CODEWITHSUNDEEP
javado-whilefactorial
Ricardo Ferreira
Ricardo Ferreira

Reputation: 1

factorial of given number

I would like to find the smallest factorial of a given long number. For example, if you enter the number 100, the code should give the factorial 5, since 5! = 1 * 2 * 3 * 4 * 5 = 120 is closer than the factorial 4! = 1 * 2 * 3 * 4 = 24. I have written the code below, but when I enter 100, I only get the factorial 3.

import java.util.Scanner;

class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        long number = scanner.nextLong();
        long factorial = 1;
        long sum;
        do {
            sum = number / factorial;
            factorial++;
        } while (number <= sum);
        System.out.println(factorial);
    }
}

What am I doing wrong here?

Upvotes: 0

Views: 639

Answers (2)

WJS
WJS

Reputation: 40034

Here is one way. This will return the smallest factorial greater than the number. A record is used to return the information.

record Factorial(long n, long nFact) {}
Integer[] data = {1,2,5,10,200,500,2520,1000, 5040, 720, 2000, 3000, 10_000};
for (long i  : data) {
     Factorial result = smallestFactorial(i);
     System.out.printf("For %-8d the smallest >= %8d is (%d! = %d)%n",
             i, i, result.n, result.nFact);
}

prints

For 1        the smallest >=        1 is (2! = 2)
For 2        the smallest >=        2 is (2! = 2)
For 5        the smallest >=        5 is (3! = 6)
For 10       the smallest >=       10 is (4! = 24)
For 200      the smallest >=      200 is (6! = 720)
For 500      the smallest >=      500 is (6! = 720)
For 2520     the smallest >=     2520 is (7! = 5040)
For 1000     the smallest >=     1000 is (7! = 5040)
For 5040     the smallest >=     5040 is (7! = 5040)
For 720      the smallest >=      720 is (6! = 720)
For 2000     the smallest >=     2000 is (7! = 5040)
For 3000     the smallest >=     3000 is (7! = 5040)
For 10000    the smallest >=    10000 is (8! = 40320)

This method calculates the factorial while comparing to the passed argument.

  • to save time it memoizes the factorials computed
  • continue calculating factorials until the factorial exceeds the argument.
  • Then return the information in a record.
public static Factorial smallestFactorial(long n) {
   if (n <= 1) {
       return new Factorial(2,2);
   }
   long fact = factorials.floorKey(n);
   int k = factorials.get(fact);
   while (n > fact) {
       fact*=++k;
       factorials.putIfAbsent(fact, k);
   }
   return new Factorial(k, fact);  
}

The output formatting is incidental and can of course be changed to suit your requirements.

Upvotes: 1

Houria Marref
Houria Marref

Reputation: 31

you should calculate the factorial of numbers, untill you find an equal or smaller number than the number you entered, as shown in the following code :

import java.util.Scanner;

class Main {
    public Long factorial (int n){
      long p = 1L;
      for(int i = 1; i<= n ; i++){
        p=p*i;
      }
    }
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        long number = scanner.nextLong();
        long i = 1;
       while (factorial(i) < number) {
           i++;
        }
        System.out.println(i);
    }

Upvotes: 2

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