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feat: add algorithm to evaluate postfix string #1441

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Merged
raklaptudirm merged 11 commits into TheAlgorithms:master from gaurovgiri:feat/evaluate-postfix-string
Oct 10, 2023
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66 changes: 66 additions & 0 deletions Data-Structures/Stack/EvaluateExpression.js
View file Open in desktop
Original file line number Diff line number Diff line change
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/**
* Evaluate a numeric operations string using a stack.
* Supports basic arithmetic operations: +, -, *, /
* Literature reference: https://www.geeksforgeeks.org/evaluation-of-postfix-expression/
*
* @param {string} expression - Numeric operations expression to evaluate.
* @returns {number|null} - Result of the expression evaluation, or null if the expression is invalid.
*/
function evaluateExpression(expression) {
const stack = [];

// Helper function to perform an operation and push the result to the stack
function performOperation(operator) {
const operand2 = stack.pop();
const operand1 = stack.pop();
if (operator === '+' || operator === '-' || operator === '*' || operator === '/') {
if (operand1 === undefined || operand2 === undefined) {
return null; // Invalid expression
}
switch (operator) {
case '+':
stack.push(operand1 + operand2);
break;
case '-':
stack.push(operand1 - operand2);
break;
case '*':
stack.push(operand1 * operand2);
break;
case '/':
if (operand2 === 0) {
return null; // Division by zero
}
stack.push(operand1 / operand2);
break;
}
} else {
return null; // Invalid operator
}
}

const tokens = expression.split(' ');

for (const token of tokens) {
if (!isNaN(parseFloat(token))) {
// If the token is a number, push it to the stack
stack.push(parseFloat(token));
} else {
// If the token is an operator, perform the operation
const result = performOperation(token);
if (result === null) {
return null; // Invalid expression
}
}
}

if (stack.length === 1) {
// The final result should be on the stack
return stack[0];
} else {
return null; // Invalid expression
}
}

export { evaluateExpression };

22 changes: 22 additions & 0 deletions Data-Structures/Stack/test/EvaluateExpression.test.js
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Original file line number Diff line number Diff line change
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import { evaluateExpression } from '../EvaluateExpression.js';

describe('evaluateExpression', () => {
it('should evaluate a valid expression', () => {
const expression = '3 4 * 2 / 5 +'; // (3 * 4) / 2 + 5 = 11
const result = evaluateExpression(expression);
expect(result).toBe(11);
});

it('should handle division by zero', () => {
const expression = '3 0 /'; // Division by zero
const result = evaluateExpression(expression);
expect(result).toBe(null);
});

it('should handle an invalid expression', () => {
const expression = '3 * 4 2 / +'; // Invalid expression
const result = evaluateExpression(expression);
expect(result).toBe(null);
});

});

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