Shell Programming and Scripting

awk '/^BC[0-9]+$/ {id=$1; next;} { sum[id]+=$0; count[id]++; } 
  END {   for (id in sum)  print id, sum[id]/count[id]; }'

awk  '
        # 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")
        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 ($0) to variable "id"
        /^BC[0-9]+$/{print 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+=$0
        #    increment the running counter for records associated with a current "id": n++
        /^[0-9]+[.0-9]*$/{s+=$0; n++} 

# at the END of processing the entire file, we still have the LAST "id" no printed out
# print the "id" value AND its average as described above.
END{print id, (n) ? s/n : "NA"}' data.txt