Hey so im trying to create a program that prints out the first 100 prime numbers. this is my code however it doesn't print out the first hundred but the primes from 2 to 100. what am i doing wrong here that make it print out only those ant not the first hundred?
import java.io.*;
import java.util.*;
import java.lang.*;
import java.text.*;
public class Prime {
public static void main(String[] args) {
int maxCount = 100;
int[] values = new int[maxCount];
int temp = 3;
boolean prime = true;
for (int j = 1; j < maxCount; ++j) {
values[j] = j + 1;
if (values[j] == 2) {
System.out.println(values[j]);
}
}
for (int j = 2; j < maxCount; ++j) {
if (values[j] % 2 == 0)
prime = false;
else {
boolean prime2 = true;
double test = Math.sqrt(values[j]);
int divisor = 3;
while (prime2 && (divisor <= test)) {
if (values[j] % divisor == 0) {
prime2 = false;
} else divisor += 2;
}
if (prime2)
System.out.println(values[j]);
}
}
}
}
You're looping until j < maxCount while you should loop until the number of elements in the array are equal to maxCount. For that you'll need another counter which is val_cnt in the following fixed code.
import java.io.*;
import java.util.*;
import java.lang.*;
import java.text.*;
public class Prime {
public static void main(String[] args) {
int maxCount = 100;
int[] values = new int[maxCount];
int temp = 3;
int val_cnt = 0;
values[val_cnt++] = 2;
System.out.println(2);
for (int j = 3; val_cnt < maxCount; ++j) {
boolean prime = true;
if (j % 2 == 0)
prime = false;
else {
double test = Math.sqrt(j);
int divisor = 3;
while (prime && (divisor <= test)) {
if (j % divisor == 0) {
prime = false;
} else divisor += 2;
}
if (prime) {
values[val_cnt++] = j;
System.out.println(j);
}
}
}
}
}
you are making your loop iterate till j becomes 100 , and at every iteration irrespective of whether a prime number has been found or not, j gets incremented.
So have something like:-
while(maxCount<100){
if(number_is_prime){
increment maxCount;
}
else{
continue;
}
You are iterating your loop till 100 only. But you want to print first 100 prime numbers, not prime numbers between 1 to 100. so every time the number is found to be a prime number just increment the count(which counts the number of prime numbers printed.) to 1 and exit from the loops when the count is 100.Here is the simple approach for printing first 100 prime numbers:
int count = 0,flag=0;
for(int i=2;count<100;i++){
flag = 0;
for(int j=2;j<i;j++){
if(i%j == 0){
flag ++;
break;
}
}
if(flag == 0){
System.out.println(i);
count++;
}
}
Only increase the value of j if prime number is found,
Re-format your codes same as this,
for (int j = 2; j < maxCount;) {
if (values[j] % 2 == 0){
prime = false;
j++;
}
else {
boolean prime2 = true;
double test = Math.sqrt(values[j]);
int divisor = 3;
while (prime2 && (divisor <= test)) {
if (values[j] % divisor == 0) {
prime2 = false;
} else divisor += 2;
}
if (prime2)
System.out.println(values[j]);
j++;
}
Try this
class PrimeNumbers {
public static void main (String[] args) {
int i =0; int num =0;
String primeNumbers = "";
for (i = 1; i <= 100; i++) {
int counter=0;
for(num =i; num>=1; num--) {
if(i%num==0) {
counter = counter + 1;
}
}
if (counter ==2) {
primeNumbers = primeNumbers + i + " ";
}
}
System.out.println("Prime numbers from 1 to 100 are :");
System.out.println(primeNumbers);
}
}
Related
i have the following code, but the answer is coming out to be 2.
public class Main {
public static void main(String[] args) {
int sum = 0;
int count = 0;
for(int i = 2; i <= 50; i++) {
for(int j = 1; j <= i; j++) {
System.out.println(j);
if(i % j == 0) {
count++;
}
if(count == 2) {
System.out.println(i);
sum = sum + i;
}
}
}
System.out.println("The sum: " + sum);
}
}
You were close. But here is all you need.
