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I have a question with the logic of a program that checks if a number is prime. (I have to use 3 methods)

I'm having trouble implementing 3 methods to create a program that checks if a number is prime. When I call my methods and run the program the only value that shows is 0. I'm assuming this has to do with variables or logic in my methods
I have tried using different variables to store user input then using that variable as an argument in my methods.
package practice;
import java.util.Scanner;
public class Practice {
static Scanner s = new Scanner(System.in);
public static void main(String[] args) {
//int result = 0 ; //stores number user picks
int numPicked = 0; //
int endResults = 0; //stores result of calculation
// Calling methods
isPrime(numPicked);
pickedNum(numPicked);
results(endResults);
}
// Method to check if numbers are prime
public static boolean isPrime(int numPicked){
if (numPicked <= 1) {
return false;
}
for (int i = 2; i <= numPicked; i++) {
if (numPicked % i == 0) {
return false;
}
}
return true;
}
// Method that asks user for a positive integer
public static int pickedNum (int userNumbers){
Scanner s = new Scanner(System.in);
System.out.println("Type a positive number that you want to know if it's prime or not.");
int numPicked = s.nextInt();
return numPicked;
}
// Method that displays result of calculation
public static int results (int numPicked){
if(numPicked == 0 && numPicked != 1 ){
System.out.println( numPicked + " is a Prime Number");
}
else{
System.out.println(numPicked + " is Not a Prime Number");
}
return numPicked;
}
}
I need to fix the logic within my methods to be able to call them correctly.

I did some correction in your code and working version is below. There is a lot of things that should be also changed but I tried to make your program working without big changes.
So first of all, You had 2 Scanners so I deleted one of them. Second thing your isPrime method was not working correctly so I replace it with working version. Other thing was that you called isPrime before loading value which was main problem that you always got 0. Other thing is that java is pass by value it means that if you put int as argument and you modify it inside method you still have old value so you have to assign returned value to your value again
static Scanner s = new Scanner(System.in);
public static void main(String[] args) {
//int result = 0 ; //stores number user picks
int numPicked = 0; //
boolean isPrime = false;
// Calling methods
numPicked = pickedNum();
isPrime = isPrime(numPicked);
results(numPicked, isPrime);
}
// Method to check if numbers are prime
public static boolean isPrime(int numPicked) {
if (numPicked == 2)
return true;
if (numPicked < 2 || numPicked % 2 == 0)
return false;
for (int i = 3; i * i <= numPicked; i += 2)
if (numPicked % i == 0)
return false;
return true;
}
// Method that asks user for a positive integer
public static int pickedNum()
{
System.out.println("Type a positive number that you want to know if it's prime or not.");
int numPicked = s.nextInt();
return numPicked;
}
// Method that displays result of calculation
public static int results (int numPicked, boolean isPrime)
{
if(isPrime)
{
System.out.println( numPicked + " is a Prime Number");
}
else
{
System.out.println(numPicked + " is Not a Prime Number");
}
return numPicked;
}

package test;
import java.util.Scanner;
public class Practice {
static Scanner s = new Scanner(System.in);
public static void main(String[] args) {
int numPicked = 0; //
Commented out endResults as it currently has no functionality.
//int endResults = 0; //stores result of calculation
Added a Boolean to record whether isPrime is true or false
boolean isPrime;
numPicked now stores the returned value of the pickedNum method and it takes no parameters.
// Calling methods
numPicked = pickedNum();
isPrime = isPrime(numPicked);
Pass the Boolean value to the results method, true or false.
results(numPicked, isPrime);
}
// Method to check if numbers are prime
public static boolean isPrime(int numPicked)
{
if (numPicked <= 1) {
return false;
}
Removed <= with just less than. This is as if numPicked equals its own value it will return false. In reality a prime number can only be divided by itself so you just need to check the numbers less than it.
for (int i = 2; i < numPicked; i++) {
if (numPicked % i == 0) {
return false;
}
}
return true;
}
// Method that asks user for a positive integer
public static int pickedNum ()
{
Scanner s = new Scanner(System.in);
System.out.println("Type a positive number that you want to know if it's prime or not.");
int numPicked = s.nextInt();
return numPicked;
}
// Method that displays result of calculation
public static int results (int numPicked, boolean isPrime)
{
Checks if isPrime is true.
if(isPrime)
{
System.out.println( numPicked + " is a Prime Number");
}
else
{
System.out.println(numPicked + " is Not a Prime Number");
}
return numPicked;
}
}

