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I read Comparing two files using Unix and Awk. It is really interesting. I read and tested it, but I can't understand it completely and use it in other cases.

I have two files. file1 has one field and the other one has 16 fields. I want to read elements of file1 and compare them with 3rd field of file2. If there was a match for each element, I sum value of field 5 in file2. As an example:

file 1


file 2

2 2 2 1 2
3 6 1 2 4 
4 1 1 2 3
6 3 3 3 4 

For element 1 in file1 I want to add values in field 5 of file2 where value of field 3 is 1. And do the same for element 2 and 3 in file1. The output for 1 is (3+4=7) and for 2 is 2 and for 3 is 4.

I don't know how I should write it with awk.

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looks like a homework problem, what's the real world use case? – bdowning Dec 26 '13 at 11:16

2 Answers 2

Here's one way. I have written it as an awk script so I can add comments:

#!/usr/local/bin/awk -f

    ## FNR is the line number of the current file, NR is the number of 
    ## lines that have bene processed. If you only give one file to
    ## awk, FNR will always equal NR. If you give more than one file,
    ## FNR will go back to 0 when the next file is reached but NR
    ## will continue incrementing. Therefore, NR == FNR only while
    ## the first file is being processed.
    if(NR == FNR){
      ## If this is the first file, save the values of $1
      ## in the array n.
      n[$1] = 0
    ## If we have moved on to the 2nd file
      ## If the 3rd field of the second file exists in
      ## the first file.
      if($3 in n){
        ## Add the value of the 5th field to the corresponding value
        ## of the n array.
## The END{} block is executed after all files have been processed.
## This is useful since you may have more than one line whose 3rd
## field was specified in the first file so you don't want to print
## as you process the files.
    ## For each element in the n array
    for (i in n){
    ## print the element itself and then its value
    print i,":",n[i];

You can save that as a file, make it executable and run it like so:

$ chmod a+x foo.awk
$ ./foo.awk file1 file2
1 : 7
2 : 2
3 : 4

Or, you can condense it into a one-liner:

awk '
     (NR == FNR){n[$1] = 0; next}
     {if($3 in n){n[$3]+=$5}}
     END{for (i in n){print i,":",n[i]} }' file1 file2
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thanks a lot especially for the comments – user55340 Dec 27 '13 at 6:35
@user55340 you're welcome. Please remember to mark one of the answers as accepted so the auestion can be marked as answered. – terdon Dec 27 '13 at 10:40
awk '
  NR == FNR {n[$3] += $5; next}
  {print $1 ": " n[$1]}' file2 file1
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It does some extra work by summing non matched fields. – Emmanuel Dec 26 '13 at 12:52
@Emmanuel, that's still one awk instructions per line of file2, which makes it shorter and faster than terdon's – Stéphane Chazelas Dec 26 '13 at 16:28
brillant solution ! – Ronald Pauffert Aug 4 at 8:19

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