We Keep Coding

Reverse an int using recursion

I want to reverse an int but it doesn't work. For example, 123 should return 321, but the printed number is 356.
public class x {
public static void main(String[] args) {
System.out.println(reverse2(123, 0));
}
static int reverse2(int a, int i) {
if(a == 0) {
return 0;
} else {
i = i*10 + a%10;
System.out.println(i);
return i += reverse2(a/10, i);
}
}
}

Your code should look like this:
public class x {
public static void main(String[] args) {
System.out.println(reverse2(123, 0));
}
static int reverse2(int a, int i) {
if(a == 0) {
return i;
} else {
i = i*10 + a%10;
System.out.println(i);
return reverse2(a/10, i);
}
}
}
You should return i when a is 0.
You shouldn't add i when you call the reverse2 function because you're adding i twice.

You are greatly complicating your recursive function for printing an integer in reverse. For one, there is no good reason for reverse2 to have two integer arguments, as you can achieve your desired results with a single argument. The trick is to access the rightmost digit with the % 10 operation then shift that digit off the number with the / 10 operation. Consider these revisions:
public class x {
public static void main(String[] args) {
System.out.println(reverse2(123));
}
static String reverse2(int number) {
if(number == 0) {
return "";
} else {
return number % 10 + reverse2(number / 10);
}
}
}

You can do it like this. You only need to pass the value you are reversing. The math computation computes 10 to the power of the number of digits in the argument.
public static int reverse(int v) {
int reversed = 0;
if (v > 0) {
int d = (int)Math.pow(10,(int)(Math.log10(v)));
reversed = reverse(v%d) * 10 + v/d;
}
return reversed;
}
Of course, if you can pass a second argument as a scratch pad, then it can be done like so. As you tear down the original value you build up the returned value.
public static int reverse(int v, int reversed) {
if (v > 0) {
return reverse(v / 10, reversed * 10 + v % 10);
}
return reversed;
}

I'm not sure if I understand correctly how to call a new method in Java

I have the following exercise that I'm trying to solve:
Write a class Summing with a method public static void sumit(). The
method computes the sum of all numbers between 1 an 200 which are
divisble by 7 and prints the result in the form
"The sum is NUMBER"
where "NUMBER" is the sum.
Here is what I've written so far:
public class Summing {
public static void main(String[] args) {
public static void sumit() {
for(int i = 0; i <= 200; i += 7) {
System.out.print("The sum is " + i);
}
}
}
}
I'm not sure how I correctly call on the sumit() method here. Can anybody point out to me how I properly create the method sumit()?

The execution of a program always starts from the main() method, so you need to call the sumit() method inside the main() method, like below:
public static void main(String[] args) {
sumit();
}
public static void sumit() {
for(int i = 0; i <= 200; i += 7) {
System.out.print("The sum is " + i);
}
}
But still there is issue with your code, which won't give you the sum of all numbers that are divisible by 7 between 0 and 200, so have a local variable which will add all numbers that are divisible by 7 in for loop
public static void sumit() {
int sum=0;
for(int i = 0; i <= 200; i += 7) {
sum+=i; //sum = sum+i;
System.out.println("The sum is " + sum);
}
}

You cannot put a method inside another method so rather do this:
-Write your method outside the main method
public class Summing
{
public static void main(String[] args)
{
sumit();
}
public static void sumit() {
for(int i = 0; i <= 200; i += 7) {
System.out.print("The sum is " + i);
}
}
}

If I understand the requirement correctly, this guy should do it.
Give it a try ;]
public class Summing {
public static void main(String[] args) {
sumit();
}
public static void sumit() {
int sum = 0;
for(int i = 0; i <= 200; i++) {
if (i % 7 == 0) {
sum = sum + i;
}
}
System.out.print("The sum is " + sum);
}
}

