Lesson 27 · Division and modulo by zero - Build an Expression Evaluator

Division and modulo by zero

Parsing errors are handled, so the focus shifts to evaluation errors. Today you turn division and modulo by zero into clear, positioned messages instead of the infinities and not-a-number values they silently produce.

The goal

Report a positioned error for division or modulo by zero.

Start here - the target

TO DO

Scenario: Dividing or taking a remainder by zero is reported

Giventhe expression string "10 / 0"

Whenit is evaluated with EvalString

Thenevaluation fails with: division by zero at position 3

Andevaluating "5 % 0" fails with: modulo by zero at position 2

Background

Floating point does not fault on division by zero; 10 / 0 quietly yields an infinity and 5 % 0 a not-a-number, both useless answers a user would rather see rejected. Guard the divisor: if the right operand evaluates to 0, return an error instead of computing. This is the first evaluation error, and it is where the position you stored on every Bin node back in the parsing chapter earns its keep, letting the message point at the operator, position 3 for the / in 10 / 0.

Storing positions on nodes at parse time, long before any error could use them, is what makes evaluation errors as precise as parse errors. The evaluator has the whole tree, but the raw string is long gone; without the position captured on the node, the best it could say is “division by zero” with no idea where. That foresight is why the front-loaded Pos field was worth carrying unused for twenty lessons.

Make it work

case "/":
  if r == 0 { return 0, fmt.Errorf("division by zero at position %d", n.Pos) }
  return l / r, nil
case "%":
  if r == 0 { return 0, fmt.Errorf("modulo by zero at position %d", n.Pos) }
  return math.Mod(l, r), nil

CheckpointDONE

Division and modulo by zero report a positioned error instead of infinity or not-a-number. Commit and stop here.