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Suggestions to improve code about primes?

I wrote a code about primes and would hear your opinion or any suggestions how i can improve my code. I'm a beginner in Java.
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
boolean a;
System.out.println("Please enter a number: ");
int zahl = s.nextInt();
if(zahl <= 0) {
System.out.println("Please enter a positive number without zero.");
return;
}
a = true;
for (int i = 2; i < zahl; i++) {
if (zahl % i == 0) {
a = false;
}
}
if (a == true) {
System.out.println("Is Prim");
}
if (a==false){
System.out.println("Not a prim");
}
}

The easiest thing to do is the following
Instead of
for (int i = 2; i < zahl; i++) {
if (zahl % i == 0) {
a = false;
}
}
change the for loop the
for (int i = 2; i < Math.sqrt(zahl); i++)
If no numbers up to the square root divide zahl, then no numbers beyond the square root will divide it either (they would have been the result of earlier divisions).
Also, for outputing the answer you could do:
System.out.println(zahl + " is " + ((a) ? "prime"
: "not prime"));
That's using the ternary operator ?:

some hints :
You do
System.out.println("Please enter a positive number without zero.");
return;
the println suggests the user can enter a new value, but no, in that case better to say the number was invalid so you exit
When you do a = false; it is useless to continue, no chance for a to be back true
It is useless to try to divide by more than sqrt the number
It is necessary to try to divide by 2 but not by an other even number, so add 2 to i rather than 1
If if (a == true) false it is useless to check if (a==false)

Your code is good. I have made three small improvements:
The input asks at once (and not only after a bad input) for a
positive int.
The input is repeated until correct.
The for loop runs only up to sqrt(zahl) which is sufficient.
public static void main(String[] args) {
Scanner s = new Scanner(System.in);
boolean a;
int zahl = 0;
while (zahl <= 0) {
System.out.println("Please enter a positive int without zero.");
zahl = s.nextInt();
}
a = true;
for (int i = 2; i <= Math.sqrt(zahl); i++) {
if (zahl % i == 0) {
a = false;
break;
}
}
if (a == true) {
System.out.println("Is Prim");
} else {
System.out.println("Not a prim");
}
}

Java code will not print out answer or read int properly [duplicate]

This question already has answers here:
What is the difference between == and equals() in Java?
(26 answers)
How do I compare strings in Java?
(23 answers)
Closed 5 years ago.
This is the whole project, it should work as far as I can see but i'm hoping that posting it here could let me see something I missed, I have a moderate amount of experience with java
package com.company;
import java.util.Random;
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
// write your code here
Scanner input = new Scanner(System.in);
int number1;
int number2;
int answer;
String operator;
This figures out what numbers and operators the user wants
System.out.println("Please Enter your first number");
number1 = input.nextInt();
System.out.println("Please enter your second number");
number2 = input.nextInt();
System.out.println("Please enter your operator: + , - , * , / ");
operator = input.next();
The if Statement should determine which operator the user wants and applys it to the two numbers and then calls the randomEquation Method to make it the wrong answer
if (operator == "+") {
answer = number1 + number2;
System.out.println("Your answer is: " + randomEquation(answer));
} else if (operator == "-") {
answer = number1 - number2;
System.out.println("Your answer is: " + randomEquation(answer));
} else if (operator == "*") {
answer = number1 * number2;
System.out.println("Your answer is: " + randomEquation(answer));
} else if (operator == "/") {
answer = number1 / number2;
System.out.println("Your answer is: " + randomEquation(answer));
}
}
This method randomly applies one of these to the answer to create the wrong answer
public static int randomEquation(int number) {
Random rand = new Random();
int random = rand.nextInt(100) + 1;
int answer = number;
if (random <= 100 && random >= 81) {
answer = number * 25;
return answer;
}
else if(random <= 80 && random >= 61){
answer += 13;
return answer;
}
else if(random <= 60 && random >= 41){
answer /= 2;
return answer;
}
else if(random <= 40 && random >= 21){
answer -= 16;
return answer;
}
else{
answer %= 4;
return answer;
}
}
}