Creating java program assistance

I have to create a java program with two classes and the challenge is =
"Enter in 10 numbers. Calculate the average and display all numbers greater than the average."
I am fairly new to java and I have no idea on what I am doing and how to send array values from one class to another.
import BreezySwing.KeyboardReader;
import javax.swing.*;
public class Average {
public static void main(String[] args) {
KeyboardReader reader = new KeyboardReader();
System.out.print('\u000C');
AverageTest at = new AverageTest();
int numberArray[];
int i;
numberArray = new int[10];
for (i = 0; i < 10; i++) {
numberArray[i] = reader.readInt("Enter a number: ");
at.setnumber(numberArray);
}
}
}
import javax.swing.*;
import BreezySwing.*;
public class AverageTest
{
private int number[];
private int a;
public void setnumber(int number)
{
number = numberArray;
}
}

import java.util.Scanner;
public class AverageTest {
public static void main(String[] args) {
int[] array = new int[10];
// Try with resources, automatically closes the reader once the work is done
// Read 10 integers from the standard input
try (Scanner reader = new Scanner(System.in);) {
for (int i = 0; i < 2; i++) {
System.out.println("Enter a number: ");
array[i] = reader.nextInt();
}
} catch (Exception e) {
e.printStackTrace();
}
// we have an array with 10 numbers, now create an average object by passing
// this array to the Average class constructor
Average averageObj = new Average(array);
// Compute the average
float average = averageObj.average();
System.out.println("Average: " + average);
System.out.println("Numbers greater than average: ");
// Print out the numbers which are greater than or equal to the average
for (int i = 0; i < array.length; i++) {
if (array[i] >= average) {
System.out.println(array[i]);
}
}
}
}
class Average {
private int[] array;
public Average(int[] array) {
if (array == null || array.length == 0) {
throw new IllegalArgumentException("Array cannot be null or empty");
}
this.array = array;
}
public int[] getArray() {
return array;
}
/**
* Computes the average of the given array and returns it.
*/
public float average() {
int sum = 0;
for (int i = 0; i < array.length; i++) {
sum += array[i];
}
return (float) sum/array.length;
}
}

there are 3 steps about this issue:
1.Enter in 10 numbers.
2.Calculate the average.
3.display all numbers greater than the average.
you have done the step 1,that's great.
and I can see that you are trying to do the step 2.
here's the suggestion of your issue:
if you want to send array values from A class to B,you just need to invoke the method of B in the A correctly.
I think I know what you are trying to do.
the problem of your code that you can't send array values from one class to another is because the method's parameter type is not matching.
the method public void setnumber(int number),the parameter is an int type,and you try to refer it to an int array,this's wrong.
first, you need to change the method's definition to public void setnumber(int[] numberarray),and try to figure out why we have to write like this.
then finish the step 2.
Hope it'll help.

java program giving blank output

The program works fine for small numbers but as soon as i take a big number like this it doesn't work
here is my code
public class Main {
public static void main(String[] args) {
long no=600851475143L,i;
int result=0;
for(i=(no/2);i>=2;i--){
if(no%i==0){
if(checkPrime(i)){
System.out.println("Longest Prime Factor is: " + i);
break;
}
}
}
}
private static boolean checkPrime(long i){
for(long j=2L;j<=(int)Math.sqrt(i);j++){
if(i%j==0)
return false;
}
return true;
}
}

to assign long variable value we does not require write L at last on value Remove L.

It will take time to display the answer. Just try with small number(1000000 ) almost 10 to 15 min for above code.
Try this
public class Main {
public static void main(String[] args) {
//long no=600851475143L,i;
System.out.println(largestPrimeFactor(600851475143L));
}
public static int largestPrimeFactor(long number) {
int i;
for (i = 2; i <= number; i++) {
if (number % i == 0) {
number /= i;
i--;
}
}
return i;
}
}
[1]https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes

StackOverFlowError in java program [duplicate]

This question already has answers here:
What is a StackOverflowError?
(16 answers)
Closed 7 years ago.
I am trying to solve a problem which asks to find the smallest prime palindrome, which comes after a given number which means that if the input is 24, the output would be 101 as it is the smallest number after 24 which is both prime and a palindrome.
Now my code works perfectly for small values but the moment I plug in something like 543212 as input, I end up with a StackOverFlowError on line 20, followed by multiple instances of StackOverFlowErrors on line 24. Here is my code :
package nisarg;
import java.util.Scanner;
public class Chef_prime_palindromes {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
long num = input.nextLong();
isPalindrome(num + 1);
}
public static boolean isPrime(long num) {
long i;
for (i = 2; i < num; i++) {
if (num % i == 0) {
return false;
}
}
return true;
}
public static void isPalindrome(long num) {
String word = Long.toString(num);
int i;
for (i = 0; i < word.length() / 2; i++) {
if (word.charAt(i) != word.charAt(word.length() - i - 1)) {
isPalindrome(num + 1);
}
}
if (i == word.length() / 2) {
if (isPrime(num)) {
System.out.println(num);
System.exit(0);
} else {
isPalindrome(num + 1);
}
}
}
}