Writing a program that determines a prime number

I'm trying to write a program that asks the user to enter a number, then I need to create a method called isPrime to create the calculation and print out the result in main. I'm sure it's something small that I'm missing, but I can't get it to produce an accurate result.
public static void main(String[] args) {
System.out.print("Enter number: ");
int num = s.nextInt();
if (isPrime(num) == true) {
System.out.println("Number is prime");
} else if (isPrime(num) == false) {
System.out.println("Number is not prime");
}
}
public static boolean isPrime(int num){
for(int i = 2; i <= num/2; i++) {
if (num%i != 0) {
return true;
}
}
return false;
}

Use an if and else (don't retest your boolean condition in the first case). And don't test for == true in an if. This
if(isPrime(num) == true)
{
System.out.println("Number is prime");
}
else if(isPrime(num) == false)
{
System.out.println("Number is not prime");
}
Should just be
if (isPrime(num)) {
System.out.println("Number is prime");
} else {
System.out.pritnln("Number is not prime");
}
or even something like
System.out.print("Number is ");
if (!isPrime(num)) {
System.out.print("not ");
}
System.out.println("prime");
If you want to put the braces on their own lines go ahead. As for your isPrime method; you have your return conditions reversed (and the test as well). Also we can optimize it a bit. Unroll the first even test, because then we can skip every other element. Also we only need to test to the square root of the input number. Like,
public static boolean isPrime(int num) {
if (num == 2) {
return true; // two is prime.
}
if (num < 1 || num % 2 == 0) {
return false; // all other even numbers are not prime.
}
for(int i = 3; i <= Math.sqrt(num); i += 2) {
if (num % i == 0) {
return false;
}
}
return true;
}

The isPrime(int num) method may need to check if num % i == 0 instead of if num % i != 0 because many non-prime numbers will pass the condition and return true.
As an example, if isPrime(9) is called, the conditional will check if 9 % 2 != 0, which is true, and the method will say that 9 is prime when it's not.
As a result, you could try changing the method to something like this:
public static boolean isPrime(int num)
{
for(int i = 2; i <= num/2; i++)
{
if (num%i==0)
{
return false;
}
}
return true;
}

You should change your "if (num%i!=0)" condition to if(num % i == 0)
See the following code:
public class Prime {
public static void main(String[] args) {
int num = 29;
boolean flag = false;
for(int i = 2; i <= num/2; ++i)
{
// condition for nonprime number
if(num % i == 0)
{
flag = true;
break;
}
}
if (!flag)
System.out.println(num + " is a prime number.");
else
System.out.println(num + " is not a prime number.");
}
}

Prime Factoring in Java - Intro Programming

Original: So for my intro to programming class, we have to find the prime factors of a range of numbers that the user inputs (i.e. 59-65). The issue with a lot of the solutions here is that they use things that we haven't discussed in class like arrays, continue, etc. It's a pretty basic class. As for the requirements, we have to use a primeFact method/function that we call in the first for loop. She instructed us to use a while and for loop in the method to get the prime factors but everytime I think I have something, it doesn't come out right. Any help is really appreciated and my code is below. I really only need help with the method portion with the algorithm for finding the factors.
Edit: Here is the final solution that I turned in. It works and will give all prime factors of all numbers in a given range of numbers.
import java.util.Scanner;
public class PrimeFact {
public static void main(String[] args) {
int start, stop;
//Get input
Scanner input = new Scanner(System.in);
System.out.println("Please enter then two values with the lower value first");
start = input.nextInt();
stop = input.nextInt();
input.close();
//Displays for the start of the loop
System.out.println("Starting value (at least two digits): "+start);
System.out.println("Ending value (at least two digits): "+stop);
System.out.println("Prime factors for numbers between "+start+" and "+stop);
//Loop for the prime factors
for (int num = start; num <= stop; num++) {
primeFact(num);
}
}
// Method for Prime Factoring
public static void primeFact(int num) {
int divisor = 2;
System.out.print(num+" = ");
while (num>1) {
if ((num%divisor) == 0) {
System.out.print(divisor+" x ");
num=num/divisor;
} else {
divisor++;
}
}
System.out.print("1");
System.out.println();
}
}

