Develop Reference

How output 10 integers per line in Java?

The problem is to display the first 50 prime numbers in 5 lines, each of which contains 10 numbers. I've created a program to output the first 50 prime numbers but I don't know how to split them so they output 10 numbers per line. I am a beginner level programmer and I really need help on this.
public class Lab4 {
public static void main(String[] args) {
int i = 0;
int num = 0;
String primeNumbers = " ";
for (i = 1; i <= 230; 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(primeNumbers);
}
}

You need to count how many items you already added and once you have 10 you need to put new line. Also I changed String to StringBuilder because concatenating in a loop is not very good, you can read about it here StringBuilder vs String concatenation
int i = 0;
int num = 0;
int lineCounter = 0;
StringBuilder primeNumbers = new StringBuilder();
for (i = 1; i <= 230; i++) {
int counter = 0;
for (num = i; num >= 1; num--) {
if (i % num == 0) {
counter = counter + 1;
}
}
if (counter == 2) {
primeNumbers.append(i).append(" ");
lineCounter++;
}
if (lineCounter == 10) {
primeNumbers.append(System.lineSeparator());
lineCounter = 0;
}
}
System.out.println(primeNumbers);

Just add this piece of code below after this line in your code: primeNumbers = primeNumbers + i + " ";
if (newLineCount == 10) {
primeNumbers += '\n';
newLineCount = 0;
}
newLineCount++;
Also init newLineCount before the loop: int newLineCount = 0;
Additionally, as mentioned in the comments, consider using StringBuilder instead of String, or even better ArrayList to store your numbers, then you can have method to print values from your ArrayList in whatever formatted way you want (with tabs, alignment, new lines...)

Here is the code that suits your needs. I haven't changed anything in your code, just added mine to suit your needs.
public class print_prime_numbers_10_per_line {
public static void main(String[] args) {
int i = 0;
int num = 0;
String primeNumbers = "";
for (i = 1; i <= 230; i++) {
int counter = 0;
for (num = i; num >= 1; num--) {
if (i % num == 0) {
counter = counter + 1;
}
}
if (counter == 2) {
primeNumbers = primeNumbers + i + " ";
}
}
String[] integerStrings = primeNumbers.split(" ");
int[] integers = new int[integerStrings.length];
for (int x = 0; x < integers.length; x++) {
integers[x] = Integer.valueOf(integerStrings[x]);
}
for (int g = 0; g < integers.length; g++) {
if (g % 11 == 0) {
System.out.println();
} else {
System.out.print(integers[g] + " ");
}
}
}
}

