Shell Programming and Scripting

FNR!=NR && /^BC[0-9]+$/ {id=$0; print $0, arr[id]}

FNR!=NR && /^BC[0-9]+$/ {id=$0; print $0}

        # for the very FIRST line (NR==1) in a file, assign the entire record ($0) to a variable "id".
        # then proceed to the next input line ("next")
        FNR==1 && NR==1{id=$0; next} 

        # if a line starts (^) with "BC" and is followed by one or more (+) numbers "[0-9]"
        # output the value of a variable "id", followed by:
        #     if "n" is non-zero, divide '"s" by "n"
        #     if "n" is 0, output string "NA"
        # "s=n=0" - assign "0" to "s" and "n"
        # assign a current record/line ($0) to variable "id"
        (FNR==NR || FNR==1) && /^BC[0-9]+$/ {arr[id]= (n) ? s/n : "NA"; s=n=0; id=$0}

        # if a line starts with one or more (+) numbers ([0-9]) optionally followed
        # by zero or more (*) numbers ([0-9]) or a dot (.)...
        #    calculate a sum (s) by adding the current record value ($0) to a running sum (s): s+=$0
        #    increment the running counter for records associated with a current "id": n++
        FNR==NR && /^[0-9]+[.0-9]*$/{s+=$0; n++}


        FNR!=NR && /^BC[0-9]+$/ {id=$0; print $0, arr[id]}
        FNR!=NR && /^[0-9]+[.0-9]*$/{print $0, $0 - arr[id] }