All shown exiting solutions use recursion and have the problem that at some point they will reach the point where a StackOverflowException will occur.
A better solution which would also be parallelizable would be to change it into a loop.
It could be something like:
package nisarg;
import java.math.BigInteger;
import java.util.Scanner;
import java.util.concurrent.CopyOnWriteArrayList;
public class Chef_prime_palindromes {
private static final CopyOnWriteArrayList<BigInteger> PRIMESCACHE
= new CopyOnWriteArrayList<>();
public static void main(String[] args) {
try (Scanner input = new Scanner(System.in)) {
BigInteger num = new BigInteger(input.nextLine());
initPrimes(num);
for (num = num.add(BigInteger.ONE);
!isPrimePalindrome(num);
num = num.add(BigInteger.ONE));
System.out.println(num.toString());
}
}
private static void initPrimes(BigInteger upTo) {
BigInteger i;
for (i = new BigInteger("2"); i.compareTo(upTo) <= 0 ; i = i.add(BigInteger.ONE)) {
isPrime(i);
}
}
public static boolean isPrimePalindrome(BigInteger num) {
return isPrime(num) && isPalindrome(num);
}
// could be optimized
public static boolean isPrime(BigInteger num) {
for (int idx = PRIMESCACHE.size() - 1; idx >= 0; --idx) {
if (num.mod(PRIMESCACHE.get(idx)).compareTo(BigInteger.ZERO) == 0) {
return false;
}
}
if (!PRIMESCACHE.contains(num)) {
PRIMESCACHE.add(num);
}
return true;
}
public static boolean isPalindrome(BigInteger num) {
String word = num.toString();
int i;
for (i = 0; i < word.length() / 2; i++) {
if (word.charAt(i) != word.charAt(word.length() - i - 1)) {
return false;
}
}
return true;
}
}

A new String object is created in each recursive call and placed onto stack (the place where all variables created in methods are stored until you leave the method), which for a deep enough recursion makes JVM reach the end of allocated stack space.
I changed the locality of the String object by placing it into a separate method, thus reducing its locality and bounding its creation and destruction (freeing of stack space) to one recursive call.
package com.company;
import java.util.Scanner;
public class Chef_prime_palindromes {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
long num = input.nextLong();
isPalindrom(num + 1);
}
public static boolean isPrime(long num) {
long i;
for (i = 2; i < num; i++) {
if (num % i == 0) {
return false;
}
}
return true;
}
private static void isPalindrom(long num) {
for (; ; ) {
if (isPalindrome(num)) {
if (isPrime(num)) {
System.out.println(num);
System.exit(0);
} else {
num++;
}
} else {
num++;
}
}
}
public static boolean isPalindrome(long num) {
String string = String.valueOf(num);
return string.equals(new StringBuilder(string).reverse().toString());
}
}

First thing you should be aware of is the fact that your resources are limited. Even if your implementation was precise and all recursive calls were correct, you may still get the error. The error indicates your JVM stack ran out of space. Try to increase the size of your JVM stack ( see here for details).
Another important thing is to look for the distribution of prime and palindrome numbers. Your code runs by testing every num+1 against palindrome property. This is incorrect. You test for palindrome only when the number is prime. This will make the computation much much easier (and reduce recursive calls). I have edited your code accordingly and got the closest palindrome number after 543212 (1003001) . Here it is:
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
long num = input.nextLong();
//isPalindrome(num+1);
nextPrimePalindrome(num+1);
}
public static void nextPrimePalindrome(long num)
{
boolean flag=true;
while(flag)
{
if(isPrime(num))
if(isPalindrome(num))
{
System.out.println(num);
flag=false;
}
num++;
}
}
public static boolean isPrime(long num){
long i;
for(i=2;i<num;i++){
if(num%i == 0){
return false;
}
}
return true;
}
public static boolean isPalindrome(long num)
{
String word=Long.toString(num);
for(int i=0;i<word.length()/2;i++)
if(word.charAt(i)!=word.charAt(word.length()-i-1))
return false;
return true;
}
}