ADAGAME4 Spoj Wrong Answer

Below is a Archive PROBLEM from SPOJ. Sample testCase is passing, but I am getting W/A on submission. I am missing some testCase(testCases). Need help to figure out what case I am missing and/or what I am doing wrong here.
Ada the Ladybug is playing Game of Divisors against her friend Velvet Mite Vinit. The game has following rules. There is a pile of N stones between them. The player who's on move can pick at least 1 an at most σ(N) stones (where σ(N) stands for number of divisors of N). Obviously, N changes after each move. The one who won't get any stones (N == 0) loses.
As Ada the Ladybug is a lady, so she moves first. Can you decide who will be the winner? Assume that both players play optimally.
Input
The first line of input will contain 1 ≤ T ≤ 10^5, the number of test-cases.
The next T lines will contain 1 ≤ N ≤ 2*10^7, the number of stones which are initially in pile.
Output
Output the name of winner, so either "Ada" or "Vinit".
Sample Input:
8
1
3
5
6
11
1000001
1000000
29
Sample Output:
Ada
Vinit
Ada
Ada
Vinit
Vinit
Ada
Ada
CODE
import java.io.*;
public class Main
{
public static int max_size = 2 * (int)Math.pow(10,7) + 1;
//public static int max_size = 25;
//public static int max_size = 2 * (int)Math.pow(10,6) + 1;
public static boolean[] dp = new boolean[max_size];
public static int[] lastPrimeDivisor = new int[max_size];
public static int[] numOfDivisors = new int[max_size];
public static void main(String[] args) throws IOException
{
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
preprocess();
int t = Integer.parseInt(br.readLine());
while(t > 0)
{
int n = Integer.parseInt(br.readLine());
if(dp[n] == true)
System.out.println("Ada");
else
System.out.println("Vinit");
t--;
}
}
public static void markLastPrimeDivisor()
{
for(int i = 0 ; i < max_size ; i++)
{
lastPrimeDivisor[i] = 1;
}
for(int i = 2 ; i < max_size ; i += 2)
{
lastPrimeDivisor[i] = 2;
}
int o = (int)Math.sqrt(max_size);
for(int i = 3 ; i < max_size; i++)
{
if(lastPrimeDivisor[i] != 1)
{
continue;
}
lastPrimeDivisor[i] = i;
if(i <= o)
{
for(int j = i * i ; j < max_size ; j += 2 * i)
{
lastPrimeDivisor[j] = i;
}
}
}
/*for(int i = 1 ; i < max_size ; i++)
System.out.println("last prime of " + i + " is " + lastPrimeDivisor[i]);*/
}
public static void countDivisors(int num)
{
int original = num;
int result = 1;
int currDivisorCount = 1;
int currDivisor = lastPrimeDivisor[num];
int nextDivisor;
while(currDivisor != 1)
{
num = num / currDivisor;
nextDivisor = lastPrimeDivisor[num];
if(nextDivisor == currDivisor)
{
currDivisorCount++;
}
else
{
result = result * (currDivisorCount + 1);
currDivisorCount = 1;
currDivisor = nextDivisor;
}
}
if(num != 1)
{
result = result * (currDivisorCount + 1);
}
//System.out.println("result for num : " + original + ", " + result);
numOfDivisors[original] = result;
}
public static void countAllDivisors()
{
markLastPrimeDivisor();
for(int i = 2 ; i < max_size ; i++)
{
countDivisors(i);
//System.out.println("num of divisors of " + i + " = " + numOfDivisors[i]);
}
}
public static void preprocess()
{
countAllDivisors();
dp[0] = dp[1] = dp[2] = true;
for(int i = 3 ; i < max_size ; i++)
{
int flag = 0;
int limit = numOfDivisors[i];
//If for any i - j, we get false,for playing optimally
//the current opponent will choose to take j stones out of the
//pile as for i - j stones, the other player is not winning.
for(int j = 1 ; j <= limit; j++)
{
if(dp[i - j] == false)
{
dp[i] = true;
flag = 1;
break;
}
}
if(flag == 0)
dp[i] = false;
}
}
}

There is a subtle bug in your countDivisors() function. It assumes
that lastPrimeDivisor[num] – as the name indicates – returns the
largest prime factor of the given argument.
However, that is not the case. For example, lastPrimeDivisor[num] = 2
for all even numbers, or lastPrimeDivisor[7 * 89] = 7.
The reason is that in
public static void markLastPrimeDivisor()
{
// ...
for(int i = 3 ; i < max_size; i++)
{
// ...
if(i <= o)
{
for(int j = i * i ; j < max_size ; j += 2 * i)
{
lastPrimeDivisor[j] = i;
}
}
}
}
only array elements starting at i * i are updated.
So lastPrimeDivisor[num] is in fact some prime divisor of num, but not
necessarily the largest. As a consequence, numOfDivisors[55447] is computed
as 8 instead of the correct value 6.
Therefore in countDivisors(), the exponent of a prime factor in num
must be determined explicitly by repeated division.
Then you can use that the divisors function is multiplicative. This leads to
the following implementation:
public static void countAllDivisors() {
// Fill the `somePrimeDivisor` array:
computePrimeDivisors();
numOfDivisors[1] = 1;
for (int num = 2 ; num < max_size ; num++) {
int divisor = somePrimeDivisor[num];
if (divisor == num) {
// `num` is a prime
numOfDivisors[num] = 2;
} else {
int n = num / divisor;
int count = 1;
while (n % divisor == 0) {
count++;
n /= divisor;
}
// `divisor^count` contributes to `count + 1` in the number of divisors,
// now use multiplicative property:
numOfDivisors[num] = (count + 1) * numOfDivisors[n];
}
}
}

