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Awk, function of underscore char.

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# 1  
Old 02-13-2014
Awk, function of underscore char.
Hello Friends,

I would appreciate so much if you could explain how the underscores works at the following code? Sorry if it sounds a bit novice question.

Code:
awk -F',' 'NR==FNR{_[$0]=1;next}!_[$4]{print}' exclude infile

KR,
Eagle
# 2  
Old 02-13-2014
It is like a letter. You can replace it with "A" for example. The array is called "_".
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# 3  
Old 02-13-2014
Quote:
Originally Posted by Scrutinizer
It is like a letter. You can replace it with "A" for example. The array is called "_".
Hmm i could not guess it, well what about

Code:
_[$0]=1,

it takes all $0 as an array here and then checks if $4 in second file equals $0 in first file. but what _[$0]=1 means?
# 4  
Old 02-13-2014
It sets an array element with index $0 to the value 1. In the second part, if checks if for $4 in the second file there is an array element with a value other than 0 or "" and if so, it does not print the record.

Another (slightly better) way of doing it would be:
Code:
awk -F, 'NR==FNR{A[$0]; next}!($4 in A)' exclude infile

# 5  
Old 02-13-2014
As per manual: The name of a variable must be a sequence of letters, digits and underscores, but it may not begin with a digit. Case is significant in variable names.

So underscore is a valid variable name.
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# 6  
Old 02-13-2014
It's just a somewhat oddly named variable.

Even more fun is when someone peppers an awk program with a variable Z or something, and never actually sets its value -- it's just a one-letter way to get a blank string, as opposed to "" !
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# 7  
Old 02-13-2014
Quote:
Originally Posted by Scrutinizer
It sets an array element with index $0 to the value 1. In the second part, if checks if for $4 in the second file there is an array element with a value other than 0 or "" and if so, it does not print the record.

Another (slightly better) way of doing it would be:
Code:
awk -F, 'NR==FNR{A[$0]; next}!($4 in A)' exclude infile

this way is really more clear, understandable for me Scrutinizer, thanks!

Corona so its a lazy (or shorter) way of using variables Smilie
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