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2 | 2 | * Evaluate a numeric operations string in postfix notation using a stack. |
3 | 3 | * Supports basic arithmetic operations: +, -, *, / |
4 | 4 | * @see https://www.geeksforgeeks.org/evaluation-of-postfix-expression/ |
5 | | - * @param {string} expression - Numeric operations expression to evaluate. |
| 5 | + * @param {string} expression - Numeric operations expression to evaluate. Must be a valid postfix expression. |
6 | 6 | * @returns {number|null} - Result of the expression evaluation, or null if the expression is invalid. |
7 | 7 | */ |
8 | 8 | function evaluatePostfixExpression(expression) { |
9 | | - const stack = []; |
10 | | - |
11 | | - // Helper function to perform an operation and push the result to the stack |
12 | | - function performOperation(operator) { |
13 | | - const operand2 = stack.pop(); |
14 | | - const operand1 = stack.pop(); |
15 | | - if (operator === '+' || operator === '-' || operator === '*' || operator === '/') { |
16 | | - if (operand1 === undefined || operand2 === undefined) { |
17 | | - return null; // Invalid expression |
18 | | - } |
19 | | - switch (operator) { |
20 | | - case '+': |
21 | | - stack.push(operand1 + operand2); |
22 | | - break; |
23 | | - case '-': |
24 | | - stack.push(operand1 - operand2); |
25 | | - break; |
26 | | - case '*': |
27 | | - stack.push(operand1 * operand2); |
28 | | - break; |
29 | | - case '/': |
30 | | - if (operand2 === 0) { |
31 | | - return null; // Division by zero |
32 | | - } |
33 | | - stack.push(operand1 / operand2); |
34 | | - break; |
35 | | - } |
36 | | - } else { |
37 | | - return null; // Invalid operator |
38 | | - } |
| 9 | + const stack = []; |
| 10 | + |
| 11 | + // Helper function to perform an operation and push the result to the stack |
| 12 | + function performOperation(operator) { |
| 13 | + const rightOp = stack.pop(); // Right operand is the top of the stack |
| 14 | + const leftOp = stack.pop(); // Left operand is the next item on the stack |
| 15 | + |
| 16 | + if (leftOp === undefined || rightOp === undefined) { |
| 17 | + return false; // Invalid expression |
39 | 18 | } |
40 | | - |
41 | | - const tokens = expression.split(' '); |
42 | | - |
43 | | - for (const token of tokens) { |
44 | | - if (!isNaN(parseFloat(token))) { |
45 | | - // If the token is a number, push it to the stack |
46 | | - stack.push(parseFloat(token)); |
47 | | - } else { |
48 | | - // If the token is an operator, perform the operation |
49 | | - const result = performOperation(token); |
50 | | - if (result === null) { |
51 | | - return null; // Invalid expression |
| 19 | + switch (operator) { |
| 20 | + case '+': |
| 21 | + stack.push(leftOp + rightOp); |
| 22 | + break; |
| 23 | + case '-': |
| 24 | + stack.push(leftOp - rightOp); |
| 25 | + break; |
| 26 | + case '*': |
| 27 | + stack.push(leftOp * rightOp); |
| 28 | + break; |
| 29 | + case '/': |
| 30 | + if (rightOp === 0) { |
| 31 | + return false; |
52 | 32 | } |
53 | | - } |
| 33 | + stack.push(leftOp / rightOp); |
| 34 | + break; |
| 35 | + default: |
| 36 | + break; |
54 | 37 | } |
55 | | - |
56 | | - if (stack.length === 1) { |
57 | | - // The final result should be on the stack |
58 | | - return stack[0]; |
| 38 | + |
| 39 | + } |
| 40 | + |
| 41 | + const tokens = expression.split(/\s+/); |
| 42 | + |
| 43 | + for (const token of tokens) { |
| 44 | + if (!isNaN(parseFloat(token))) { |
| 45 | + // If the token is a number, push it to the stack |
| 46 | + stack.push(parseFloat(token)); |
59 | 47 | } else { |
60 | | - return null; // Invalid expression |
| 48 | + // If the token is an operator, perform the operation |
| 49 | + const result = performOperation(token); |
| 50 | + if (result === false) { |
| 51 | + return null; // Invalid expression |
| 52 | + } |
61 | 53 | } |
62 | 54 | } |
63 | | - |
64 | | - export { evaluatePostfixExpression }; |
65 | | - |
| 55 | + |
| 56 | + return (stack.length === 1) ? stack[0] : null; |
| 57 | +} |
| 58 | + |
| 59 | +export { evaluatePostfixExpression }; |
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