Recursion

I know what this function will do, but I do not comprehend
why.
1   int factor(int x)
2   {
3       if (x==10)
4           return 1;
5       else
6       {
7           x *= factor(x - 1);
8           return x;
9       }
10  }
       
programAsked:
Who is Participating?
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

vsinhaCommented:
I am assuming x >= 10

rewriting your function to make it simpler
int factor(int x)
 {
     if (x==10)
         return 1;
     else
         return x * factor(x - 1);
 }

factor(10) = 1
factor(11) = 11 * factor(10) = 11 * 1 = 11
factor(12) = 12 * factor(11) = 12 * 11
factor(13) = 13 * factor(12) = 13 * 12 * 11
factor(14) = 14 * factor(13) = 14 * 13 * 12 * 11
factor(15) = 15 * factor(14) = 15 * 14 * 13 * 12 * 11

let me know if you don't understand anything, or if I have misunderstood you.
0

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
nietodCommented:
Its unusual that it stops at 10 and not 1.  Ussually the line

if (x == 10)

would be

if (x == 1)

The recursive algorithm just says:  Given a number, If the number is one (10?) then factor of the number is one.  For any other number,  factor of the number is the number times factor of one less than the number.  This process continues calling factor recursivly using a number one less each time until it calls it with a value of 1 (10?).  Then the innermost factor call returns (it returns 1) which the caller multiplies by 2 and then returns.  Which the caller multiplies by 3 and then returns and so on until the outermost call returns.
0
vsinhaCommented:
Actually I think to calc. the factorial the safest soln. would be to replace:
if (x == 10)
by
if (x <= 1)

but this would depend on what you want to do in the function.
0
Cloud Class® Course: Microsoft Office 2010

This course will introduce you to the interfaces and features of Microsoft Office 2010 Word, Excel, PowerPoint, Outlook, and Access. You will learn about the features that are shared between all products in the Office suite, as well as the new features that are product specific.

ozoCommented:
I'd say
if( x==0 )
(-1)! is supposed to blow up

0
programAuthor Commented:
I misstyped the line: if(x==10). It is supposed to be: if(x==1),
like nietod & vsinha mentioned. All appologies. Could I please
get a new explanation.
0
vsinhaCommented:
rewording, my previous answer:

your func. can be written as:
     int factor(int x)
      {
          if (x==1)
              return 1;
          else
              return x * factor(x - 1);
      }

for small values:
     factor(1) = 1
     factor(2) = 2 * factor(1) = 2 * 1
     factor(3) = 3 * factor(2) = 3 * 2 * 1
     factor(4) = 4 * factor(3) = 4 * 3 * 2 * 1
     factor(5) = 5 * factor(4) = 5 * 4 * 3 * 2 * 1

the best way to think of it is how factorial is defined normally:
n! = n * (n-1)!            except for n=1 where 1! = 1
0
programAuthor Commented:
Thank you much vsinha for the clear explanation of this
function.
0
vsinhaCommented:
You are welcome
0
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
C++

From novice to tech pro — start learning today.