Lesson 22 · Evaluating comparisons - Build a Spreadsheet Engine

Evaluating comparisons

The parser builds comparison nodes; now the evaluator turns them into booleans. Today you evaluate the six comparison operators to a Bool value, which IF will branch on next.

The goal

Evaluate a comparison node to a Bool value by comparing its two numeric sides.

Start here - the target

TO DO

Scenario: Comparisons evaluate to booleans

Givena sheet with A1 set to 5

Whencomparison formulas are evaluated

Theneval('=A1>0') is the Bool true and eval('=A1<0') is the Bool false

Andeval('=A1>=5') is the Bool true and eval('=A1<>5') is the Bool false

Background

The comparison nodes have been parsing since Chapter 2; now they get meaning. Evaluating a comparison is like evaluating arithmetic - walk both sides to values - but instead of returning a number, applyBin compares the two numbers and returns a Bool. All six operators live here: greater/less, the or-equal variants, equality (=), and not-equal (<>). Over A1 = 5, the formula A1>0 is true and A1<0 is false.

Producing a real boolean value (not a 1 or 0) is what lets the next lesson’s IF branch cleanly on a condition. It also means the Bool kind you defined all the way back in the value type is finally being generated, not just reserved. Comparisons here coerce their sides to numbers before comparing, which keeps the behavior simple and predictable for the numeric spreadsheets this engine targets; comparing text or booleans directly is a refinement a later extension could add.

Make it work

// in applyBin, the comparison operators return a Bool
case ">":  return Value{Kind: Bool, B: toNum(l) > toNum(r)}
case "<":  return Value{Kind: Bool, B: toNum(l) < toNum(r)}
case ">=": return Value{Kind: Bool, B: toNum(l) >= toNum(r)}
case "<=": return Value{Kind: Bool, B: toNum(l) <= toNum(r)}
case "=":  return Value{Kind: Bool, B: toNum(l) == toNum(r)}
case "<>": return Value{Kind: Bool, B: toNum(l) != toNum(r)}

CheckpointDONE

Comparison formulas now evaluate to true or false. Commit and stop here.