Here is my program which is supposed to create an array and initialize prime numbers to it. The prime numbers should then be printed but the program just keeps running.
import java.util.*;
public class primes
{
public static void main(String[] args)
{
Scanner scan = new Scanner(System.in);
System.out.println("Enter number of primes ");
int x = scan.nextInt();
int[] prime = new int[x];
int div=2,hold=2;
int c=0;
while (prime[x-1]==0)
{
for(int a=2; div>a;a++)
{
if(div>a && div%a==0)
a=div;
else if(div==(a-1))
hold=div;
}
if(div==2||hold!=prime[c-1])
{
prime[c]=hold;
c++;
}
div++;
}
for(int f =0; f<x;f++)
System.out.print(" "+prime[f]+" ");
}
}
I tried changing my loops but I just don't know whats wrong
Like the others mentioned your logic is not right, try something like:
public static void main(String args[]) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter number of primes ");
int x = scan.nextInt();
List<Integer> primes = getPrimes(x);
Integer[] primeArray = primes.toArray(new Integer[primes.size()]);
for(int i :primes.toArray(primeArray)){ // you could just use for(int i :primes){ if you don't need array
System.out.print(i + " ");
}
}
private static List<Integer> getPrimes(int upperLimit) {
ArrayList primes = new ArrayList();
for (int i = 2; i < upperLimit; i++) {
boolean isPrime = true;
// Is it prime?
for (int j = 2; j < i; j++) {
if (i % j == 0) {
isPrime = false;
break;
}
}
if (isPrime)
primes.add(i);
}
return primes;
}
The above will print out up to the numbers entered so if you type 5 it will print out 2 3 but not 5.
The following is an other example with Java 8, this one will print as many prime numbers based on the input, if you input 5 you will get 2 3 5 7 11
public static void main(String args[]) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter number of primes ");
int x = scan.nextInt();
long[] prime = primes(x).toArray();
Arrays.stream(prime).forEach(value -> System.out.print(value + " " ));
}
private static LongStream primes(long max) {
return LongStream.iterate(2, i -> i + 1)
.filter(PrimeNumber::isPrime)
.limit(max);
}
private static boolean isPrime(long x) {
return LongStream.rangeClosed(2, (long)(Math.sqrt(x)))
.allMatch(n -> x % n != 0);
}
Your code is wrong. First correct it, And i think you want to store prime numbers coming in range of 1 to N where N is user provided number. Use arrayList (growable) to store it.
It will keep on running because you have this: while (prime[x-1]==0). Where x is an input from the user. Say 5 for instance, then prime[5-1] initially is going to contain a 0 always, and you are running your while loop on this condition which is always going to turn true, thus never ending. Also, your prime number generation logic is not right!
I ran your code in debugger mode and I found the problem.
I tested your program with x=5.
At the end of the first while loop iteration you have :
prime[0] = 2
div = 3
hold = 2
c = 1
And here's the problem :
if(div==2||hold!=prime[c-1])
{
prime[c]=hold;
c++;
}
This part won't ever be reached anymore because :
div is never decrement, so it will always be superior to 2.
hold is
equal to prime[c-1], and never change value.
So prime will always stick to be : 2 0 0 0 0, and your while loop will never end.
I found what was wrong and rewrote the code, it works now. The program asks the user for the number primes they want to see and it prints them after storing them in a basic integer array.
import java.util.*;
public class Prime
{
public static void main(String [] args)
{
Scanner scan= new Scanner(System.in);
int i=0, hold=2, d=2;
boolean flag = true;
System.out.println("Enter the number of primes.");
int[] prime= new int[scan.nextInt()];
for(;flag;){
for(int a=2;d>a;a++){
if(d==(a)||d%a==0){
break;
}
if((d-1)==a){
hold = d;
}
}
d++;
if(hold==2 || hold!=prime[i-1]){
prime[i] = hold;
i++;
}
if(i==prime.length)
flag= false;
}
for(int x=0;x<prime.length;x++)
System.out.print(prime[x]+" ");
System.out.println("");
}
}
Related
After writing 1 on scanner I want a random dice number generated and after pressing 1 again I want another random number generated but now I want it added with previous number. I want to make a loop, I want to keep pressing 1 and keep adding random numbers till I reach a certain number.
