Menilai ungkapan matematik dengan mengambil kira operator keutamaan dalam JavaScript

Soalan

Kita perlu menulis fungsi JavaScript yang menerima ungkapan matematik sebagai rentetan dan mengembalikan hasilnya sebagai nombor.

Kita perlu menyokong pengendali matematik berikut -

• Pembahagian / (sebagai pembahagian titik terapung)

• Tambahan + p>

• Penolakan -

• Penolakan -

Operator sentiasa berdasarkan kiri Ke kanan, * dan / mesti dinilai sebelum + dan -.

Contoh

Berikut ialah kod -

Demonstrasi masa nyata

const exp = '6 - 4';
const findResult = (exp = '') => {
   const digits = '0123456789.';
   const operators = ['+', '-', '*', '/', 'negate'];
   const legend = {
      '+': { pred: 2, func: (a, b) => { return a + b; }, assoc: "left" },
      '-'&: { pred: 2, func: (a, b) => { return a - b; }, assoc: "left" },
      '*': { pred: 3, func: (a, b) => { return a * b; }, assoc: "left" },
      '/': { pred: 3, func: (a, b) => {
      if (b != 0) { return a / b; } else { return 0; }
   }
   }, assoc: "left",
   'negate': { pred: 4, func: (a) => { return -1 * a; }, assoc: "right" }
};
exp = exp.replace(/\s/g, '');
let operations = [];
let outputQueue = [];
let ind = 0;
let str = '';
while (ind < exp.length) {
   let ch = exp[ind];
   if (operators.includes(ch)) {
      if (str !== &#39;&#39;) {
         outputQueue.push(new Number(str));
         str = &#39;&#39;;
      }
      if (ch === &#39;-&#39;) {
         if (ind == 0) {
            ch = &#39;negate&#39;;
         } else {
            let nextCh = exp[ind+1];
            let prevCh = exp[ind-1];
            if ((digits.includes(nextCh) || nextCh === &#39;(&#39; || nextCh === &#39;-&#39;) &&
               (operators.includes(prevCh) || exp[ind-1] === &#39;(&#39;)) {
                  ch = &#39;negate&#39;;
            }
         }
      }
      if (operations.length > 0) {
         let topOper = operations[operations.length - 1];
         while (operations.length > 0 && legend[topOper] &&
         ((legend[ch].assoc === &#39;left&#39; && legend[ch].pred <= legend[topOper].pred) ||
         (legend[ch].assoc === &#39;right&#39; && legend[ch].pred < legend[topOper].pred))) {
            outputQueue.push(operations.pop());
            topOper = operations[operations.length - 1];
         }
      }
      operations.push(ch);
   } else if (digits.includes(ch)) {
      str += ch
   } else if (ch === &#39;(&#39;) {
      operations.push(ch);
   } else if (ch === &#39;)&#39;) {
      if (str !== &#39;&#39;) {
         outputQueue.push(new Number(str));
         str = &#39;&#39;;
      }
      while (operations.length > 0 && operations[operations.length - 1] !== &#39;(&#39;) {
         outputQueue.push(operations.pop());
      }
      if (operations.length > 0) { operations.pop(); }
   }
   ind++;
}
if (str !== &#39;&#39;) { outputQueue.push(new Number(str)); }
   outputQueue = outputQueue.concat(operations.reverse())
   let res = [];
   while (outputQueue.length > 0) {
      let ch = outputQueue.shift();
      if (operators.includes(ch)) {
         let num1, num2, subResult;
         if (ch === &#39;negate&#39;) {
            res.push(legend[ch].func(res.pop()));
         } else {
            let [num2, num1] = [res.pop(), res.pop()];
            res.push(legend[ch].func(num1, num2));
         }
      } else {
         res.push(ch);
      }
   }
   return res.pop().valueOf();
};
console.log(findResult(exp));

Output

2

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