[英]Finding prime numbers in Java
我有一個java代碼檢查素數,然后顯示它們。 但是,有些情況下沒有素數(例如14-17之間)。 在這些情況下,我想要顯示一條消息。 例如"No primes found"
。 我不知道如何將其添加到我的代碼中。
這是我的全班:
import java.io.*;
public class Prime {
BufferedReader input = new BufferedReader(new InputStreamReader(System.in));
public static int lowerBound;
public static int higherBound;
public void getInput() throws IOException {
System.out.println("Please enter the lower and upper bound");
String line1 = input.readLine();
String line2 = input.readLine();
int lowInput = Integer.parseInt(line1);
int highInput = Integer.parseInt(line2);
lowerBound = lowInput;
higherBound = highInput;
}
public void validatedata() throws IOException {
do {
getInput();
if (lowerBound < 2) {
System.out.println("Finish");
break;
} else if (higherBound < lowerBound) {
System.out
.println("The upper bound should be at least as big as the lower bound.");
}
} while (higherBound < lowerBound);
}
public void prime_calculation() throws IOException {
while ((lowerBound >= 2) && (higherBound > lowerBound)) {
int k;
for (k = lowerBound; k < higherBound; k++) {
boolean primecheck = true;
for (int j = 2; j < k; j++) {
if (k % j == 0) {
primecheck = false;
}
}
if (primecheck)
System.out.println(k);
}
validatedata();
}
}
}
這是我的主要虛空方法:
import java.io.IOException;
public class PrimeUser extends Prime {
public static void main(String argv[]) throws IOException {
Prime isprime = new Prime();
isprime.validatedata();
isprime.prime_calculation();
}
}
ArrayList<Integer> primes = new ArrayList<Integer>();
if ((lowerBound >= 2) && (higherBound > lowerBound))
{
int k;
for (k = lowerBound; k < higherBound; k++)
{
primecheck = true;
for (int j=2; j < k/2; j++) // you dont have to check beyond k/2, since (k/2 + 1) times anything whole number cannot be equal to k
{
if (k % j == 0) {
primecheck = false;
break; // no need for more iterations of the current number, since you already know it is not a prime
}
}
if (primecheck) {
primes.add(k);
}
}
} else {
// invalid bounds
return;
}
if (primes.size() > 0) {
for (int num : primes) {
System.out.println(num);
}
} else {
System.out.println("No primes exist");
}
// Without an array
boolean y = false;
String x = new String();
if ((lowerBound >= 2) && (higherBound > lowerBound))
{
int k;
for (k = lowerBound; k < higherBound; k++)
{
primecheck = true;
for (int j=2; j < k/2; j++) // you dont have to check beyond k/2, since (k/2 + 1) times anything whole number cannot be equal to k
{
if (k % j == 0) {
primecheck = false;
break; // no need for more iterations of the current number, since you already know it is not a prime
}
}
if (primecheck) {
x += Integer.toString(k) + "\n";
y = true; // yes, there exists atleast 1 prime
}
}
} else {
// invalid bounds
return;
}
if (y == true) {
System.out.println(x);
} else {
System.out.println("No primes exist");
}
假設2和3是已知素數:將在不到900毫秒內找到100萬個素數
public class FindPrime {private Integer [] arr_num = new Integer [1000000];
private void arr_init()
{
for(int i=0; i< arr_num.length; i++)
{
arr_num[i] = i;
}
}
private void find_prime()
{
for (int i=3; i<arr_num.length; i++)
{
find_prm(arr_num[i]);
}
}
private void find_prm(Integer chk_prm)
{
boolean isprime = false;
if(chk_prm == null)
return;
if(chk_prm %2 !=0 )
{
double sqrt = Math.sqrt(chk_prm);
for(Integer k=arr_num[2]; (k!= null) && (k<sqrt + 1) ; k++)
{
if(chk_prm % k == 0)
{
isprime=false;
break;
}
else
isprime = true;
}
}
else
{
setNull(chk_prm);
}
if(!isprime)
{
setNull(chk_prm);
}
}
private void setNull(int num_to_null)
{
arr_num [num_to_null] = null;
}
private void print_prime()
{
for(int i=2; i < arr_num.length; i++)
{
if(null != arr_num[i])
{
System.out.println(arr_num[i]);
}
}
}
public static void main(String [] args)
{
FindPrime findPrime = new FindPrime();
findPrime.arr_init();
long timeStart = System.currentTimeMillis();
findPrime.find_prime();
long diff = System.currentTimeMillis()- timeStart;
System.out.println("Running time :: " + diff);
}
}
看看http://rosettacode.org/wiki/Miller-Rabin_primality_test#Java ,它解釋了如何確定數字是否為素數。
然后處理你的數組值並保持一個數字是否是一個素數。
我找到了一種解決問題的簡便方法。 我只是添加了一個計數器來計算素數的出現次數。 如果計數器等於= 0,則顯示消息。 代碼如下。 不管怎樣,謝謝你的幫助。 如果我有任何問題,我會再次發布。 順便說一句,我想知道是否有人可以告訴我或分享任何來源/網頁,或者只是以任何方式訓練我的算法技巧來尋找解決方案?
最后的代碼在這里
public void prime_calculation() throws IOException
{
while ((lowerBound >= 2) && (higherBound > lowerBound))
{
int k;
int m = 0;
for (k = lowerBound; k < higherBound; k++)
{
boolean primecheck = true;
for (int j=2; j < k; j++)
{
if (k % j == 0)
{
primecheck = false;
}
}
if (primecheck)
{
System.out.println(k);
m++;
}
}
if (m == 0)
System.out.println("No primes found");
validatedata();
}
}
}
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