Lesson 04 · Formatting a reference back to A1

Formatting a reference back to A1

The engine stores cells by coordinate, but people and error messages want A1 addresses. Today you build the inverse of parsing so a (column, row) pair prints back as its A1 reference, completing the round-trip.

The goal

Format a (column, row) coordinate back into its A1 reference string.

Start here - the target

TO DO

Scenario: A coordinate becomes an A1 reference

Givena zero-based (column, row) coordinate

WhenformatRef renders it

ThenformatRef(col 0, row 0) is "A1" and formatRef(col 1, row 2) is "B3"

AndformatRef(col 25, row 0) is "Z1" and formatRef(col 26, row 9) is "AA10", and parseRef and formatRef are inverses

Background

This closes the addressing loop. formatRef is the mirror of parseRef: render the column index back to letters with indexToCol, and turn the zero-based row back into a one-based row number by adding one. So the coordinate (0, 0) prints as A1, and (26, 9) prints as AA10.

The property to hold onto is that parsing and formatting are exact inverses: formatRef(parseRef("AA10")) is "AA10", and parseRef(formatRef(c)) is c. That round-trip means the engine can freely move between the internal coordinate it computes with and the A1 string it shows a user. Every error message, cycle report, and recalculation trace later in the project will use formatRef to name a cell.

Make it work

// column via indexToCol, row is 0-based so add 1 back
func formatRef(r Ref) string {
  return indexToCol(r.Col) + itoa(r.Row+1)
}

CheckpointDONE

Coordinates and A1 references now round-trip cleanly. Commit and stop here.