Factorial(s) of N numbers in for loop

Question

I am working on a problem from CodeChef where I need to calculate the factorial of n numbers.

The user inputs a number which determines how many ints to perform a factorial calculation on and then inputs the numbers to calculate.

My problem is with the multiplication itself. For example if I have an int == 5 then the result will be 20 (it will calculate n by the last factorial only, not by all of them)

Here is where the problem exists:

for(int x = 0; x < _numbersToProcess.Length; x++) {// Loop throuigh Array by index
    for (int y = 1; y < _numbersToProcess[x]; y++) {// Y is equal to less than index x
         _result[x] = _numbersToProcess[x] * y;// Multiply x by y then add to array
    }
}

The outer loop defines how many calculations to perform.

The inner loop calulates the factorials by iterating through each index of _numberToProcess and multiplying it by every number less than the number to be calculated on.

The problem is that the factorial calculation overwrites itself,

for example:

factorial of 5 result: 20 but it should be 120 (it overwrites itself until it reaches the last multiplier)

So I tried the following:

_result[x] = _numbersToProcess[x] *= y;

This is obviously the same as _numbersToProcess[x] = _numbersToProcess[x] * y;

But this gives a completley different result:

If we again input 5 then this will result in the output of -1899959296.

I know I can easily copy and paste from other submissions but I want to know why my method does not result in the correct output.

Here is the method in its entirety:

int _numbers = int.Parse(Console.ReadLine());// Get number of ints to calculate
        int[] _numbersToProcess = new int[_numbers];// Array of inputs
        int[] _result = new int[_numbers];
        int i = 0;

        while(i < _numbersToProcess.Length) {
            _numbersToProcess[i] = int.Parse(Console.ReadLine());
            i++;
        }

        for(int x = 0; x < _numbersToProcess.Length; x++) {// Loop throuigh Array by index
            for (int y = 1; y < _numbersToProcess[x]; y++) {// Y is equal to less than index x
                _result[x] = _numbersToProcess[x] *= y;// Multiply x by y then add to array
            }
        }

        for (int n = 0; n < _result.Length; n++) {// Y is equal to less than index x
            Console.WriteLine(_result[n]);// Write to console
        }

        Console.ReadLine();

Answer 1

int _numbers = int.Parse(Console.ReadLine());// Get number of ints to calculate
    int[] _numbersToProcess = new int[_numbers];// Array of inputs
    int[] _result = new int[_numbers];
    int i = 0;

    while(i < _numbersToProcess.Length) {
        _numbersToProcess[i] = int.Parse(Console.ReadLine());
        i++;
    }

    for (int x = 0; x < _numbersToProcess.Length; x++)
        {// Loop throuigh Array by index
            int fact = 1;
            for (int y = 1; y <= _numbersToProcess[x]; y++)
            {// Y is equal to less than index x
                fact = fact*y;
            }
            _result[x] = fact;
        }


    for (int n = 0; n < _result.Length; n++) {// Y is equal to less than index x
        Console.WriteLine(_result[n]);// Write to console
    }

    Console.ReadLine();

Problem is with your inner for loop. here, you are always, overriding result array.

i.e for y=5; inner for loop executes for 5 times.

iteration -1 : 
  y=1,
  _numbersToProcess[5]=5
  _result[x]=5

  iteration -2 : 
  y=2,
  _numbersToProcess[5]=10
  _result[x]=10

iteration -3 : 
  y=3,
  _numbersToProcess[5]=30
  _result[x]=30

.
.
.
.
.

thus it goes for 12 iteration as your _numbertoprocess[5] is changing and stops once it reaches less than 0 i.e -1899959296.

iteration 12:
  _numbertoprocess[5] = -1899959296.

i.e you are changing numbertoprocess everytime in your inner for loop.

you can verify it by adding

Console.WriteLine(y);
Console.WriteLine(_numbersToProcess[x]);
Console.WriteLine(_result[x]);

in your inner for loop.