first, set sum to 2. It is the only even prime and doing this allows a better increment below
now increment the test cases starting at 3 and incrementing by 2. Skip even values.
now divide by each test case by numbers from 2 to the sqrt(i). No need to go beyond that.
as soon as you have a successful division, end the inner loop and start the next of the outer loop.
otherwise, if the inner loop completes, add i to sum.
int sum = 2;
outer:
for(int i = 3; i <= 50; i+= 2) {
for(int j = 2; j <= Math.sqrt(i); j++) {
if(i % j == 0) {
continue outer;
}
}
sum += i;
}
System.out.println("The sum: " + sum);
An improvement would be to store the primes as they are found and then only divide by those instead of arbitrary numbers.
Disclaimer: this code was written by Github Copilot with minimal supervision.
class Scratch {
// find the sum of all prime numbers between 2 and n
public static int sumPrimes(int n) {
int sum = 0;
for (int i = 2; i < n; i++) {
if (isPrime(i)) {
sum += i;
}
}
return sum;
}
// return true if n is prime
public static boolean isPrime(int n) {
if (n == 2) {
return true;
}
if (n % 2 == 0) {
return false;
}
for (int i = 3; i * i <= n; i += 2) {
if (n % i == 0) {
return false;
}
}
return true;
}
public static void main(String[] args) {
// print sum of primes between 2 and 50
System.out.println("The sum: " + sumPrimes(50));
}
}
Prints:
The sum: 328
Maybe a minimal version with recursive method based on Aivean response:
class Scratch {
// find the sum of all prime numbers between 2 and n
public static int sumPrimes(int n) {
if (n >= 2) {
return (isPrime(n) ? n : 0) + sumPrimes(n-1);
}
return 0;
}
// return true if n is prime
public static boolean isPrime(int n) {
for (int i = 2; i < n; i++) {
if (n % i == 0) {
return false;
}
}
return true;
}
public static void main(String[] args) {
// print sum of primes between 2 and 50
System.out.println("The sum: " + sumPrimes(50));
}
}
I wanted to use a for and a while loop to obtain the prime factors of a number. My while loop example works fine which I have posted below my for loop example. However, my for loop does not work, and i am guessing that I can't use a continue in the same manner that I used it in the while loop. If this is true, then how would I accomplish this. I have not been able to find a basic beginners example of this using a for loop. Thanks
// My getting largest prime factor using a for loop
public class LargestPrime{
public static void main(String[] args) {
int number = 36;
int largestPrime = 0;
for ( int i = 2; i <= number; i++){
if (number % i == 0){
largestPrime = i;
number /= i;
continue;
}
System.out.println(" largest prime = " + i);
}
}
}
//*******************************************************************
//*******************************************************************
public class LargestPrime {
// gettting largest prime using a while loop
public static int getLargestPrime(int number) {
if (number <= 1) {
return -1;
}
int largestPrime = 0;
int count = 2;
while (count <= number) {
if (number % count == 0) {
largestPrime = count;
number = number / count;
continue;
}
count++;
}
return largestPrime;
}
}
The problem is that continue in a for loop executes the update part (i++), which your while loop didn't.
The other problem is that you're printing inside the loop.
There are multiple way to fix this:
Do i-- before continue, so it evens out to nothing with the i++. This is a fairly common way to handle this.
Since you don't have any code after the if statement, you don't need the continue.
for (int i = 2; i <= number; i++) {
if (number % i == 0) {
largestPrime = i;
number /= i;
i--; // to retry same `i` value
}
}
Do the i++ "yourself", i.e. not as part of for loop:
for (int i = 2; i <= number; ) {
if (number % i == 0) {
largestPrime = i;
number /= i;
continue;
}
i++;
}
Or:
for (int i = 2; i <= number; ) {
if (number % i == 0) {
largestPrime = i;
number /= i;
} else {
i++;
}
}
Use a while loop inside the for loop:
for (int i = 2; i <= number; i++) {
while (number % i == 0) {
largestPrime = i;
number /= i;
}
}
That can be shortened to:
for (int i = 2; i <= number; i++)
for (; number % i == 0; number /= i)
largestPrime = i;
Though rather than assign largestPrime repeatedly, you could do this:
for (int i = 2; i <= number; i++) {
if (number % i == 0) {
largestPrime = i;
do {
number /= i;
} while (number % i == 0);
}
}
Write a program to read n numbers. The first number specified as input will be n. Next, the program should read n integer numbers.
