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how can i check if the number is perfect square by not using math.sqrt

i want to make a program which related to this question:
An integer that can be expressed as the square of another integer is called a perfect square, such as 4,9,16,25, etc. Write a progran that checks if a number is a perfect square.
I did built something goes like:
import java.util.Scanner;
class Q3{
public static void main(String[] args){
Scanner sc = new Scanner(System.in);
int num = 0;
int a = 0;
System.out.println("Type a number to check if it has square");
num = sc.nextInt();
for(a = 1;a<num;a++){ }
if (a*a == num){
System.out.println("Ok");
break;
}
else if (a*a != num){
System.out.println("Not ok");
}
}
}
So it doesn’t give what i want when i run it. What should i change or add ?

I think your for loop interpretation might be wrong, I made up something that might just work. Give this code a try.. You can make the method return a boolean too if you want.
static void perfectSquare(int number) {
for (int i = 1; i < i * number; ++i) {
// 'i' is the divisor, making sure
// it is equal to the quotient
if ((number % i == 0) && (number / i == i)) {
System.out.println(i);
}
}

If you want to brute force every number then you are on the right track but you should only print "Not ok" if all numbers in the loop have failed otherwise you may have a perfect square but "Ok" will be hidden within many "Not ok" lines. Also there is nothing in your for loop so the if statement always checks if 0*0 == num.
This looks like it may be a homework question so I won't give a full answer for you but think about how you can make this more efficient.
If you have found an integer number that matches do you need to keep going?
Do you need to check every number? (a starting point may be following the principles of a binary search)

I ended up like this:
import java.util.Scanner;
class Q3{
public static void main(String[] args){
Scanner sc = new Scanner(System.in);
int num = 0;
int a = 0;
int b = 0;
System.out.println("Type a number to check if it has square");
num = sc.nextInt();
for(a = 1;a<num;a++){
if (a*a == num){
b = 1;
break;
}
}
if(b==1){
System.out.println("Perfect Square");
}
else {
System.out.println("Not ok");
}
}
}
Thanks for support !

How to pass method output into an array element?

The basis of my problem is here: https://github.com/experiencethebridge1/primeGap
Bottom line, I want to create an array in which the output of a method will populate the elements of the new array.
This is not homework.
package primenumbermethod;
import java.util.Scanner;
public class PrimeNumberMethod {
public static void main(String[] args) {
System.out.print("How many prime numbers do you want to work with? ");
Scanner input = new Scanner(System.in);
int arraySize = input.nextInt();
// Invoke printPrimeNumbers method
System.out.println("If I can ever get it to work, the number of the "
+ "elements in the array I want to build will be " + arraySize +".");
System.out.println();
printPrimeNumbers(arraySize);
// How can I read parts of a method into elements of an array?
int[] myList = new int[arraySize];
}
public static int printPrimeNumbers(int numberOfPrimes) {
final int NUMBER_OF_PRIMES_PER_LINE = 10; // Display 10 per line
Scanner input = new Scanner(System.in);
System.out.print("What number do you want to start from? ");
int number = input.nextInt();
int count = 0; // Count the number of prime numbers
// Repeatedly find prime numbers
while (count < numberOfPrimes) {
// Print the prime number and increase the count
if (isPrime(number)) {
count++; // Increase the count
if (count % NUMBER_OF_PRIMES_PER_LINE == 0) {
// Print the number and advance to the new line
System.out.printf("%-15d\n", number);
} else {
System.out.printf("%-15d", number);
}
}
number++;
}
return 0;
}
// Method for checking if number is prime
public static boolean isPrime(int number) {
for (int divisor = 2; divisor <= number / 2; divisor++) {
if (number % divisor == 0) {// If true, number is not prime
return false; // Number is not a prime
}
}
return true; // Number is prime
}
}
Tried using global variables, abstraction does not apply (but could).
The main method initiates the program, then traces to method printPrimeNumbers, then into method boolean isPrime. I want to return the output of that method into a new array...
The array size will be defined by the user input <"How many prime numbers do you want to work with? ">, and then <"What number do you want to start with?>
Problem, I can't seem to pass the output of a method into the elements of an array.
Thoughts?

I would suggest you should restructure your code in the following way:
public static void main(String[] args) {
int numberOfPrimes = readIntFromCommandLine...;
int numberToStartWith = readIntFromCommandLine...;
int[] primeNumbers = getPrimeNumbers(numberOfPrimes, numberToStartWith);
// maybe extract this to another method as well
for (int prime : primeNumbers) {
// do whatever you want with prime, e.g. print it - or sum it, or multiply or whatever
}
}
public static int[] getPrimeNumbers(int amount, int from) {
int[] primes = new int[amount];
int count = 0;
/* now put your current prime logic here and whenever you
find a prime set primes[count] = newlyFoundPrime; */
}
public static boolean isPrime(int number) { /* stays the same */ }
It is generally a good idea to only ask for user input at a well defined point in your code, not all over the place. Therefore I placed the two inputs at the front. Another generally good idea is to make every method (maybe except for the main method) only do one thing. Your isPrime is a good example of that. Moving the printing logic out of getPrimeNumbers simplifies that method and lets you handle the printing at another, dedicated place.

