E1 — Penilai Ungkapan (bahagian 1: + dan - LTR)

// Evaluate tokens left-to-right for + and - only
function evalPlusMinus(tokens) {
  if (!Array.isArray(tokens) || tokens.length === 0) return null;

  // result starts with first number
  let acc = tokens[0];
  if (typeof acc !== "number") return null;

  // step through [op, num] pairs
  for (let i = 1; i < tokens.length; i = i + 2) {
    const op = tokens[i];
    const val = tokens[i + 1];

    if ((op !== "+" && op !== "-") || typeof val !== "number") {
      return null;
    }

    if (op === "+") acc = acc + val;
    else acc = acc - val;
  }

  return acc;
}

console.log("eval 10-3-2:", evalPlusMinus([10, "-", 3, "-", 2]));

// Glue the pieces: from string to result
function evalExprSimple(expr) {
  const tokens = tokenize(expr);
  if (tokens === null) {
    return { ok: false, error: "Invalid character" };
  }
  const result = evalPlusMinus(tokens);
  if (typeof result !== "number") {
    return { ok: false, error: "Invalid sequence" };
  }
  return { ok: true, value: result };
}

console.log("12 + 3 - 4 =", evalExprSimple("12 + 3 - 4"));
console.log("bad char   =", evalExprSimple("2 + a"));

// A few quick checks
const cases = [
  "0-3",
  "7+5",
  "10-3-2",
  "2 + 2 + 2",
  "9 - 1 + 1"
];

for (const c of cases) {
  const r = evalExprSimple(c);
  console.log(c, "=>", r.ok ? r.value : r.error);
}

// Note: unary minus (like "-3+2") not handled yet
// This simple version expects a number first