public static void primeFact(int num) {
System.out.print(num+" = ");
for(int i=2;i<num;i++)
{
if(num%i==0)
{
if(isPrime(i))
{
System.out.println("Prime Factor for "+num+" is:"+i);
}
}
}
if(num==2)
System.out.println("Prime Factor for "+num+" is:"+num);
}
static boolean isPrime(int num){
for(int i=2;i<num;i++)
{
if(num%i==0)
{
return false;
}
}
return true;
}
//if u want use while replace the for loop like
public static void primeFact(int num) {
System.out.print(num+" = ");
int i=2;
while(i<num)
{
if(num%i==0)
{
if(isPrime(i))
{
System.out.println("Prime Factor for "+num+" is:"+i);
}
}
i=i+1;
}
if(num==2)
System.out.println("Prime Factor for "+num+" is:"+num);
}
static boolean isPrime(int num){
int i=2;
while(i<num)
{
if(num%i==0)
{
return false;
}
i=i+1;
}
return true;
}

you're method is pretty close to a solution. but your while loop is invalid.
remember it should be like this:
while(<boolean statement>){
//code here
}
I will reference the variable we are checking "primality" for as "num"
here's how to check for factors with a while loop:
//remember, num is given to us in the parameters of primeFact(int num).
boolean isPrime = true; //we are optimistic.
//we start at 2 because everything is divisible by one
int posFact = 2; //short for "possible factor"
while(posFact < num){ //this will start at 2, and go to 1 less than num
if(num % posFact == 0){ // "%" gives us remainder
isPrime = false; // we now know it's not prime
System.out.println("The number " + posFact + " is a factor of " + num + ".");
}
posFact++; //increments posFact up by 1
}
if(isPrime){ //This will only be true if the number is prime
System.out.println("The number " + num + " is prime!");
}
We can do the exact same thing with a for loop:
//remember, num is given to us in the parameters.
boolean isPrime = true; //we are optimistic.
for(int posFact = 2; posFact < num; posFact++){ //posFact's initialization and incrementation was moved here.
if(num % posFact == 0){ // "%" gives us remainder
isPrime = false; // we now know it's not prime
System.out.println("The number " + posFact + " is a factor of " + num + ".");
}
}
if(isPrime){ //This will only be true if the number is prime
System.out.println("The number " + num + " is prime!");
}

Write a method called isOdd() in a class called OddTest, which takes an int as an argument and returns true if the int is odd

I'm in my first year of Java and am having trouble with with a boolean method to test if even or odd. I have most of the code, but when I enter an odd number it returns it as even. My professor gave us the signature of the method: public static boolean isOdd(int number)
package homeWork1;
import java.util.Scanner;
public class OddTest {
public static void main(String args[])
{
int number;
System.out.println("Enter an integer to check if it is odd or even: ");
Scanner input = new Scanner(System.in);
number = input.nextInt();
isOdd(number);
boolean answer=isOdd(number);
if (answer=true)
{
System.out.println("EVEN");
}
if (answer=false)
{
System.out.println("ODD");
}
}
public static boolean isOdd(int number)
{
if(number % 2 == 0)
{
return true;
}
return false;
}

if (number % 2 == 0) means that the number is divisible by 2, and therefore it is even.
You want to change your condition to:
if (number % 2 != 0)
And, as a conditional statement is a boolean value, you can simplify it further like:
public static boolean isOdd(int number)
return number % 2 != 0;
}
Also, you have an error in the code that uses your method.
boolean answer=isOdd(number);
if (answer=true)
{
System.out.println("ODD");
}
if (answer=false)
{
System.out.println("EVEN");
}
Here, answer=true is an assignment, not comparison. You can fix it by writing:
if (isOdd(number))
{
System.out.println("ODD");
}
else
{
System.out.println("EVEN");
}