Thank you.
import java.util.Random;
import java.util.Scanner;
public class dice {
public static void main(String[] args) {
// TODO Auto-generated method stub
for (int k = 0; k < 100; k++) {
Scanner scan = new Scanner(System.in);
int s = scan.nextInt();
System.out.println(s);
int previous = s;
if (s == 1) {
Random ran = new Random();
int n = ran.nextInt(6) + 1;
System.out.print(n);
int next;
while (true) {
next = scan.nextInt();
if (next == 1) {
System.out.println(previous);
}
previous = n + 10;
}
}
}
}
}
Define previous outside the for loop, and replace
int previous = s;
previous = n + 10;
with
previous += s;
previous += n + 10;
Scanner sc=new Scanner(System.in);
int sum=0;
for(;;)
{
if(sc.nextInt()==1)
{
int num = (int)(Math.random()*6); // using the pre-defined random function in java.lang.Math class
System.out.println("Dice Value: "+num);
sum+=num; // shorthand adding the number for each iteration
}
//if(sum>100)
// break;
//if statement to check if value of sum is greater/lesser than a specific number
}
System.out.println("Final Answer: "+sum)
Something like this might work (not yet tested): an infinite loop that can be terminated as per choice.
If you are looking for a way that the program works as soon as you physically press the '1' key on your keyboard, without having to press the enter key, something like a keyevent might work:
https://docs.oracle.com/javase/7/docs/api/java/awt/event/KeyEvent.html
Please do let me know if there are any errors or doubts :)
So I've written this code in java which should output numbers on the screen from 1 to n(given by the user) and it should write "-prime" near the ones that are prime.
import java.util.Scanner;
public class primeMass {
public static void main(String args[])
{
Scanner sc = new Scanner(System.in);
int n;
int i,j;
System.out.print("Dati n: ");
n = sc.nextInt();
for(i=1;i<=n;i++)
for(j=2;j<=n/2;j++)
{
if(i%j==0)
System.out.println(i);
else System.out.println(i +"-prime");
}}
}
If I input 6 for example i get :
Dati n: 6
1-prime
1-prime
2
2-prime
3-prime
3
4
4-prime
5-prime
5-prime
6
6
I'm new to this, and i'm really struggling with my algorithmic, could you tell me how should i change my program so it outputs correct values, and explain to me what i did wrong ? Thank you
UPDATE:
I've done it, thank you everyone for helping me out : this is the outcome:
import java.util.Scanner;
public class primeMass {
public static void main(String args[])
{
Scanner sc = new Scanner(System.in);
int n;
int i,j;
boolean gasit = false;
System.out.print("Dati n: ");
n = sc.nextInt();
for(i=1;i<=n;i++) {
gasit=false;
for(j=2;j*j<=i;j++)
{
if(i%j==0) gasit=true;}
if(!gasit) {System.out.println(i+"-prime");}
else {
System.out.println(i);}
}
}
}
You print something on every iteration of the inner loop.
Instead, you should print something after all iterations have completed, e.g.
boolean found = false;
for(j=2;j<=n/2;j++) {
if(i%j==0) found = true;
}
if (!found) {
System.out.println(i + "-prime");
} else {
System.out.println(i);
}
Additionally, you shouldn't be going up to n/2: you perhaps mean i/2 (a number doesn't have any factors greater than itself); but you can make it even tighter, since you don't have to check for factors greater than sqrt(i). Or, stated another way, that j * j <= i.
So you can make your loop declaration:
for(j=2; j*j<=i; j++) {
The problem is that the second loop goes until n/2, it should go until i/2 to check if i is prime. A more optimzed version of the primality check algorithm goes until sqrt(i), as suggested in comments.
The next issue is that you are concluding, in a false way, that if in the first case if(i%j==0) you say not prime, otherwise you say it is prime, which is not true necessairly. You should iterate the whole interval of values between [2:i/2] to conclude that i is prime.
The point of this program is to take a three digit number from the command line and then reversing it. After that it is supposed to subtract the reverse from the original number and add the original to the reversed.
This is supposed to only take numbers that are three digits and the first digit of the number has to be greater than the last so that it is not negative when the numbers are subtracted.
The code compiles correctly but when I put a number in the command line prints out the line "Enter a three digit number, with the first digit larger than the third" only.
