Merge pull request #13 from code4tomorrow/JoshJ-IsPrime

isPrime Problem

Author: JJ27

google_checks_mod.xml

@@ -322,8 +322,13 @@
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+    <!--
+      Modification: Disabled the commented out checks
+      https://checkstyle.sourceforge.io/config_javadoc.html
+
     <module name="JavadocParagraph"/>
     <module name="RequireEmptyLineBeforeBlockTagGroup"/>
+    -->
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src/com/codefortomorrow/advanced/chapter13/practice/PrimePractice.java

@@ -0,0 +1,19 @@
+package com.codefortomorrow.advanced.chapter13.practice;
+
+/*
+Write a method called isPrime which returns
+true if the given integer is prime and false otherwise.
+
+This is similar to the Chapter 11 problem, but this time write
+your method using recursion.
+
+In your main method, include a Scanner so the user can check
+as many numbers as they want until they enter -1.
+
+Note: There are more complex solutions, but this is the fastest one
+within the scope of this chapter.
+*/
+
+public class PrimePractice {
+    // write code here
+}

src/com/codefortomorrow/advanced/chapter13/solutions/PrimePractice.java

@@ -0,0 +1,68 @@
+package com.codefortomorrow.advanced.chapter13.solutions;
+
+import java.util.Scanner;
+
+/*
+Write a method called isPrime which returns
+true if the given integer is prime and false otherwise.
+
+This is similar to the Chapter 11 problem, but this time write
+your method using recursion.
+
+In your main method, include a Scanner so the user can check
+as many numbers as they want until they enter -1.
+
+Note: There are more complex solutions, but this is the fastest one
+within the scope of this chapter.
+*/
+
+public class PrimePractice {
+
+    public static void main(String[] args) {
+        Scanner reader = new Scanner(System.in);
+        int number = 0;
+        while (number != -1) {
+            System.out.print("Enter an integer to check: ");
+            number = reader.nextInt();
+            if (number != -1) {
+                if (isPrime(number, 2)) {
+                    System.out.println("That is a prime!");
+                } else {
+                    System.out.println("Not a prime!");
+                }
+            }
+        }
+        reader.close();
+    }
+
+    /**
+     * isPrime checks the primality of a given integer.
+     * @param n  The integer to check
+     * @param z  Current divisor (used for recursion)
+     * @return boolean true if prime and false if not
+     */
+    public static boolean isPrime(int n, int z) {
+        // Check base cases
+        if (n <= 2) {
+            return n == 2;
+        }
+
+        // If n is divisible by the current divisor,
+        // it has a factor other than 1 and thus is
+        // not prime
+        if (n % z == 0) {
+            return false;
+        }
+
+        // If z gets high enough that z > sqrt(n), then n is prime,
+        // because factors just repeat after
+        if (z > Math.sqrt(n)) {
+            return true;
+        }
+
+        // If none of the above work,
+        // keep calling isPrime recursively
+        // with a larger divisor
+        return isPrime(n, z + 1);
+    }
+}