awk, array indexing

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# 1  
awk, array indexing

cat filename|nawk ' { FS="="; if (!a[$1]++ == 0) print $0 } '

can anyone plz explain how does array inexing works,how it is evaluating if (!a[$1]++ == 0)??
# 2  
Just the indexing? I will try to explain all of it.

Firstly, it is often (and better) written:

nawk -F= '!a[$1]++' filename

Each record (a line here) is split into fields. The first field is addressed with $1, the second with $2, and so on. The entire record is $0. So it uses $1 as the index into the array called a. Stored there is the count of occurances (the ++ operator increments the value). The first time it is seen though, it evaluates as 0. (It returns 0 but is then set to 1, called post-increment). So the ! operator will negate that. So it will only print the first line where $1 is unique, because each successive iteration of that same $1 will then become 0 after !.
This User Gave Thanks to neutronscott For This Post:
# 3  
thanx neutronscott.....but if we are equating '!a[$1]++-==0' , its only printing the duplicate value and if we are doing '!a[$1]++==1' it is printing the first occurrence. how is that happening???

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