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# 1  
Old 05-20-2009
nawk strange behavior
Dear guys;
when deleting repeated lines using nawk as below ;
Why the below syntax works?

nawk ' !a[$1]++' infile > outfile


and when using the other below syntax the nawk doesn't work?

nawk ' { !a[$1]++ } ' infile > outfile
or
nawk '
{
!a[$1]++
} ' infile > outfile

BR
# 2  
Old 05-20-2009
You really should read some awk tutorial to get a grasp on how it works and what are the defaults.

awk 'expression'
is equivalent to
awk 'expression{print $0}'
which in turn means do print $0 when for the current line expression is true.

awk '{expression}'
is equivalent to
awk '1{expression}'
which in turn means do expression (this can have side effects or not) when for the current line 1 is true (that is for each and every line).
# 3  
Old 05-20-2009
nawk strange behavior
do you mean that the first expression is equivalent to
nawk ' {if (!a[$1]++) print $0}' infile > outfile

and the other expression is equivalent to
nawk '{ if (!a[$1]++) "do nothing"}' infile > outfile


Thanks colemar very much and kindly if you have any links to awk tutorial to provide me with Smilie

BR Smilie
# 4  
Old 05-20-2009
Explanation:

Code:
nawk '!a[$1]++'

1. If the value of the element a[$1] is empty, print $0
2. Increase the value of the element a[$1] with 1
3. Read next line

Code:
nawk '{ !a[$1]++ }'

1. Increase the value of the element of the array with 1, the ! doesn't have any meaning here and can be omitted
2. Read next line

Hope this helps
# 5  
Old 05-20-2009
The second is equivalent to:
nawk '{if (1) !a[$1]++}' infile > outfile
therefore it does nothing besides accessing the array a by the key $1 and incrementing the array element value.

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