number of records read so far from the current input file.

So, when you read from the 1st input file a[FNR] will be the default uninitialized empty string and length(a[FNR]) will be zero which will then cause a[FNR] to be set to the contents of field 2 from that line, field 3 from that line, and the field separator ($2$3FS). And, when the awk reads the corresponding line from subsequent input files, length(a[FNR]) will not be zero and a[FNR] will be set to the current a[FNR] with field 2 from the current line in the new file, field 3 from the current line in the new file, and FS appended to the end of it (a[FNR]$2$3FS).

Is this any clearer for you?
This User Gave Thanks to Don Cragun For This Post:
# 5  
Old 05-21-2015
Thank Don and Corona!
Realized there was a precedence in the script, and I get it now. I rewrote that part:
awk '{a[FNR]=(length(a[FNR])?a[FNR]$2$3FS:$2$3FS); print FNR, a[FNR], length(a[FNR])}
END{print "-------------"; for (i =0; i++<FNR;) print i, a[i], length(a[i])}' *.tab

and the output is:
1 AA  3
2 AA  3
3 CC  3
4 CC  3
1 AA TT  6
2 AA TT  6
3 CC GG  6
4 CC GG  6
1 AA TT BB  9
2 AA TT BB  9
3 CC GG LL  9
4 CC GG LL  9
1 AA TT BB  9
2 AA TT BB  9
3 CC GG LL  9
4 CC GG LL  9

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