Instead of checking whether answer is true or false, you're assigning these values. Checking for equality is == in Java. By the way, I'd recommend using just if (answer) and if (!answer) for booleans - looks simpler and less room for a mistake :)

Getting the prime numbers [closed]

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Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 5 years ago.
Improve this question
Please have a look at the following code
public class Prime
{
public static void main(String[]args)
{
int i = 2;
int counter = 0;
while(true)
{
if(counter==6)//Count still 6
{
break;
}
else
{
if(getPrimes(i)==true)
{
i++;
counter++;
System.out.println("Counter: "+counter);
}
else
{
System.out.println("No");
}
}
}
}
static boolean getPrimes(int num)
{
boolean result = false;
int i = 2;
while(true)
{
if((num%i) != 0) //if the number cannot be divided by any other number (except 1 and it self) it is prime
{
result = true;
System.out.println(num);
System.out.println("I is: "+i);
i=2;
break;
}
else //Not a prime. Repeat the process
{
result = false;
i++;
}
}
return result;
}
}
In here, I am trying to get all the prime numbers between 0-6. This is the test case to get thousands of prime numbers from a really big number. However, it is not showing the only primes, it is showing each and every number!
What am I doing here wrong? Please help!

try this answer...in ur loop
static boolean getPrimes(int num)
{
boolean result=true; // incase u gave 1 or 2 as input.....
int i = 2;
int mid=num/2;
while(i<mid)
{
if(num%i==0)
{
result=false; // not a prime and breaks...
break;
}
else
{
result=true; // the loop make result as true.....
}
i++;
}
return result;
}

I think you need something like this:
public void getPrimes(int a){
for(int i = 2; i < a; i++){
int inCounter = 0;
if(counter%i==0){
System.out.println("false");
inCounter++;
}
if(inCounter == 0){
System.out.println("Prime: "+counter);
}
}
}

Use a sieve, either Eratosthenes or Atkins
Here's the Eratosthenes implementation by Robert Sedgewick in Java:
http://introcs.cs.princeton.edu/java/14array/PrimeSieve.java.html

I guess I found the solution. At least, I found the answer I need. Here is my answer
import java.math.BigInteger;
public class Problem7
{
public static void main(String[]args)
{
int i = 0;
int counter = 0;
while(true)
{
if(i>6)
{
break;
}
if(i>1)
{
String str = String.valueOf(i);
if (new BigInteger(str).isProbablePrime(i/2))
{
System.out.println(str);
counter++;
}
}
i++;
}
}
}
I guess this is the easiest way...

import java.util.Scanner;
class PrimeNumbers2
{
public static void main (String[] args)
{
Scanner scanner = new Scanner(System.in);
int i =0;
int num =0;
//Empty String
String primeNumbers = "";
System.out.println("Enter the value of n:");
int n = scanner.nextInt();
for (i = 1; i <= n; i++)
{
int counter=0;
for(num =i; num>=1; num--)
{
if(i%num==0)
{
counter = counter + 1;
}
}
if (counter ==2)
{
//Appended the Prime number to the String
primeNumbers = primeNumbers + i + " ";
}
}
System.out.println("Prime numbers from 1 to n are :");
System.out.println(primeNumbers);
}
}
it will display all the prime number.

Find Prime number with minimum iteration
boolean IsPrimeNumber(int num) {
boolean isPrime = true;
if (num == 1 || num ==0)
isPrime = false;
else{
int limit = (int) Math.sqrt(num);
for (long i = 2L; i <=limit ; i++)
if (num % i == 0) {
isPrime = false;
break;
}
}
return isPrime;
}