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125 lines (112 loc) · 4.2 KB
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package problem07;
/* This program is based on the following "Project Euler" problem:
By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
What is the 10,001st prime number?
The program prompts the user to test either a number's primality, or find a certain prime number.
*/
import java.util.Scanner;
public class PrimeNumbers {
private static Scanner scan = new Scanner(System.in);
public static void main(String[] args) {
boolean run = true;
if (args.length == 0) {
while (run) {
printInstructions();
String choice = scan.next();
System.out.println();
if (choice.equalsIgnoreCase("t")) {
System.out.print("Please enter a number to test: ");
int testNumber = scan.nextInt();
System.out.println("Is " + testNumber + " a prime number? "
+ primeTest(testNumber));
} else if (choice.equalsIgnoreCase("f")) {
System.out.print("Which prime number do you wish to find?(#) ");
int numberOfPrimes = scan.nextInt();
int answer = findPrime(numberOfPrimes);
System.out.println(answer + " is the " + numberOfPrimes
+ numberType(numberOfPrimes) + " prime number. ");
} else if (choice.equalsIgnoreCase("s")) {
System.out.println("Find the sum of all primes below: ");
int max = scan.nextInt();
System.out.println("Answer: " + findPrimeSumUnderValue(max));
} else if (choice.equalsIgnoreCase("q")) {
run = false;
} else {
System.out.println("I'm sorry. Please choose one of the options.");
}
}
} else {
if (args[0].equals("0")) { // problem 7
System.out.println(findPrime(10001));
} else if (args[0].equals("1")) {
System.out.println(findPrimeSumUnderValue(2000000));
}
}
}
// returns whether or not a given number is prime
public static boolean primeTest(int number) {
if (number == 2) {
return true;
}
// tests up to the square root of the number
int root = (int) Math.ceil(Math.sqrt(number));
for (int i = 2; i < root + 1; i++) {
if ((double) number % i == 0) {
return false;
} else if (i == root) {
return true;
}
}
return false;
}
// answers Euler problem 10, "find the sum of all primes below n"
private static long findPrimeSumUnderValue(int max) {
long sum = 0;
for (int i = 2; i < max; i++) {
if (primeTest(i)) {
sum += i;
}
}
return sum;
}
// TODO: Replace with Sieve of Eratosthenes
private static int findPrime(int numberOfPrimes) {
if (numberOfPrimes == 1) {
return 2;
}
int primeCount = 1;
int testNumber = 3;
while (primeCount < numberOfPrimes) {
if (primeTest(testNumber)) {
primeCount++;
}
if (primeCount == numberOfPrimes) {
return testNumber;
}
testNumber += 2; // only odd numbers are tested
}
return -1;
}
// PROGRAM PROMPT ITEMS
private static void printInstructions() {
System.out.println("This program deals with prime numbers. You can do one of four things.");
System.out.println("\t't' to test for primality\n" +
"\t'f' to find a prime number\n"+
"\t's' to find the summation of all prime numbers below a given value\n" +
"\t'q' to quit the program.");
System.out.print("Choice: ");
}
private static String numberType(int number) {
if (number % 100 == 10 || number % 100 == 11 || number % 100 == 13) {
return "th";
} else if (number % 10 == 1) {
return "st";
} else if (number % 10 == 2) {
return "nd";
} else if (number % 10 == 3) {
return "rd";
} else {
return "th";
}
}
}