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# 8  
Old 10-06-2009
The A[$1] = $1 statement simply makes A[$1] exist by giving it a value. Assigning the value to the same index is just a convenience.
# 9  
Old 10-06-2009
I try the command below:
awk 'NR == FNR { A[$1] = $2; next } A[$3]'
awk 'NR == FNR { A[$1] = $3; next } A[$3]'
awk 'NR == FNR { A[$1] = $4; next } A[$3]'
All fail to get my desired output result.
Thus I'm interesting about the reason why need to set like A[$1] = $1

Quote:
Originally Posted by jp2542a
The quick answer:

First realize that awk arrays are indexed by a value not a number. While array indexes may look like a number, that is not how awk sees them.

The first clause sets up an array (A) indexed by values from the pattern file.
Indexes to A are cd003, cd005, etc... so A["cd003"] is a valid entry in the A array.
NR is the number records awk has ever read. NR is set to 1 when awk starts.
FNR is the number of records read from the current file. FNR is reset to 1 when a new file is opened.
Both are incremented when a record is read.

So, if NR is equal to FNR, then we are reading from the first file (pattern file) since the record counts are the same.
If NR is not equal to FNR, then we are reading from a subsequent file (i.e. data file).

The A[$3] (where $3 is the third field from the data file) says if the entry exists (i.e A["cd003"] then do the default action (print the line), else ignore that entry.

This clause is not executed on the pattern file because the next statement says "skip all following code. read next line, and start processing clauses from the top". The 'next' statement adds to the robustness of the code.


---------- Post updated at 11:45 PM ---------- Previous update was at 11:41 PM ----------

Actually I got try the command below:
awk 'NR == FNR { A[$1] = $2; next } A[$3]'
awk 'NR == FNR { A[$1] = $3; next } A[$3]'
awk 'NR == FNR { A[$1] = $4; next } A[$3]'
All fail to get my desired output result.
Thus I'm interesting about the reason why need to set like A[$1] = $1
Really thanks ya.
I'm the new user of awk ^^
But I like awk Smilie
Quote:
Originally Posted by jp2542a
The A[$1] = $1 statement simply makes A[$1] exist by giving it a value. Assigning the value to the same index is just a convenience.
# 10  
Old 10-06-2009
Scottn's srcript works fine based on your spec. What exactly are you trying to do?

Changing the A[$1] rvalue will not change the programs execution. If what you posted represents the command line you put to the shell, then you forgot to specify the pattern and data files..
# 11  
Old 10-06-2009
Hi,
Sorry that I missed some hints..
Like what you said, "If what you posted represents the command line you put to the shell, then you forgot to specify the pattern and data files"
How can I do that to specify the pattern and data files?

My work script

[patrick@home]$ awk 'NR == FNR { A[$1] = $1; next } A[$3]' pattern_file data_file
bca cd002 cd003 cza
bac cd004 cd005 zac
acb cd006 cd007 caz
cab cd007 cd008 azc
[patrick@home]$ awk 'NR == FNR { A[$1] = $2; next } A[$3]' pattern_file data_file
[patrick@home]$ awk 'NR == FNR { A[$1] = $3; next } A[$3]' pattern_file data_file
[patrick@home]$ awk 'NR == FNR { A[$1] = $4; next } A[$3]' pattern_file data_file
[patrick@home]$ awk 'NR == FNR { A[$1] = $5; next } A[$3]' pattern_file data_file

If I change the A[$1] = $1 to $2/$3/$4, I can't get any output result.
Thanks a lot for your guide and sharing Smilie

Quote:
Originally Posted by jp2542a
Scottn's srcript works fine based on your spec. What exactly are you trying to do?

Changing the A[$1] rvalue will not change the programs execution. If what you posted represents the command line you put to the shell, then you forgot to specify the pattern and data files..
# 12  
Old 10-06-2009
pattern_file and data_file are just place holders for the actual paths to your files.

For instance if /export/home/user/real.txt is the path to the file with the patterns and /export/home/user/thedata.txt is the path to the data file, then the command line would be:

Code:
awk 'NR == FNR { A[$1] = $1; next } A[$3]' /export/home/user/real.txt /export/home/user/thedata.txt

And I just noticed something. $2..$n don't exist. So A[$1] will have no value and hence not exist either.
# 13  
Old 10-06-2009
Hi,
thanks for your explanation Smilie
Actually I just confusing about this "A[$1] = $1"
The $1 I not sure how to fill it and its refer to what ?! Smilie
Quote:
Originally Posted by jp2542a
Scottn's srcript works fine based on your spec. What exactly are you trying to do?

Changing the A[$1] rvalue will not change the programs execution. If what you posted represents the command line you put to the shell, then you forgot to specify the pattern and data files..
Quote:
Originally Posted by jp2542a
pattern_file and data_file are just place holders for the actual paths to your files.

For instance if /export/home/user/real.txt is the path to the file with the patterns and /export/home/user/thedata.txt is the path to the data file, then the command line would be:

Code:
awk 'NR == FNR { A[$1] = $1; next } A[$3]' /export/home/user/real.txt /export/home/user/thedata.txt

And I just noticed something. $2..$n don't exist. So A[$1] will have no value and hence not exist either.
# 14  
Old 10-06-2009
A[$1] = $1 is used to set up the array of patterns. It is set when NR is equal to FNR, in other words when the first file is read i.e. the pattern file. It means fill the associative array "$1" to the value of "$1", $1 being the first field of your pattern file So with the OP's provided inputs it gets filled like so:
Code:
A[cd003]=cd003
A[cd005]=cd005
A[cd007]=cd007
A[cd008]=cd008

So once the pattern file is done it starts reading the input file FNR is no longer equal to NR, so it will just execute the part "A[$3]" for each line, which means: print the current line if field 3 exist as a key in the array.

The script is only testing the existence of the array elements, not using its contents. So IMO the use of $1 is a tiny bit superfluous. I think we could also just set it to 1 instead:

Code:
awk 'NR == FNR { A[$1]=1; next } A[$3]' pattern_file input_file

or, since pattern file does not have a third column.
Code:
awk 'NR == FNR { A[$1]=1 } A[$3]' pattern_file input_file

Last edited by Scrutinizer; 10-06-2009 at 03:28 AM..
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