Why doesn't my program run correctly?

for my school project I have to create a program that outputs perfect numbers based on how many perfect numbers the user(teacher) want. The user can pick any number from 1-4 and it should display however many number the user chooses. Here is my current code. Please ignore the sumupTo, factorial, isprime, and the testGoldbach methods, please only look at the Perfect numbers method/code.
import java.util.Scanner;
public class MyMathB
{
public static int sumUpTo(int n)
{
int sum = 0;
for (int k = 1; k <= n; k++)
sum += k;
return sum;
}
public static long factorial(int n)
{
long f = 1;
for (int k = 2; k <= n; k++)
f *= k;
return f;
}
public static boolean isPrime(int n)
{
if (n <= 1)
return false;
int m = 2;
while (m * m <= n)
{
if (n % m == 0)
return false;
m++;
}
return true;
}
public static void PerfectNumbers(int number)
{
System.out.println("How many perfect numbers would you like to see? Please enter an integer from 1 to 4");
Scanner s = new Scanner(System.in);
int numbersToSee = s.nextInt();
int counts = 0;
for(counts = 0; counts <= numbersToSee; counts++)
{
for (int n = 5; n <= 10000; n++)
{
int temp = 0;
for(int i = 1; i <= number / 2; i++)
{
if (number % i == 0)
{
temp += i;
}
if (temp == number)
{
System.out.println(number);
}
}
}
}
}
public static boolean testGoldbach(int bigNum)
{
for (int n = 6; n <= bigNum; n += 2)
{
boolean found2primes = false;
for (int p = 3; p <= n/2; p += 2)
{
if (isPrime(p) && isPrime(n - p))
found2primes = true;
}
if (!found2primes)
{
System.out.println(n + " is not a sum of two primes!");
return false;
}
}
return true;
}
public static void main(String[] args)
{
Scanner kb = new Scanner(System.in);
int n;
do
{
System.out.print("Enter an integer from 4 to 20: ");
n = kb.nextInt();
} while (n < 4 || n > 20);
kb.close();
System.out.println();
System.out.println("1 + ... + " + n + " = " + sumUpTo(n));
System.out.println(n + "! = " + factorial(n));
System.out.println("Primes: ");
for (int k = 1; k <= n; k++)
if (isPrime(k))
System.out.print(k + " ");
System.out.println();
System.out.println("Goldbach conjecture up to " + n + ": " + testGoldbach(n));
}
}

you didn't declare the variable "number" in your method.
Edit: you didn't SET the variable number to anything, I misworded my last statement.

How do I terminate this loop?

"Only one while loop should be used to determine all even and odd numbers between 50 and 100."
public class EvenOdd {
public static void main(String args[]) {
int x = 50;
int y = 50;
int i = 0;
int j = 0;
int n = 0;
System.out.print("Even numbers between 50 and 100: ");
while((i != x) || (j != y)) {
n++;
System.out.print(i + x + ", ");
i += 2;
if(i != x)
continue;
System.out.println("100");
System.out.print("\nOdd numbers between 50 and 100: ");
System.out.print((j+1) + y + ", ");
j += 2;
if(j != y)
continue;
}
}
}
The evens print fine but the odds continue on forever. This may be a dumb question, but I'm having the biggest brainfart right now, and I would really appreciate help on this.

Simply try this and let me know. It's based on your code, with just a few minor adjustments:
public class Teste4 {
public static void main(String args[]) {
int x = 50;
int y = 50;
int i = 0;
int j = 1;
int n = 0;
System.out.print("Even numbers between 50 and 100: ");
while((i < x) || (j < y)) {
if(i < x){
System.out.print(i + x + ", ");
i += 2;
continue;
}else if(i == x){
System.out.println("100");
i++;
}
if(j == 1){
System.out.print("\nOdd numbers between 50 and 100: ");
}
System.out.print(j + y + ", ");
j += 2;
}
}
}

