Stack • Applications

Evaluate Prefix Expression

Prefix expressions (Polish Notation) are evaluated using a Stack similarly to Postfix, but we process the expression backwards (Right-to-Left).

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Right-to-Left

Instead of reading the expression left-to-right (like Postfix), we read a Prefix expression strictly from right-to-left.

Pop Order

Because we process backwards, when an operator is found, the first pop is `val1` (left operand) and the second pop is `val2` (right operand). This is the exact opposite of Postfix!

Why Prefix?

LISP and similar languages use Polish Notation. It removes the need for order-of-operations logic or parentheses entirely, just like Postfix.

-+2*319

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Code Tracker

:::

1function evaluatePrefix(exp) {

2 let stack = [];

3 // Read right-to-left

4 for (let i = exp.length - 1; i >= 0; i--) {

5 let token = exp[i];

6 if (!isNaN(token)) {

7 stack.push(Number(token));

8 } else {

9 let val1 = stack.pop();

10 let val2 = stack.pop();

11 if (token === '+') stack.push(val1 + val2);

12 else if (token === '-') stack.push(val1 - val2);

13 else if (token === '*') stack.push(val1 * val2);

14 else if (token === '/') stack.push(Math.floor(val1 / val2));

15 }

16 }

17 return stack.pop();

18}