Re: [Dev-C++] recursion help

[Richard G. <use...@ya...>]

Chris Irish wrote:

> Richard Gelling wrote:
>
>> Hi,
>>
>> I am working my way through Sams Teach yourself c++ in 21 days and am 
>> having a bit fof difficulty with recursion, could someone explain to 
>> me how the following oiece of code works:
>>
>> #ifdef HAVE_CONFIG_H
>> #include <config.h>
>> #endif
>>
>> #include <iostream>
>> #include <cstdlib>
>>
>> using namespace std;
>>
>> int toPowerOf( int , int );
>> int main(int argc, char *argv[])
>> {
>>    int powerOf, number, result;
>>    cout << "\nEnter the number: ";
>>    cin >> number;
>>    cout << "\nEnter the exponential: ";
>>    cin >> powerOf;
>>      result = toPowerOf( number, powerOf );
>>      cout << "\nThe result of " << number << "  to the power of " << 
>> powerOf << " is " << result << endl;
>>      return 0;
>> }
>> int toPowerOf( int number, int powerOf )
>> {
>>     if ( powerOf == 1 )
>>        return number;
>>     else
>>    {
>>        cout << "number is: " << number << " and the power of is " << 
>> powerOf << endl;
>>        return ( number  * toPowerOf( number, powerOf - 1 ));
>>    }
>>   }
>>
>> If someone could explain how the recusion side workd I would be 
>> gratefull, as I don't seem to be getting anywhere.
>>
>> Thanks a lot
>> Richard G.
>>
>>
> Note: Expand this window till you have the columns all lined up in my 
> explanation below.
>
> Well I'm by no means a master of recursion but I believe this example 
> works like this...... someone correct me if I'm wrong.  Recursion uses 
> the a stack for each call to the recursive function.  So the first 
> call is made, lets say with the values (2,3).  Then the first call 
> calls itself but with the values (2,2) now, pushing its call onto the 
> stack (like a stack of plates). Then the second call calls itself 
> again with the values (2,1).... now in the third call..... powerOf == 
> 1, so this stops the recursive calls and the returned value is the 
> 2......that call is popped off the stack and that returned 2 it 
> multiplied by the 2 in the second call, then the returned value is 
> (2*2), the second call is now popped off the stack and that returned 
> (2*2) is multiplied by the last 2 (which is the first call's 2)....... 
> so then "result" is set to 8 which is 2^3.
>
> //push                    //push                     //push then 
> pop'd                 //pop                                           
> //pop
> toPowerOf(2,3)    toPowerOf(2,2)      toPowerOf(2,1) //returns 2   
> toPowerOf(2,2) //returns (2*2)   toPowerOf(2,3)
>                             toPowerOf(2,3)      toPowerOf(2,2)      
>              toPowerOf(2,3)
>                                                            toPowerOf(2,3)
>
> The last one returns 2*2*2
>
> Hope that helps...... I find recursion difficult too :)
>
>
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Thanks for that everyone, that's made it clearer, I think I was 
suffering from a mental block,