Perhaps you should try this
int even_counter = 50;
int odd_counter=51;
System.out.println("Even numbers between 50 and 100: ");
while((even_counter < 100 ) || (odd_counter < 100)){
if(even_counter < 100){
System.out.print(even_counter+ " ");
even_counter+=2;
continue;
}
if(odd_counter < 100){
if(odd_counter == 51){
System.out.println("\nOdd numbers between 50 and 100: ");
}
System.out.print(odd_counter+ " ");
odd_counter+=2;
}
}

I'm sure this is an assignment but i'll add my 2 cents since it's solved. This one will give you an array.
final int EVEN = 0, ODD = 1;
int low = 50, high = 100, current = low;
int[][] numbers = new int[2][];
numbers[EVEN] = new int[((high - low) / 2) + ((high - low) % 2)];
numbers[ODD] = new int[((high - low) / 2)];
while(current < high){
numbers[current % 2][(current - low) / 2] = current++;
}
System.out.println("EVEN" + Arrays.toString(numbers[EVEN]));
System.out.println("ODD " + Arrays.toString(numbers[ODD]));

Alternative approach ,
int current = 50 , end = 100 ;
String odd , even ;
while(current <= 100){
if (current % 2 == 0)
even = even.concat ("," + current);
else
odd = odd.concat("," + current);
current++;
}
System.out.println("Even no are : " + even);
System.out.println("Odd no are : " + odd);
I dont have a compiler now . I think this should be right :) .

Luhn formula implementation in Java

I'm trying to implement a luhn formula in my java servlet application. I tried other 'valid' credit cards numbers scattering in the internet and didn't work. I just want to know if I got it correctly. Any help would be appreaciate!
public static boolean luhn(String input){
char[] creditCard = input.toCharArray();
int checkSum = 0;
boolean alternate = false;
for (int i = creditCard.length - 1; i >= 0; i --){
int m = (int)Integer.parseInt(Character.toString(creditCard[i]));
if (alternate){
m *= 2;
if (m > 9){
m = (m & 10) + 1;
}
}
checkSum += m;
alternate = true;
}
if ( (checkSum % 10) == 0){
return true;
}else{
return false;
}
}

here the working code
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
boolean repeat;
List<Integer> digits = new ArrayList<Integer>();
do {
repeat = false;
System.out.print("Enter your Credit Card Number : ");
String input = in.next();
for (int i = 0; i < input.length(); i++) {
char c = input.charAt(i);
if (c < '0' || c > '9') {
repeat = true;
digits.clear();
break;
} else {
digits.add(Integer.valueOf(c - '0'));
}
}
} while (repeat);
int[] array = new int[digits.size()];
for (int i = 0; i < array.length; i++) {
array[i] = Integer.valueOf(digits.get(i));
}
boolean valid = check(array);
System.out.println("Valid: " + valid);
}
to check for luhn algo
public static boolean check(int[] digits) {
int sum = 0;
int length = digits.length;
for (int i = 0; i < length; i++) {
// get digits in reverse order
int digit = digits[length - i - 1];
// every 2nd number multiply with 2
if (i % 2 == 1) {
digit *= 2;
}
sum += digit > 9 ? digit - 9 : digit;
}
return sum % 10 == 0;
}
or a more refracted program might be as below
import java.util.Scanner;
public class Luhn {
private static Scanner input;
public static void main(String... args) {
input = new Scanner(System.in);
System.out.print("Enter number to validate:\n");
String pnr = input.nextLine();
boolean result = luhn(pnr);
printMessage(result);
input.close();
}
static boolean luhn(String pnr){
// this only works if you are certain all input will be at least 10 characters
int extraChars = pnr.length() - 10;
if (extraChars < 0) {
throw new IllegalArgumentException("Number length must be at least 10 characters!");
}
pnr = pnr.substring(extraChars, 10 + extraChars);
int sum = 0;
for (int i = 0; i < pnr.length(); i++){
char tmp = pnr.charAt(i);
int num = tmp - '0';
int product;
if (i % 2 != 0){
product = num * 1;
}
else{
product = num * 2;
}
if (product > 9)
product -= 9;
sum+= product;
}
return (sum % 10 == 0);
}
private static void printMessage(boolean valid) {
if (valid){
System.out.print("Valid!\r");
}
else{
System.out.print("Invalid!");
}
}
}