From bae7f8915618c396b677428b314188ef566c6db5 Mon Sep 17 00:00:00 2001
From: Hardik Pawar <97388607+Hardvan@users.noreply.github.com>
Date: Sun, 13 Oct 2024 16:27:05 +0530
Subject: [PATCH] feat: Add `PrefixEvaluator` new algorithm with Junit tests
(#5755)
---
DIRECTORY.md | 2 +
.../thealgorithms/stacks/PrefixEvaluator.java | 76 +++++++++++++++++++
.../stacks/PrefixEvaluatorTest.java | 27 +++++++
3 files changed, 105 insertions(+)
create mode 100644 src/main/java/com/thealgorithms/stacks/PrefixEvaluator.java
create mode 100644 src/test/java/com/thealgorithms/stacks/PrefixEvaluatorTest.java
diff --git a/DIRECTORY.md b/DIRECTORY.md
index a78efffb8..d9beaadcb 100644
--- a/DIRECTORY.md
+++ b/DIRECTORY.md
@@ -600,6 +600,7 @@
* [NextSmallerElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextSmallerElement.java)
* [PostfixEvaluator](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixEvaluator.java)
* [PostfixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixToInfix.java)
+ * [PrefixEvaluator](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PrefixEvaluator.java)
* [PrefixToInfix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PrefixToInfix.java)
* [SortStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/SortStack.java)
* [StackPostfixNotation](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/StackPostfixNotation.java)
@@ -1131,6 +1132,7 @@
* [NextSmallerElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java)
* [PostfixEvaluatorTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixEvaluatorTest.java)
* [PostfixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixToInfixTest.java)
+ * [PrefixEvaluatorTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PrefixEvaluatorTest.java)
* [PrefixToInfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PrefixToInfixTest.java)
* [SortStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/SortStackTest.java)
* [StackPostfixNotationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/StackPostfixNotationTest.java)
diff --git a/src/main/java/com/thealgorithms/stacks/PrefixEvaluator.java b/src/main/java/com/thealgorithms/stacks/PrefixEvaluator.java
new file mode 100644
index 000000000..eb2ce95e1
--- /dev/null
+++ b/src/main/java/com/thealgorithms/stacks/PrefixEvaluator.java
@@ -0,0 +1,76 @@
+package com.thealgorithms.stacks;
+
+import java.util.Set;
+import java.util.Stack;
+
+/**
+ * Evaluate a prefix (Polish) expression using a stack.
+ *
+ * Example: Expression "+ * 2 3 4" results in 10.
+ *
Applications: Useful for implementing compilers and interpreters.
+ *
+ * @author Hardvan
+ */
+public final class PrefixEvaluator {
+ private PrefixEvaluator() {
+ }
+
+ private static final Set OPERATORS = Set.of("+", "-", "*", "/");
+
+ /**
+ * Evaluates the given prefix expression and returns the result.
+ *
+ * @param expression The prefix expression as a string with operands and operators separated by spaces.
+ * @return The result of evaluating the prefix expression.
+ * @throws IllegalArgumentException if the expression is invalid.
+ */
+ public static int evaluatePrefix(String expression) {
+ Stack stack = new Stack<>();
+ String[] tokens = expression.split("\\s+");
+
+ for (int i = tokens.length - 1; i >= 0; i--) {
+ String token = tokens[i];
+ if (isOperator(token)) {
+ int operand1 = stack.pop();
+ int operand2 = stack.pop();
+ stack.push(applyOperator(token, operand1, operand2));
+ } else {
+ stack.push(Integer.valueOf(token));
+ }
+ }
+
+ if (stack.size() != 1) {
+ throw new IllegalArgumentException("Invalid expression");
+ }
+
+ return stack.pop();
+ }
+
+ /**
+ * Checks if the given token is an operator.
+ *
+ * @param token The token to check.
+ * @return true if the token is an operator, false otherwise.
+ */
+ private static boolean isOperator(String token) {
+ return OPERATORS.contains(token);
+ }
+
+ /**
+ * Applies the given operator to the two operands.
+ *
+ * @param operator The operator to apply.
+ * @param a The first operand.
+ * @param b The second operand.
+ * @return The result of applying the operator to the operands.
+ */
+ private static int applyOperator(String operator, int a, int b) {
+ return switch (operator) {
+ case "+" -> a + b;
+ case "-" -> a - b;
+ case "*" -> a * b;
+ case "/" -> a / b;
+ default -> throw new IllegalArgumentException("Invalid operator");
+ };
+ }
+}
diff --git a/src/test/java/com/thealgorithms/stacks/PrefixEvaluatorTest.java b/src/test/java/com/thealgorithms/stacks/PrefixEvaluatorTest.java
new file mode 100644
index 000000000..e2faa6195
--- /dev/null
+++ b/src/test/java/com/thealgorithms/stacks/PrefixEvaluatorTest.java
@@ -0,0 +1,27 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.EmptyStackException;
+import org.junit.jupiter.api.Test;
+
+public class PrefixEvaluatorTest {
+
+ @Test
+ public void testValidExpressions() {
+ assertEquals(10, PrefixEvaluator.evaluatePrefix("+ * 2 3 4"));
+ assertEquals(5, PrefixEvaluator.evaluatePrefix("- + 7 3 5"));
+ assertEquals(6, PrefixEvaluator.evaluatePrefix("/ * 3 2 1"));
+ }
+
+ @Test
+ public void testInvalidExpression() {
+ assertThrows(EmptyStackException.class, () -> PrefixEvaluator.evaluatePrefix("+ 3"));
+ }
+
+ @Test
+ public void testMoreThanOneStackSizeAfterEvaluation() {
+ assertThrows(IllegalArgumentException.class, () -> PrefixEvaluator.evaluatePrefix("+ 3 4 5"));
+ }
+}