E1 — 式評価器（パート1：+と-の左から右への処理）

作成するもの

トークナイザー: 数値と演算子

// Tokenize: split into numbers and operators; ignore spaces
function tokenize(expr) {
  if (typeof expr !== "string") return null;

  const tokens = [];
  let num = "";

  for (let i = 0; i < expr.length; i = i + 1) {
    const ch = expr[i];

    if (ch === " ") {
      // skip spaces
      continue;
    }

    if (ch >= "0" && ch <= "9") {
      // build number
      num = num + ch;
    } else if (ch === "+" || ch === "-") {
      // flush pending number
      if (num.length > 0) {
        tokens.push(Number(num));
        num = "";
      }
      tokens.push(ch);
    } else {
      // unsupported character
      return null;
    }
  }

  if (num.length > 0) tokens.push(Number(num));
  return tokens;
}

console.log("tokens:", tokenize("12 + 3 - 4"));

左から右に評価（+/-）

// 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]));

安全なラッパー

// 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"));

小さなケースを実行する

// 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

初心者向けガイド

左から右の動作クイズ

まとめ