Lesson 33 · The - Build a Spreadsheet Engine

The

Some formulas are perfectly valid until you run them - dividing by zero is the classic case. Today you make division by zero produce a

The goal

Make division by zero evaluate to a

Start here - the target

TO DO

Scenario: Division by zero produces an error value

Givena sheet with A1 set to the formula '=1/0'

WhenRecalculate runs and A1 is read

ThenGet("A1") is the Error value '#DIV/0!'

Anda nonzero divisor still works normally, so '=6/2' evaluates to the Number 3

Background

Not every error is a structural one like a cycle; some come from the arithmetic itself. Division by zero is the archetype: =1/0 is a well-formed formula that simply has no numeric answer. Rather than return infinity, a wrong number, or panic, the evaluator produces an Err value carrying the code #DIV/0!. A normal divide with a nonzero divisor is unaffected and still returns a number.

This is the second source of error values, and it is different from #CIRC! in an important way: it is produced deep inside evaluation, at a single operator, not by the graph structure. That raises a new question the moment you have it: if A1 is #DIV/0!, what should a cell that reads A1 show? A number would be a lie. The answer every spreadsheet gives is that the error spreads - a formula built on an error is itself an error. Making that propagation happen is the next lesson.

Make it work

case "/":
  if toNum(r) == 0 {
    return Value{Kind: Err, Code: "#DIV/0!"}
  }
  return Value{Kind: Number, Num: toNum(l) / toNum(r)}

CheckpointDONE

Dividing by zero yields a