﻿ Java - Check whether a number is a Lucky Number or not

# Java: Check whether a number is a Lucky Number or not.

## Java Numbers: Exercise-29 with Solution

Write a Java program to check whether a number is a Luck number or not.

Lucky numbers are defined via a sieve as follows.
Begin with a list of integers starting with 1 :
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, . . . .
Now eliminate every second number :
1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, ...
The second remaining number is 3, so remove every 3rd number:
1, 3, 7, 9, 13, 15, 19, 21, 25, ...
The next remaining number is 7, so remove every 7th number:
1, 3, 7, 9, 13, 15, 21, 25, ...
Next, remove every 9th number and so on.
Finally, the resulting sequence is the lucky numbers.

Following animation demonstrating the lucky number sieve. The numbers in red are lucky numbers. Animation Credits: Celtic Minstrel

Sample Solution:

Java Code:

``````import java.util.*;
public class solution {
public static boolean isLucky(int n) {
//Create an array of size 10 to initialize all elements as
//false to check if a digit is already seen or not.
boolean temp[]=new boolean;
for (int i = 0; i < 10; i++)
temp[i] = false;
while (n > 0)
{
// Find the last digit
int digit = n % 10;
// Return false if digit is already seen,
if (temp[digit])
return false;
temp[digit] = true;
n = n / 10;
}
return true;
}

public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("Input an integer: ");
String input = scanner.nextLine();
int number = Integer.parseInt(input);
System.out.println("Is Lucky number? "+isLucky(number));
}
}
```
```

Sample Output:

```Input an integer:  25
Is Lucky number? true
```

Flowchart: Java Code Editor:

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﻿

## Java: Tips of the Day

IsPowerOfTwo

Checks if a value is positive power of two.

To understand how it works let's assume we made a call IsPowerOfTwo(4).

As value is greater than 0, so right side of the && operator will be evaluated.

The result of (~value + 1) is equal to value itself. ~100 + 001 => 011 + 001 => 100. This is equal to value.

The result of (value & value) is value. 100 & 100 => 100.

This will value the expression to true as value is equal to value.

```public static boolean isPowerOfTwo(final int value) {
return value > 0 && ((value & (~value + 1)) == value);
}
```

Ref: https://bit.ly/3sA5d4I

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