You need to use .equals with Strings.
Instead of:
operator == "+"
Try:
"+".equals(operator)

Global Int variable method for multiple methods

I have to set up a math program that supplies random questions to the user based on their year level, and if they're doing well raise the difficulty. I've set up this code here that runs through questions and if they meet the requirements it displays the harder questions, however if "i" that i use to determinate how member questions they've done if separate the program will run the harder questions then go back and finish the easier questions
So basically I've tried to write a method for global "i" which all other methods will use, however when i replace "i" with the method it stops counting and continues to display questions infinitely and i don't know how to fix this.
import java.util.Scanner;
import java.util.*;
import java.util.Date;
public class Quiz {
public static void main(String[] args) {
int answer;
int correct;
double current_score = 100.00;
// int i = 0;
while (questionsDone() < 10) { // start of question loop
int random = (int) (Math.random() * 9 + 0);
int random2 = (int) (Math.random() * 9 + 0);
System.out.print("What is the sum of" + " ");
System.out.print(random);
System.out.print(" + " + random2 + " ");
System.out.print("=" + " ");
Scanner scan = new Scanner(System.in);
//answer
answer = scan.nextInt();
correct = random + random2;
if (answer == correct) { // start of result display
System.out.println("You are correct");
} else if (answer != correct) {
System.out.println("That wasn't right");
current_score = (current_score - 10.00);
}
System.out.println("Your current percentage is " + current_score); // end of result display
// i++; // raise number of questions given by 1
if (questionsDone() == 5 && current_score >= 75) { // code to move up or down year level
System.out.println("You are doing well! Let's increase the difficulty a little");
Year1_10Questions();
}
}
}
public static void Year1_10Questions() {
int i = 0;
int answer;
int correct;
double current_score = 100.00;
while (i < 10) { // start of question loop
int random = (int) (Math.random() * 9 + 0);
int random2 = (int) (Math.random() * 9 + 0);
int random3 = (int) (Math.random() * 2 + 1);
String operator = "";
switch (random3) {
case 1:
operator = "+";
break;
case 2:
operator = "-";
break;
}
System.out.print("What is the sum of ");
System.out.print(" " + random + " ");
System.out.print(operator + " ");
System.out.print(random2 + " ");
Scanner scan = new Scanner(System.in);
//answer
answer = scan.nextInt();
if (random3 == 1) {
correct = random + random2;
if (answer == correct) { // start of result display
System.out.println("You are correct");
} else if (answer != correct) {
System.out.println("That wasn't right");
current_score = (current_score - 10);
}
} else if (random3 == 2) {
correct = random - random2;
if (answer == correct) { // start of result display
System.out.println("You are correct");
} else if (answer != correct) {
System.out.println("That wasn't right");
current_score = (current_score - 10);
}
}
System.out.println("Your current percentage is " + current_score); // end of result display
i++; // raise number of questions given by 1
}
} // end of year 1_10 questions
public static int questionsDone() {
int i = 0;
i++;
return i;
}
}

Since all the methods are in the same class, you can define i on class level:
public class Quiz {
static int i;
...
}

Using a do-while to restart a game in java

Please help with the swtich case need for a game
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.print("Please Enter a number");
int day = input.nextInt();
switch(day)
{
case 1: System.out.println("1 Microphone");
break;
case 2: System.out.println("2 Loud Speakers 1 Microphone ");
break;
case 3: System.out.println("3 Keyboards 2 Loudspeakers 1 Microphone ");
break;
case 4: System.out.println("4 Java Books 3 Keyboards 2 Loudspeakers 1 Microphone");
break;
case 5: System.out.println("5 Iphones 4 Java Books 3 Keyboards 2 Loudspeakers 1 Microphone");
break;
default: System.out.println("Enter A Valid Prize Day");
}
}