The program should check for each number if it is prime as well as if its reverse is prime.
Display all such numbers in ascending order.
Consider below example for input and output:
Input:
7
11
12
23
19
7
113
101
Output:
7
11
101
113
My code
public class Prime {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int x = sc.nextInt();
int temp;
int[] a = new int [x];
int[] r = new int [x];
int[]c = new int[a.length+r.length];
int[] rev = new int [x];
for(int i=0;i<x;i++){
a[i] = sc.nextInt();
rev[i]=a[i];
}
for(int i = 0; i < a.length; i++) {
while(rev[i] != 0) {
r[i] = r[i] * 10;
r[i] = r[i] + rev[i]%10;
rev[i] = rev[i]/10;
}
}
for(int i = 0; i < a.length; i++) {
boolean isPrime = true;
for (int j = 2; j < i; j++) {
if((a[i]%j==0) || (r[i]%j==0)) {
isPrime = false;
break;
}
}
if(isPrime)
System.out.println(a[i]);
System.out.println(r[i]);
}
}
}
Somewhere I am stuck I don't know how to eliminate repeated no, how to merge the array at last and also it is printing 1 and 2 as prime no when I give input and 2
You need to use TreeSet - which will contain only distinct elements and give result in sorted form. You can refer to following code-
Set<Integer> set = new TreeSet<>();
for(int i = 0; i < a.length; i++) {
boolean isPrime = true;
if(isPrime(a[i]) && isPrime(r[i]))
set.add(a[i]);
}
Iterator it = set.iterator();
while(it.hasNext())
System.out.print(it.next() + " ");
Also create a function for checking prime numbers -
private static boolean isPrime(int num) {
if(num==1) return false;
for(int i = 2; i <= num/2; ++i)
{
if(num % i == 0)
{
return false;
}
}
return true;
}
You can try below code. Hope it helps you,
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;
public class PrimeNumberTest {
public static void main(String[] args) {
List<Integer> list = new ArrayList<>(Arrays.asList(7, 11, 12, 23, 19, 7 ,113, 101));
//To remove duplicates
Set<Integer> set = new TreeSet<>(list);
System.out.println(getPrimeNumbers(set).toString().replaceAll(",", "").replace("]", "").replace("[", ""));
}
//Method to get unique ordered set of prime numbers
private static Set<Integer> getPrimeNumbers(Set<Integer> set) {
Set<Integer> resultList=new TreeSet<>();
set.forEach(ele->{
//check for prime
if(isPrime(ele)){
//if prime number check for reverse and if true, add to result
if(isPrime(reverserNumb(ele)))
resultList.add(ele);
}
});
return resultList;
}
private static boolean isPrime(int num){
if(num<2)
return false;
// Check for even numbers
if (num % 2 == 0) {
return num == 2;
}
// Check for odd numbers
for (int i = 3; i*i <= num; i += 2) {
if (num % i == 0) {
return false;
}
}
return true;
}
private static int reverserNumb(int num) {
return Integer.valueOf(new StringBuilder(String.valueOf(num)).reverse().toString());
}
}
Here is the code for prime test using √n approach
static boolean isPrime(int n){
//corner case
if (n <= 1) return false;
if (n <= 3) return true;
//middle 5 number
if (n % 2 == 0 || n % 3 == 0) return false;
for (int i = 5; i * i <= n; i = i + 6)
if (n % i == 0 || n % (i + 2) == 0)
return false;
return true;
}
Use can use set for remove duplicate element
Try if this code works, hope it helps
Code:
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
