What's wrong in calculating npr ?

Question

1.00/5 (2 votes)

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Java

calculate nPr n!/(n-r)!.
input :- 5 2
output :- 20

What I have tried:

Java

public class Main{
  
  public static void main(String[] args) {
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      int b = sc.nextInt();
      int dn = Value(n, b);
      System.out.println(dn);
   }
   
   public static int Value(int n, int r){
    // Your code here
	    int fact = n;  
            for (int i= n-1; i>1; i--)  
            {  
                fact = fact * i;  
            } 
	    
           int num = (n - r);  
           int fact2 = num;  
            for (int i = num - 1; i >= 1; i--)  
            {  
                fact2 = fact2 * i;  
            }  
           int per = fact / fact2;  

	   return per;
   }
  }

Posted 19-Dec-22 22:43pm

ak343

Updated 20-Dec-22 7:45am

Graeme_Grant

v2

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3 solutions

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CPallini · Answer 1 · 2022-12-20T01:57:00

Actually your code is logically correct, as you may easily verify providing small input values.
Problem arises when n > 12, because 13! = 6,227,020,800 a number bigger than Java int datatype maximum value (2,147,483,647).
In order to ameliorate the problem you might:

Use Java long datatype. This way your code would compute factorial correctly for n up to 20

Note that (with n>k)
```
n!/k! = (n * (n-1) * .. * k * (k-1) * .. 1) / (k * k-1) * .. 1) = (n * (n-1) * .. * (k+1))
```
This way, for example
```
50! / 48! = 50 * 49 = 2450
```
You could also use the Stirling's approximation - Wikipedia[^] in order to validate inputs (i.e. using the double datatype estimate if the given inputs would overflow your integral computations),

Graeme_Grant · Answer 2 · 2022-12-19T22:52:00

Solution 1

Did you try Googling? how to calculate npr using java - Google Search[^]

First result was: Program to calculate the value of nPr - GeeksforGeeks[^]

There are many others to choose from.

Posted 19-Dec-22 22:52pm

Graeme_Grant

OriginalGriff · Answer 3 · 2022-12-19T23:16:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

private int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

What's wrong in calculating npr ?

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