As #AlexandreSantos pointed out, you need to reinitialise the values of maxRolls and sum every time you restart the game. That is, these initialisations should be the first things executed in your do {} while () loop.
do {
int maxRolls = 7;
int sum = 0;
// ...
} while (option);
I'd also give you other recommendations:
in Java, the class names, by convention, start with an upper-case letter. Thus, I'd name your class Game instead of game.
The following code (and its equivalent with "no"):
(userInputTwo.equals("Yes") || userInputTwo.equals("yes") || userInputTwo.equals("YES"))
... can be replaced by:
userInputTwo.equalsIgnoreCase("yes")
... since, as you mentioned in your question, you're actually simply trying to ignore the case ;)
You're doing all that asking the user whether is wants to restart or not in two places. You could (should) actually simply do it once, after having printed either "You won" or "You lost".
I'd suggest to replace:
if (sum >= 43) {
System.out.println("You Win");
System.out.print("Would You Like To Play Again . Yes or No?");
final String userInput = input.nextLine();
if (userInput.equals("Yes") || userInput.equals("yes") || userInput.equals("YES")) {
// MISSING CODE TO RESTART THE PROGRAM
option = true;
} else if (userInput.equals("No") || userInput.equals("no") || userInput.equals("NO")) {
System.exit(0);
}
}
if (sum < 43 || sum % 10 == 0) {
System.out.println("You Lose");
System.out.print("Would You Like To Play Again . Yes or No?");
final String userInputTwo = input.nextLine();
if (userInputTwo.equals("Yes") || userInputTwo.equals("yes") || userInputTwo.equals("YES")) {
option = true;
// MISSING CODE TO RESTART THE PROGRAM
} else if (userInputTwo.equals("No") || userInputTwo.equals("no") || userInputTwo.equals("NO")) {
System.exit(0);
}
}
... by:
if (sum >= 43) {
System.out.println("You Win");
}
if (sum < 43 || sum % 10 == 0) {
System.out.println("You Lose");
}
System.out.print("Would You Like To Play Again . Yes or No?");
final String userInput = input.nextLine();
if ("yes".equalsIgnoreCase(userInput) {
// MISSING CODE TO RESTART THE PROGRAM
option = true;
} else if ("no".equalsIgnoreCase(userInput)) {
System.exit(0);
}
... or, even better, extracting this into an other method.
Or, even better, not even checking for one of the possibilities and make it the default one, in case the user enters something that's neither "yes" nor "no":
private static boolean restart(final Scanner input) {
// I choose to interpret any input that's different from "yes" as a "no".
System.out.print("Would You Like To Play Again. Yes or No? (default: No)");
final String userInput = input.nextLine();
if ("yes".equalsIgnoreCase(userInput)) {
return true;
}
return false;
}
... which can obviously then become:
private static boolean restart(final Scanner input) {
// I choose to interpret any input that's different from "yes" as a "no".
System.out.print("Would you like to play again? [Yes/No] (default: No)");
return "yes".equalsIgnoreCase(input.nextLine());
}
... and the option variable could disappear:
do {
...
} while (Game.restart(input));
You could (should) use Random instead of Math.random(), it's just way more convenient.
For example:
final int dieOne = (int) (Math.random() * faces) + 1;
final int dieTwo = (int) (Math.random() * faces) + 1;
final int totalRollForRound = dieOne + dieTwo;
... could become:
// Outside of the do {} while ():
final Random r = new Random();
// Inside the do {} while ():
final int totalRollForRound = r.nextInt(faces) + r.nextInt(faces) + 2;
You should always close the Scanner before leaving the program.
Use the try-with-resources syntax:
private static boolean restart() {
try (final Scanner input = new Scanner(System.in) {
// I choose to interpret any input that's different from "yes" as a "no".
System.out.print("Would you like to play again? [Yes/No] (default: No)");
return "yes".equalsIgnoreCase(input.nextLine());
}
}
One last thing: your sum % 10 == 0 is weird: you've already told the user that he won if he scored at least 43, and he's gonna lose if he scored less than 43... You should either:
Test that condition before checking whether the user has scored more than 43 (and therefore also rejecting scores like 50, 60, 70, 80...)
... or:
Forget about that rule that only aims to reject 10, 20, 30 and 40, which are already covered by the score < 43 rule.
Cheers ;)
Just 'cause I felt bored, I actually applied my own advices (and a few more) to your code:
import java.util.Random;
import java.util.Scanner;
public class Game {
private static final int FACES = 6;
private static final int MAX_ROLLS = 7;
private static final Random R = new Random();
public static void main(final String[] args) {
try (final Scanner input = new Scanner(System.in)) {
do {
if (Game.roll() >= 43) {
System.out.println("You won!");
} else {
System.out.println("You lost.");
}
} while (Game.restart(input));
}
}
private static int roll() {
int maxRolls = MAX_ROLLS;
int sum = 0;
for (int i = 1; i < maxRolls; i++) {
final int dieOne = R.nextInt(FACES) + 1;
final int dieTwo = R.nextInt(FACES) + 1;
sum += dieOne + dieTwo;
System.out.println("Roll #" + i + ": You rolled " + dieOne + " and " + dieTwo + ".\tYour new total is: " + sum);
if (dieOne == dieTwo) {
System.out.println("DOUBLES! You get an extra roll.");
maxRolls++;
}
}
return sum;
}
private static boolean restart(final Scanner input) {
System.out.print("Play again? [Yes/No] (default: No): ");
return "yes".equalsIgnoreCase(input.nextLine());
}
}

