average data in third column per month and round to the nearest hundredth

I have a file called output.txt and I want to average data per month. The original attached file should create month in the first column average value in the second column. Each row should look like this below. The averages are bogus just for example. See attached file.

output2.txt sample file rows with bogus data.
198001 120.22
198002 133.42
output.txt
libertyforall2Asked:
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bazikaCommented:
Could You please clarify the stucture of Your input file (which is called "output.txt")?

Sometimes there are 4 columns there, and later columns have 5 columns. Which exactly fields should be analyzed?
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bazikaCommented:
Here is the example of shell script.
I suppose, that the first field in output.txt is "YYYYMMDD", so selecting "substr($1, 1, 6 )" gives us "YYYYMM" (year+month).

Also, the summary of all fields from the 3-rd is taken (i.e. field3+field4+field5 ...)

I do not know, which awk version is used, therefor, I do additional initialization of the arrays.

#!/bin/ksh

cat output.txt | awk '
$3 != "" { cur_mon = substr($1, 1, 6 ) ;
        if( a_sum[cur_mon] == "" ) { a_sum[cur_mon] = 0 ; a_cou[cur_mon]  = 0 }
        for(i=3;i<=NF;i++) {
                a_sum[cur_mon] += $i;
                a_cou[cur_mon] ++ ;
        }
}
END {
        for (cur_mon in a_sum )
                { printf "%s %.2f\n", cur_mon, a_sum[cur_mon]/a_cou[cur_mon] ; }
        }
' | sort -k 1

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Shell Scripting

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