What it is supposed to print out like
$ java Rev 351
Reverse and subtract:
351
153 -
---
198
Reverse and add:
198
891 +
---
1089
This is my code:
public class Rev
{
public static void main(String[] args)
{
int num = 0;
for (int i = 0; i < args.length; i++)
{
System.out.println("Enter a three digit number, with the first digit larger than the third");
num = Integer.parseInt(args[i]);
reverseNum(num);
subtractNum(num);
addNum(num);
}
}
static boolean checkDigits(int number) // checks if numbers are correct
{
int reverse = reverseNum(number);
if(number > reverse)
{
throw new Error("Reverse number needs to be less than the original number!");
}
else
{
return true;
}
}
static int reverseNum(int number) //reverses number
{
int reverse = 0;
int r = 0;
while (number != 0)
{
if(number < 1000 || number > 99)
{
r = number % 10;
reverse = (reverse*10) + r;
number = number/10;
}
}
return reverse;
}
static void subtractNum(int number) // subtracts
{
int reverse = reverseNum(number);
int total = number - reverse;
System.out.println("Reverse and subtract: ");
System.out.println(number);
System.out.println(reverse + " - ");
System.out.println("---");
System.out.println(total);
System.out.println();
number = total;
}
static void addNum(int number) // adds
{
int reverse = reverseNum(number);
int total = number + reverse;
System.out.println("Reverse and add: ");
System.out.println(number);
System.out.println(reverse + " + ");
System.out.println("---");
System.out.println(total);
number = total;
}
}
public static void main(String[] args)
{
int num = 0;
for (int i = 0; i < args.length; i++)
{
System.out.println("Enter a three digit number, with the first digit larger than the third");
num = Integer.parseInt(args[i]);
reverseNum(num);
subtractNum(num);
addNum(num);
}
}
So the args variable is the command line argument. So if you're compiling via command line, you would call something like java Rev.class 321 where 321 is your 3 digit number. If you want to use the Java console to take inputs, use a Scanner.
To take inputs, use something like this:
Scanner sc = new Scanner(System.in);
num = sc.nextInt();
You're never actually getting a number from the input. You need to do this in your main():
public static void main(String[] args)
{
int num = 0;
for (int i = 0; i < args.length; i++)
{
System.out.println("Enter a three digit number, with the first digit larger than the third");
try (Scanner s = new Scanner(System.in)){
num = s.nextInt();
}
reverseNum(num);
subtractNum(num);
addNum(num);
}
}
The Scanner reads the main input stream (from the keyboard). Otherwise, when you pass in the argument on the command line, it hasn't yet asked you for the input, and prints out the request for input.
Your other problem is that you don't call checkDigits() after getting your number, so you should probably do a while loop using it until you get a number you'll accept, like this:
public static void main(String[] args)
{
int num = -1;
while (num < 0 || !checkDigits(num)){
System.out.println("Enter a three digit number, with the first digit larger than the third");
try (Scanner s = new Scanner(System.in)){
num = s.nextInt();
}
}
reverseNum(num);
subtractNum(num);
addNum(num);
}
Also, your other methods are incorrect in that they are acting on an input parameter (which is possible for objects but not primitives, and is bad practice in any case).
Instead write them as functions which take in values and return them, then modify your main again to look like this:
public static void main(String[] args)
{
int num = Integer.parseInt(args[0]);
if (checkDigits(num)){
num = subtractNum(num);
addNum(num);
} else {
System.out.println("Enter only a three digit number, with the first digit larger than the third");
}
}
Hi I have to make a program(java) on which you input a number and it outputs the number of factors it has(EX: 4 ---> 3)
Code:
package Class;
import java.util.Scanner;
public class Profgrams {
public static void main(String[] args) {
System.out.println("Enter the number;");
Scanner scan = new Scanner(System.in);
long n = scan.nextLong();
for(int i=1, f=0; i <= n; i++){
if(n % i == 0){
f++;
}
System.out.println(f);
}
}
}
Thanks for help.
Declare and initialize f before the loop, and then print the result after the loop terminates. That way printing will not occur at each iteration of the loop.
int f = 0;
// ...
System.out.println(f);
I want to get input from stdin in the for of
3
10 20 30
the first number is the amount of numbers in the second line. Here's what I got, but it is stuck in the while loop... so I believe. I ran in debug mode and the array is not getting assign any values...
import java.util.*;
public class Tester {
public static void main (String[] args)
{
int testNum;
int[] testCases;
Scanner in = new Scanner(System.in);
System.out.println("Enter test number");
testNum = in.nextInt();
testCases = new int[testNum];
int i = 0;
while(in.hasNextInt()) {
testCases[i] = in.nextInt();
i++;
}
for(Integer t : testCases) {
if(t != null)
System.out.println(t.toString());
}
}
}
It has to do with the condition.
in.hasNextInt()
It lets you keep looping and then after three iterations 'i' value equals to 4 and testCases[4] throws ArrayIndexOutOfBoundException.
The Solution to do this could be
for (int i = 0; i < testNum; i++) {
*//do something*
}
Update your while to read only desired numbers as below:
while(i < testNum && in.hasNextInt()) {
The additional condition && i < testNum added in while will stop reading the numbers once your have read the numbers equivalent to your array size, otherwise it will go indefininte and you will get ArrayIndexOutOfBoundException when number array testCases is full i.e. you are done reading with testNum numbers.