Sounds like you want an outer loop; each time through the loop the user plays one game. At the top of that loop, you initialize the values that you need to play one game:
boolean playingMoreGames = false;
do
{
int sum = 0;
int maxRolls = 6;
int rollsMade = 0;
boolean gameOver = false;
do
{
// roll dice
// determine win or loss
// and determine whether game is over
// include testing rollsMade against maxRolls
}
while (!gameOver)
// ask user whether he wants to play again and set playingMoreGames accordingly
}
while (playingMoreGames);
I have suggested a change to a while loop that executes as long as the maxRolls has not been reached. It is not a good idea to modify the target of a for loop within the loop; in some languages, at least, the behavior is undefined, and it confuses the reader. Since maxRolls can change, you need a different looping form there.
And you don't really need to call System.exit(); if you "fall out of" the bottom of your main routine, your program will just exit since it has no more instructions to execute.
I don't recommend do while(true) in this case; the (small) problem with it is that it makes it harder for the reader to determine when the loop exits. Not a big deal.
Good luck.

Determine prime number by user input using recursive method

I need to create a program in Java that determines if a number is prime.
The user should enter any number, and the program will determine if it's prime or not, and display "not prime" or "prime." My code now compiles and runs but it always says a number isn't prime even if it is.
import java.util.Scanner;
public class PrimeNumber
{
public static void main(String[] args)
{
Scanner input = new Scanner(System.in);
int constant = 0;
int variable = 0;
System.out.println("Enter a Number to test if Prime or Not");
constant = input.nextInt();
variable = constant;
double answer = 0.0;
answer = testPrime(constant, variable);
System.out.println(+answer);
if (answer == 1)
{
System.out.println(+constant + " is a prime number.");
}
else
{
System.out.println(+constant + " is NOT a prime number.");
}
}
public static double testPrime(int number, int divide)
{
double prime = 0.0;
prime = number%divide;
if (prime > 0 && divide != number)
{
return testPrime(number, divide - 1);
}
else
{
return prime;
}
}
}

if (prime > 0 && divide != number)
This will never be true. Because your divide and number are always equal.
See that you have assigned variable=constant and that's what you pass to the method
constant = input.nextInt();
variable = constant;
answer = testPrime(constant, variable);
That said, you need go so complex to find out if a number is prime or not. Check the web for simple algorithms. See http://www.mkyong.com/java/how-to-determine-a-prime-number-in-java/ for example.