public class PrimeNumbers {
static ArrayList<Integer> prime = new ArrayList<>();
public static void main(String[] args) {
Scanner user_input = new Scanner(System.in);
System.out.print("Enter Number of Integers: ");
int number_count = user_input.nextInt();
int[] numbers = new int[number_count];
for (int i = 0; i < numbers.length; i++) {
System.out.print("Enter Integer: ");
numbers[i] = user_input.nextInt();
}
System.out.print("Values Entered: ");
for (int i = 0; i < numbers.length; i++) {
System.out.print(numbers[i] + " ");
checkPrime(numbers[i],false); //false don't reverse
checkPrime(numbers[i],true); //true reverse value to check if it is also prime
}
System.out.print("\nList of prime numbers: ");
Collections.sort(prime);
for(int n : prime){
System.out.print(n + " ");
}
if(prime.isEmpty()){
System.out.print("no prime numbers on list\n");
}
}
//check for duplicates
static void insertValueToPrime(int n){
for(int p : prime){
if(n == p){
return;
}
}
prime.add(n);
}
static void checkPrime(int n,boolean isReverse) {
int i, m = 0, flag = 0,realn = n;
if(isReverse){
n = reverseNumber(n);
}
m = n / 2;
if (n == 0 || n == 1) {
//no a prime number
} else {
for (i = 2; i <= m; i++) {
if (n % i == 0) {
// not a prime number
flag = 1;
break;
}
}
if (flag == 0) {
insertValueToPrime(realn);
}
}
}
static int reverseNumber(int n){
String reverse = "",str=""+n;
for(int i = str.length() - 1; i >= 0; i--)
{
reverse = reverse + str.charAt(i);
}
return Integer.parseInt(reverse);
}
}
I wrote some code to find prime numbers up to a given number. Could you guys let me know ways to make my code more efficient or better? Or give insight on how I did? In addition, there is a problem in my code where certain numbers repeats twice or three times in a pattern. How do I fix this?
public class PrimeNumber2 {
public static void main(String[] args)
{
int max_prime = 10000;
for(int i = 3; i < max_prime; i+=2)
{
for(int j = 1; j < Math.sqrt(i); j++)
{
if(i % j == 0)
{
System.out.println(i);
}
}
}
}
}
Have a look at:
https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
This is quite a nice way of doing it.
Here is some code for you to look at:
public void runEratosthenesSieve(int upperBound) {
int upperBoundSquareRoot = (int) Math.sqrt(upperBound);
boolean[] isComposite = new boolean[upperBound + 1];
for (int m = 2; m <= upperBoundSquareRoot; m++) {
if (!isComposite[m]) {
System.out.print(m + " ");
for (int k = m * m; k <= upperBound; k += m)
isComposite[k] = true;
}
}
for (int m = upperBoundSquareRoot; m <= upperBound; m++) {
if (!isComposite[m]) {
System.out.print(m + " ");
}
}
Here´s a tuned version of yours.
I did put the check if the number is prime into a seperate method. That´s also a part of the reason why your version did print values multiple times, since if it found out that it has a divisor, then it would print the values. (Also you´r algorythm would print basicly everything despite it beeing prime or not, for example it prints 15 and 27).
The reason why it did print multiple values was, that once you found a divisor it would have printed i, but it would continue looping. If it would have found another divisor, it would print i again(you can notice that it does not only print prime numbers).
