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Simplify StackPostfixNotation.postfixEvaluate (#4264)

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Piotr Idzik 2023-07-26 19:16:52 +02:00 committed by GitHub
parent fc274c84f8
commit de50fc0294
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1 changed files with 43 additions and 20 deletions
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63
src/main/java/com/thealgorithms/others/StackPostfixNotation.java
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@ -2,38 +2,61 @@ package com.thealgorithms.others;
import java.util.Scanner;
import java.util.Stack;
import java.util.function.BiFunction;
/**
* @brief Utility class evaluating postix expressions, cf. https://en.wikipedia.org/wiki/Reverse_Polish_notation
* @details The computation is done using Integers.
*/
public final class StackPostfixNotation {
private StackPostfixNotation() {
}
// Evaluates the given postfix expression string and returns the result.
public static int postfixEvaluate(final String exp) {
Stack<Integer> s = new Stack<Integer>();
private static BiFunction<Integer, Integer, Integer> getOperator(final String operationSymbol) {
// note the order of operands
switch (operationSymbol) {
case "+":
return (a, b) -> b + a;
case "-":
return (a, b) -> b - a;
case "*":
return (a, b) -> b * a;
case "/":
return (a, b) -> b / a;
default:
throw new IllegalArgumentException("exp contains an unknown operation.");
}
}
private static void performOperation(Stack<Integer> s, final String operationSymbol) {
if (s.size() < 2) {
throw new IllegalArgumentException("exp is not a proper postfix expression (too few arguments).");
}
s.push(getOperator(operationSymbol).apply(s.pop(), s.pop()));
}
private static void consumeExpression(Stack<Integer> s, final String exp) {
Scanner tokens = new Scanner(exp);
while (tokens.hasNext()) {
if (tokens.hasNextInt()) {
s.push(tokens.nextInt()); // If int then push to stack
} else { // else pop top two values and perform the operation
if (s.size() < 2) {
throw new IllegalArgumentException("exp is not a proper postfix expression (too few arguments).");
}
int num2 = s.pop();
int num1 = s.pop();
String op = tokens.next();
switch (op) {
case "+" -> s.push(num1 + num2);
case "-" -> s.push(num1 - num2);
case "*" -> s.push(num1 * num2);
case "/" -> s.push(num1 / num2);
default -> throw new IllegalArgumentException("exp contains an unknown operation.");
}
// "+", "-", "*", "/"
s.push(tokens.nextInt());
} else {
performOperation(s, tokens.next());
}
}
tokens.close();
}
/**
* @brief Evaluates the given postfix expression.
* @param exp the expression to evaluate.
* @return the value of the given expression.
* @exception IllegalArgumentException exp is not a valid postix expression.
*/
public static int postfixEvaluate(final String exp) {
Stack<Integer> s = new Stack<Integer>();
consumeExpression(s, exp);
if (s.size() != 1) {
throw new IllegalArgumentException("exp is not a proper postfix expression.");
}