Thread: Need suggestion on sum of factorials.

This is my solution:

A quick test shows that your solution is incorrect.

Any suggestion to make this program better.

For starters, change void main() to int main(), and remove your dependence on the non-standard <conio.h> by removing the unnecessary calls to clrscr() and getch().

Now, for actually solving the problem, you can make use of the fact that for any integer n > 0, n!/(2n) = (n-1)!/2.

I never know about this relation.

It is just simple arithmetic. n! = (n-1)! * n, therefore you can simplify as such. Actually, there is a further simplification, but that may actually complicate your code a slightly instead, so perhaps it is better to leave it for later.

Could you show me how to implement it in my code ?

First, implement a loop to sum from 1 to (n-1)! inclusive.

Originally Posted by noops

I suggest creating a function that returns the factorial of a number.

n! = (n - 1)! * n

A recursive function might be:

Code:

int fact(int n)
{
     if (n <= 1)
          return(1);
     return(fact(n - 1) * n);
}

But it is often better to let the poster figure those things out him-/herself, as that is touching many more parts of the brain, and thus makes it easier to remember, v.s. selecting an area of text and pasting it into their editor.

--
Mats

Not to mention that using such a function is inefficient.

It's not efficient, but it's quite simple. I mean, if the OP has already problems getting a correct solution, we shouldn't ask about getting the most efficient solution, shouldn't we ?

The only wrong thing I see with this function is the parameter and return type; I would definitely use an unsigned integer instead of a signed one, since the definition of factorial is only good for non-negative number.

By writing the factorial as a seperate function I believe that the OP would not learn everything that they should be learning. They will get a solution, but will not have learned how to properly implement an algorithm that performs according the more desireable and easily achieveable O(n) Big-Oh notation.

Tested and it works!

it is really strange - it should not

Tested and it works!

Sorry, it still does not work. For example, an input of 5 gives the result 0.96875, but clearly the answer is 17.

Originally Posted by chhay

I don't know why it become like this, but when in typed in 3, it gives the result 2, which is the correct answer.

And do you also get the correct result for 4 or 5? It may be coincidence that 3 gives the correct result, despite the calculation itself being incorrect [and I agree that it most likely is].

--
Mats

Originally Posted by chhay

I don't know why it become like this, but when in typed in 3, it gives the result 2, which is the correct answer.

In the words of Dijkstra: "Testing can only reveal the presence of bugs, not their absence". In other words, getting a correct answer for a single test doesn't mean that the whole program works.

QuantumPete