Factorial

[bowlbase]

I'm brand spanking new to C++ so any help/suggestions would be great:

I'm suppose to be making a program that shows factorial for 1-5 using "for".

I have: for ( int n = 1; n <= 5; n++ ) but the output is killing me trying to figure out how I can manipulate 'n' to give me the factorial for each new integer.

I thought that I would do an if (n = 1) cout << 1 << endl;
and then else if for each integer after that with just the proper multiplication of each. However, when I run the program I just get an endless loop of 1. I assume this is because I am not returning back to the top to the 'for'. I don't know for sure though.

I'm being forced to use this method (for) to create the increments as I have already done it another way that works but isn't acceptable for a grade.

I appreciate the help.

#include <iostream>

using namespace std;


int main()
{
    
    for ( int n = 1; n <= 5; n++ )
      if (counter = 1)
         cout << 1 << endl;
        else if (n = 2)
             cout << 1 * 2 << endl;
        else if (n = 3)
             cout << 1 * 2 * 3 << endl;
        else if (n = 4)
             cout << 1 * 2 * 3 *4 << endl;
        else
            cout << 1 * 2 * 3 * 4 * 5 << endl;              
             
        
    cout << "press enter to close\n";
    cin.get();
    
    return 0;
}

[bradw]

This is where recursion is good for you.

int Factorial(int n);

int main()
{
    cout << "facoral of 5 is: " << Factorial (5)  << endl;
}

int Fatorial(int n)
{
    if (n == 1)
        return n;
   return n * Factorial(n - 1);
}

if you really want to use a for loop:

int main()
{
    int n = 5;
    int fact = 1;
    for(int i = 1; i <=n; i++)
    {
        fact *= i;
    }
    cout << "factorial is: " << fact << endl;
}

[bowlbase]

This works to give me a factorial of some number but it doesn't do it for all n up to a limit (which is what I need). The assignment is to have a program that spits out:
n! x
1 1
2 2
3 6
4 24
5 120

Maybe the misunderstanding comes from what I wrote for my program before since I'm likely using the if/else if wrong. I thought that the for would increment the digit once and output it to the if function and it would then make a decision what to do with it. Once the decision was made it would return to the for function to be incremented 1 more time until n=5.

[kaseron]

so use

int main()
{
  int limit,factor;
  factor=1;
  cout<<"Input number of factorials to display: "<<endl;
  cin>>limit;
  for(int fact; fact<=limit; fact++)
  {
    cout<<fact<<" ";
    factor*=fact;
    cout<<factor<<endl;
  }
  cin.get();
}

[vlad from moscow]

Factorials start from 0. 0! is equal to 1.
If to use a recursive function as suggested by @bradw it could look the following way

#include <iostream>

unsigned int factorial( unsigned int n = 5 )
{
   int f = ( n == 0 ) ? 1 : n * factorial( n - 1 );
   
   std::cout << n << ' ' << f << std::endl;

   return ( f );
}

int main()
{
   factorial();

   return 0;
}

[bowlbase]

This basically works but misses the first digit (fixed that part). Can you explain a little bit why it works? I understand everything till:

cout<<fact<<" "; <-- output of whatever digit it has counted to
factor*=fact; <-- no idea
cout<<factor<<endl; <-- output of factor value

i s '*=' actually a way of computing factorials that I didn't know about?? Never seen this symbol.

[kaseron]

The factor *= fact; means that factor is now equal to the previous value of factor times the number fact.
It could also be written with an int called temp like this:

int temp;
temp =factor * fact;
factor=temp;

That would accomplish the same thing, but we're lazy, so we use the *= instead.

[bowlbase]

I got it now. So it is just storing the old number then multiplying by the new number. I never even thought of doing it this way, so much easier than trying to do if.

For my own understanding, why did my first set up not work? I should expect that I'll be writing a lot of else and if code later on this semester so I'd like to get a better grasp of it.

[vlad from moscow]

You shall change the assignment operator (=) to the comparision operator (==) in your code

     if (counter = 1)
         cout << 1 << endl;
        else if (n = 2)
             cout << 1 * 2 << endl;
        else if (n = 3)
             cout << 1 * 2 * 3 << endl;
        else if (n = 4)
             cout << 1 * 2 * 3 *4 << endl;
        else
            cout << 1 * 2 * 3 * 4 * 5 << endl;     

[bowlbase]

Ah, I missed the comparison operator in my book. I got it to work that way now as well.

Thanks everyone I learned quite a bit!