I wrote the AWK version of this calculator when I volunteered to work out the expenses for our 1981 ski trip to Jackson Hole. webpage 
I wasn't even sure how to approach the calculation so I just started writing facts into a file. Pretty soon I realized that if I could just sum the groups and then bring those sums into other groups then I'd pretty much have it.
I wrote AWK to do the calculation and print a report. I made it more general than need be because that was the easiest way to get it right. I finished the script and the report together.
A key notion is that there is a running sum. This is saved to or restored from a hash when a name is encountered. The first occurrence defines the name; later occurrences recall its value.
$1 ~ /^[A-Z]+[A-Z0-9]*$/ { if (sums[$1] == "" || $1 == "SUM") { # Define Symbol sums[$1] = sum $1 = sum sum = 0 } else { # Dereference Symbol $1 = sums[$1] } } # Pretty Print ($1+0) != 0 {$1 = sprintf("%7.2f", $1)} {print} # Explicit Calculations $2 == "*" {$1 *= $3} $2 == "/" {$1 /= $3} $2 == "DB" {$1 = -$1} $2 == "CR" {$1 = -$1} # Implicit Summation NF == 0 {sum = 0} {sum += $1}
My friend Jim Bessemer recognized the similarity of my approach with spreadsheet programs which I had not yet used. The common feature is calculation in the context of a document.
Years later I revived the program and recoded it in CoffeeScript as a step toward the calculation methodology for Federated Wiki.