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Full Discussion: comparing two fields from two different files in AWK
Top Forums Programming comparing two fields from two different files in AWK Post 302580111 by Cybex2011 on Wednesday 7th of December 2011 02:24:58 PM
Old 12-07-2011
Hi, all:

I roughly understand that this code first reads in a smaller file(file_2) and stores every line into an array f2.

Next, when it reads in the other bigger one, file_1, line by line, it checks each line's 2nd field against the array f2. If there is a match, it prints out the current line in process and an element from the array f2.

Code:
awk > file3 'NR == FNR {
  f2[$1] = $2; next 
  }
$2 in f2 {
  print $0, f2[$2]
  }' file_2 file_1

#==> file_2 <==
#cust_034_60 2
#cust_406_4 3
#
#==> file_1 <==
#1 cust_034_60
#2 cust_80_91
#3 cust_406_4
#
#==> OUTPUT:
#1 cust_034_60 2
#3 cust_406_4 3


But I have some difficulty to figure out:

1. how file_2's lines being stored into f2?

Use the 1st line of file_2 as an example, "fcust_034_60 2"
==> f2[$1]=$2

It looks to me, the code only put the 2nd field, which is the number 2, into the array's position $1, not the entire current line goes in there.

2. how f2's element being retrieved by the code
==> f2[$2]?

If it had just put a field in the 1st step,
how could the code use "fcust_034_60" now read in from file_1 to search against the array f2?

To me, "fcust_034_60" is even not in f2.
And in the previous step, seems to me, the code "f2[$1]=$2", just put $2 from the line in the $1 position of f2. How come when retrieved, it is from the position $2 of f2?

I am new here and awk is new to me too.
The book I have at hand is talking about using awk to process one single file only.

Using awk to process 2 or more files at a time seems will take me some get used to it.

Thanks to the other person here on the other page clarified the NR/FNR issue very well,
Now I have a vague impression about how awk uses NR/FNR to handle 2 files's comparison at a time.

Hopefully someone here will give me some hints to my questions above too. Thank you in advance.

Regards,
 

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LEARN ABOUT OSX

ppmtosixel

ppmtosixel(1)						      General Commands Manual						     ppmtosixel(1)

NAME

       ppmtosixel - convert a portable pixmap into DEC sixel format

SYNOPSIS

       ppmtosixel [-raw] [-margin] [ppmfile]

DESCRIPTION

       Reads  a  portable  pixmap  as input.  Produces sixel commands (SIX) as output.	The output is formatted for color printing, e.g. for a DEC
       LJ250 color inkjet printer.

       If RGB values from the PPM file do not have maxval=100, the RGB values are rescaled.  A printer control header and a color assignment table
       begin the SIX file.  Image data is written in a compressed format by default.  A printer control footer ends the image file.

OPTIONS

       -raw   If  specified,  each  pixel  will  be explicitly described in the image file.  If -raw is not specified, output will default to com-
	      pressed format in which identical adjacent pixels are replaced by "repeat pixel" commands.  A raw file is often an order	of  magni-
	      tude larger than a compressed file and prints much slower.

       -margin
	      If  -margin  is not specified, the image will be start at the left margin (of the window, paper, or whatever).  If -margin is speci-
	      fied, a 1.5 inch left margin will offset the image.

PRINTING

       Generally, sixel files must reach the printer unfiltered.  Use the lpr -x option or cat filename > /dev/tty0?.

BUGS

       Upon rescaling, truncation of the least significant bits of RGB values may result in poor color conversion.  If the original PPM maxval was
       greater	than  100,  rescaling  also reduces the image depth.  While the actual RGB values from the ppm file are more or less retained, the
       color palette of the LJ250 may not match the colors on your screen.  This seems to be a printer limitation.

SEE ALSO

       ppm(5)

AUTHOR

       Copyright (C) 1991 by Rick Vinci.

								   26 April 1991						     ppmtosixel(1)
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