Using arrays to store primes

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("");
}
}

How to recurse back to start?

I'm a noob programmer, but I've been stuck on this one bit of code. How do you recurse back to start? I've tried several different methods but they all either take a ridiculous amount of code or don't work properly. I've been trying to implement this "simple" piece of code in all of my programming assignments, but it hasn't been working out. Thanks!
p.s. I've already finished the assignment. I'm just trying to make it more "complete".
public class OddProduct {
public static void main(String[] args) {
//Inputs from user
System.out.println("Enter an odd number");
Scanner input_odd = new Scanner(System.in);
int odd = input_odd.nextInt();
int oddproduct = 1;
//Multiplies all odd integers
for (int counter = 1; counter <= odd; counter = counter + 2){
oddproduct = oddproduct * counter;
}//end of for- loop
System.out.printf("\nThe product of all the odd integers up to %d is %d\n",
odd, oddproduct);
/* MY NOTES FOR RECURSE
if (odd%2 == 1){ proceed normally}
else if (odd%2 != 1) { HOW TO LOOP BACK???}
else { println = "Application closed"}
*/
}//end of main method
}//end of OddProduct class

Based upon your Notes I think this is what you require
Scanner input_odd = new Scanner(System.in);
int odd = 0;
while (odd % 2 != 1) { // fails first time && if user enters even number
System.out.println("Enter an odd number");
odd = input_odd.nextInt();
}

Java - No Duplicate number array

I am in the process of creating of a Lottery Program using Java via BlueJ and I am having trouble with the user inputted numbers and the number being generated by the program (up to and including 1-49), I need the numbers that are entered by the user to not be duplicate i.e. the user cannot enter 1 and 1.
I am not really sure how to get the numbers to not be duplicate i was thinking of using an Array but im not sure what type or where to begin im rather new to the whole programming thing.
import java.util.Arrays;
import java.util.Random;
import java.util.Scanner;
public class JavaApplication8 {
public static void main(String[] args) {
Scanner user_input = new Scanner (System.in);
Scanner keyIn = new Scanner(System.in);
int[] LotteryNumbers = new int[6];
int input;
int count = 0;
System.out.print("Welcome to my lottery program which takes\nyour lottery numbers and compares\nthem to this weeks lottery numbers!");
System.out.print("\n\nPress the enter key to continue");
keyIn.nextLine();
for (int i = 0; i < LotteryNumbers.length; i++)
{
count ++;
System.out.println("Enter your five Lottery Numbers now " + count + " (must be between 1 and 49): ");
input = Integer.parseInt(user_input.next());
if (input < 1 || input > 49)
{
while (input < 1 || input > 49)
{
System.out.println("Invalid number entered! \nPlease enter lottery number (between 1 and 49) " + count);
input = Integer.parseInt(user_input.next());
if (input >= 1 || input <= 49)
{
LotteryNumbers[i] = input;
}
}
}
else
{
LotteryNumbers[i] = input;
}
}
System.out.println("Thank you for your numbers.\nThe system will now check if you have any matching numbers");
System.out.print("Press the enter key to continue");
keyIn.nextLine();
Random randNumGenerator = new Random();
StringBuilder output = new StringBuilder();
int[] ActLotteryNumbers = new int[6];
for (int j = 0; j < ActLotteryNumbers.length; j++)
{
int roll = randNumGenerator.nextInt(49);
ActLotteryNumbers[j] = roll;
}
System.out.println(Arrays.toString(ActLotteryNumbers));
int counter = 0;
for (int i = 0; i < LotteryNumbers.length; i++)
{
for (int j = 0; j < ActLotteryNumbers.length; j++)
{
if (LotteryNumbers[i] == ActLotteryNumbers [j])
{
counter ++;
System.out.println("The numbers that match up are: \n" + LotteryNumbers[i]);
}
}
}
if (counter == 0)
{
System.out.println("You had no matching numbers this week ... Try Again next week!");
}
}
}

As "fge" mentioned, use Set to add all the values that you are getting from the user.
Get the user inputs and add it to Set.
Use a Iterator to check the user entered values and generated random numbers.
Set myset = new HashSet();
myset.add(user_input1);
myset.add(user_input1);
To retrive use the iterator'
Iterator iterator = myset.iterator();
while(iterator.hasNext(){
int value= iterator.next();
if(randomValue==value)
//do your logic here
}

I am assuming this is for a school project/lab? (This is due to the JavaApplication8 class name) If that is the case, what the instructor is most likely looking for is a contains method.
For a contains method you write a method that takes an integer and checks to see if it is already in your LotteryNumbers array and returns a boolean. It would return true if it is in the array, false if it is not in it. This method would be called before inserting the number into LotteryNumbers. You could use your count variable that doesn't appear to be used anywhere else as the limit on your loop in the contains method to avoid checking uninitialized entries.
If there is no restriction on type, the set idea suggested by others works and is more efficient, it just depends on what you are supposed to be using for your requirements.
Additionally, the logic you use should most likely be applied to ActLotteryNumbers as well. If you can't have duplicates incoming, you shouldn't have duplicate values in the comparing array. Lottery isn't fair in real life, but not that unfair ;-)
First step should be checking your restrictions on this project.