Here is the fixed version of your´s
public static void main(String[] args) {
if(isPrime(2)) {
System.out.println(2);
}
int max_prime = 10000;
for(int i = 3; i < max_prime; i+=2)
{
if(isPrime(i)) {
System.out.println(i);
}
}
}
private static boolean isPrime(int n) {
if(n<=1) return false;
if(n == 2) return true;
for(int i = 2;i*i<=n;++i) {
if(n%i == 0) return false;
}
return true;
}
Try this.
public class PrimeNumber2 {
public static void main(String[] args)
{
int max_prime = 10000;
System.out.println(2);
L: for (int i = 3; i < max_prime; i += 2)
{
for (int j = 3, max = (int)Math.sqrt(i); j <= max ; j += 2)
{
if(i % j == 0)
{
continue L;
}
}
System.out.println(i);
}
}
}
Here a way using parallelStream
System.out.println(2);
IntStream.range(1, 10000000)
.map(i -> i * 2 + 1)
.filter(i -> (i & 1) != 0 && IntStream.range(1, (int) (Math.sqrt(i)-1)/2)
.map(j -> j * 2 + 1)
.noneMatching(j -> i % j == 0)
.forEach(System.out::println);
Note: the ranges test the n-th odd number.
public static void main(String[] args) {
int upperBound = 30;
List<Integer> primes = new ArrayList<>();
// loop through the numbers one by one
for (int number = 2; number < upperBound; number++) {
boolean isPrimeNumber = true;
// check to see if the number is prime
for (int j = 2; j < number; j++) {
if (number % j == 0) {
isPrimeNumber = false;
break; // exit the inner for loop
}
}
// print the number if prime
if (isPrimeNumber) {
primes.add(number);
}
}
System.out.println("The number of prime is: " + primes.size() + ", and they are: " + primes.toString());
}
possible to dupplicate
get prime numbers and total prime numbers in range
public boolean isPrimeNumber(int number) {
for (int i=2; i<=number/2; i++) {
if (number % i == 0) {
return false;
}
}
return true;
}
I am new here. I am trying to solve this exercise Problem 18 just for reinforcing my solving skills. I've already coded the answer. The task asks for "How many of the primes below 1,000,000 have the sum of their digits equal to the number of days in a fortnight?" (a fortnight is 14 days). My answers is 16708, but it is wrong. I hope you can help me. I don't know what my error is. I have 2 methods, 1 for generating the primes, and another for counting the digits of each prime.
This is my code:
import java.util.ArrayList;
import java.util.List;
public class Problema18 {
public static void main(String args[]) {
ArrayList<Integer> num = primes();
System.out.println(num);
count(primes());
}
public static ArrayList<Integer> primes() {
List<Integer> primes = new ArrayList<Integer>();
primes.add(2);
for (int i = 3; i <= 1000000; i += 2) {
boolean isPrime = true;
int stoppingPoint = (int) (Math.pow(i, 0.5) + 1);
for (int p : primes) {
if (i % p == 0) {
isPrime = false;
break;
}
if (p > stoppingPoint) { break; }
}
if (isPrime) { primes.add(i); }
}
// System.out.println(primes);
return (ArrayList<Integer>) primes;
//System.out.println(primes.size());
}
public static void count(ArrayList<Integer> num) {
int count = 0;
for (int i = 0; i <= num.size() - 1; i++) {
int number = num.get(i);
String num1 = String.valueOf(number);
int sum = 0;
for (int j = 0; j < num1.length(); j++) {
sum = Integer.parseInt(num1.charAt(j) + "") + sum;
if (sum == 14) { count++; }
}
System.out.println(sum);
}
System.out.println(count);
}
}
You should check whether sum == 14 outside the inner for loop. What happens now is that you also count those primes for which the sum of digits is larger than 14 but the sum of the digits in some prefix of the prime is equal to 14.
This part...
if (sum == 14) {
count++;
}
should be outside the inner for-loop - i.e. you want to do it each time you pass through the i for-loop, but not each time you pass through the j for-loop.
Like this:
public static void count(ArrayList<Integer> num) {
int count = 0;
for (int i = 0; i <= num.size() - 1; i++) {
int number = num.get(i);
String num1 = String.valueOf(number);
int sum = 0;
for (int j = 0; j < num1.length(); j++) {
sum = Integer.parseInt(num1.charAt(j) + "") + sum;
}
System.out.println(sum);
if (sum == 14) {
count++;
}
}
System.out.println(count);
}