Thread: recurive function cannot understand one line plz have a look

Code:

//recursive factorial function
#include <iostream>
#include <iomanip>
using namespace std;

unsigned long factorial( unsigned long );

int main()
{

   for (int i = 1; i <= 10; i++ )
       cout << setw( 2 ) << i << "! = " << factorial( i ) << endl; // this part <<factorial(i)  calls function factorial with the parameter of i?

   return 0;

}

// recursive definition of function factorial
unsigned long factorial( unsigned long number )
{
   if ( number <= 1 )   // base case
      return 1;
   else                 // recursive case
      return number * factorial( number -1 );  //this part here i have problem also number* factorial(number-1 ,so what happens here factorial has no value? so why multiply by number? i'm lost please help.
}

paramenter i, in that loop u set i to 1 and loop till it reaches 10. So whenever u have the call as factorial(i) ..... the present value of i is sent down to the function.


okay here is a test case that will help you understand the function ( im guessing thats what ur having trouble with )

lets assume i = 5 when the function is called.

so it goes through the check and since 5 is greater than 1 it goes to the next step that is the recursice call.

so now in reality things look like 5*factorial(4)
it will go through the steps like till it hits 1, and will look something like this on the stack

5*4*factorial(3) //second call
5*4*3*factorial(2)//third call
5*4**3*2*factorial(1)
now it will return 1 and things will start to unwind and will go on multiplying.
5*4*3*2*1 == 120

and that is the answer....

so what happens here factorial has no value?

factorial always returns a value, so you are multiplying number * the value returned by the function call. The fact that it is the same function is irrelevant, because each call is separate.

Code:

unsigned long square( unsigned long number )
{
    return number * number;
}

unsigned long myFunc( unsigned long number )
{
    return number * square( number - 1 );
}

The code in myFunc follows the same idea, multiply a number by the result of a function. Even though it is not recursive, it works the same as your recursive one.

Guys thank you so much i understand now how this works.
Thanks alot !