Lesson 06 · The sheet grid - Build a Spreadsheet Engine

The sheet grid

A spreadsheet is a grid of cells you can write to and read from by address. Today you build that grid so literal values can be stored at an A1 address and read back, with any address you never set reading as empty.

The goal

Store literal values at A1 addresses and read them back, defaulting to Empty for anything unset.

Start here - the target

TO DO

Scenario: The sheet stores and returns cell values

Givena new empty sheet

WhenSetNumber("A1", 5) and SetText("B2", "hi") are called and cells are read back

ThenGet("A1") is the Number 5 and Get("B2") is the Text "hi"

AndGet("C3"), never set, is the Empty value

Background

Now the addressing work pays off: a sheet is just a map from a cell coordinate to its value. Writing a cell parses the A1 address to a Ref and stores the value; reading a cell parses the address and looks it up. Because the store is sparse - we only keep cells that were actually set - any address you have never written reads back as the Empty value rather than an error or a zero.

That “unset means Empty” rule matters more than it looks. Real spreadsheets are mostly blank, and formulas routinely reference cells that hold nothing yet; treating an unset cell as Empty (which arithmetic will later read as zero) is what lets a formula like =A1+B1 work before you have filled in B1. Today the sheet only holds literals you set directly; next lesson it learns to tell a literal apart from a formula.

Make it work

type Sheet struct{ cells map[Ref]Value }
func NewSheet() *Sheet { return &Sheet{cells: map[Ref]Value{}} }
func (s *Sheet) SetNumber(a string, n float64) {
  s.cells[parseRef(a)] = Value{Kind: Number, Num: n}
}
func (s *Sheet) Get(a string) Value {
  v, ok := s.cells[parseRef(a)]
  if !ok { return Value{Kind: Empty} }
  return v
}

CheckpointDONE

The sheet stores literals by address and returns Empty for anything unset. Commit and stop here.