Not the answer as the OP wants recursion (homework I guess).
You need to only go till the square root of n to see if it has a divisor (divisor besides self will be < sqrt(n))
boolean isPrime(int n) {
if(n % 2 == 0)return false;
int till = (int)java.lang.Math.pow(n, 0.5); //(int)n / 2;
for(int i= 3;i<till;i+=2) {
if(n % i == 0)
return false;
}
return true;
}

I see you want recursion for this, so I converted tgkprog's answer to a recursive method (although his is definitely more efficient). Additionally, I think you may want to return a prime factor if the input isn't prime? I'm just speculating this judging from the OP's return value of a double instead of a boolean. Mine will return an int though, because returning a double is silly.
int isPrime(int n){ //starter function
if(n<=1) return n; //sanity check for weird inputs
if(n % 2 == 0) return 2; //2 is a prime factor
int start = (int)java.lang.Math.pow(n, 0.5);
return isPrime(n,start-(start%2)); //makes start odd if it wasn't already
}
int isPrime(int n, int testval){ //recursive function
if(testval<=1) return 1; //n is prime, return n since it has no prime factors
if(n % i == 0)
return i; //found a prime factor!
return isPrime(n,i-2);
}

with recursion
import java.util.Scanner;
public class PrimeRecursion
{
static int dbg;
public static void main(String[] args)
{
Scanner input = new Scanner(System.in);
System.out.println("Enter non 0 if you want debug :");
dbg = input.nextInt();
long constant = 3;
long variable = 0;
while(true){
System.out.println("Enter a Number to test if Prime or Not (1 to exit)");
constant = input.nextLong();
if(constant < 3){
if(constant == 2){
System.out.println("2 is a special prime");
}
return;
}
if(constant % 2 == 0){
System.out.println(constant + " is NOT a prime number, even number.");
}else{
variable = (long)Math.pow(constant, 0.5);
variable = (variable % 2 == 1) ? variable : variable + 1;//odd number
double answer = 0.0;
answer = testPrime(constant, variable);
System.out.println("End answer : " + answer);
if (answer == 1){
System.out.println(+constant + " is a prime number. answer : " + answer);
}
else{
System.out.println(constant + " is NOT a prime number.answer : " + answer);
}
}
System.out.println();
}
}
static double testPrime(long number, long divide)
{
double prime = 0.0;
prime = (double)number / divide;
if(dbg > 0){
System.out.println("Debug number " + number + " | divide " + divide + " |prime : " + prime + " | Math.abs(prime) " + Math.abs(prime));
}
if (prime == ((long)prime))//whats the best way to do this?
{
//divided evenly
return divide;
}
else{
return testPrime(number, divide - 2);
}
}
}

the recursive function for me goes like-Correct me if i am wrong.Thank you.calling statement >Boolean b=isPrime(number,number-1);
recursive function-
void isPrime(int p,int d);
{
int prime=p%d;
if((p==0&&d>1)||p%2==0)
return true;//that the number is not prime
if(d==1)
return false;
else
return isPrime(p,d-2);//calls the function again
}

Well I am directly giving you all the code instead of writing snippet. Hope you all may like this one as I have tried my best to make it as simple as possible.
The code is :>
import java.util.*;
class Prime_Number
{
static int c=0;
public static void main(String args[])
{
int n,i,sq=0;
Scanner in=new Scanner(System.in);
Prime_Number ob=new Prime_Number();
System.out.print("Enter a no.:>");
n=in.nextInt();
i=n;
sq=(int)(Math.sqrt(n));//square root is been taken since a no. cannot have factors greater than its square root
int k=ob.prime_Chk(n,i,sq);
if(k==1)
{
System.out.println("Prime");
}
else
{
System.out.println("Non-Prime");
}
}
public int prime_Chk(int g,int i,int sq)
{
if(i==sq)
{
return c;
}
else
{
if(g%i==0)
{
c++;
}
i--;
return(prime_Chk(g,i,sq));
}
}
}
Well in the prime() I have taken int i , int sq and int g as arguments.Instead of those if you wish then you can take other variables also.

import java.util.Scanner;
public class HW8R_T03 {
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner sc = new Scanner(System.in);
System.out.print("Enter number: ");
int n = sc.nextInt();
int simple = prime(n,n-1);
System.out.println(simple==1 ? "prime" : "not prime");
}
static int prime(int x, int y){
int div = 1;
if (y==0 || y==1){
return div;
}
if (x%y==0){
div = y;
} else {
div = prime (x, --y);
}
return div;
}
}