# Simply factorial problem; returning factorial call stack

Hello all,

Practising my poor scala skills (and general programming) and in order to better understantig of recursion, I just want to build and return list of strings with the call stack.
For the well known factorial program:

factStringList(5) should return:

5*fact(4)
5*4*fact(3)
5*4*3*fact(2)
5*4*3*2*fact(1)
5*4*3*2*1

Here is my piece of code that does output:
5*4*3*2*fact(1)
4*3*2*fact(1)
3*2*fact(1)
2*fact(1)
fact(1)

def factString(n : Int): String = n match {
case 1 => "f(1)"
case _ =>  n.toString() + " * " + factString(n - 1)
}

def factStringList(n : Int): List[String] = n match {
case 0 => Nil
case _ => {
val s = factString( n )
s :: factStringList(n - 1)
}
}

Any help?

Thanks.

### Re: Simply factorial problem; returning factorial call stack

You need to think step-by-step through the code you've written.

For example, could your code possibly print f(n) for any n other than 1?

If you recurse from 5 down to three, could your code possibly know that it should have a 5*4* at the beginning of whatever comes next (that is, is there anything in your code that could generate that substring?).

You're going to need to pass some information forward into the recursion, in this style:

def passItForward(n: Int, s: String = ""): String = {
if (n <= 0) s else passItForward(n-1, s + "," + n.toString)
}

--Rex

On Mon, Feb 20, 2012 at 1:44 PM, csg <carlossg00 [at] gmail [dot] com> wrote:
Hello all,

Practising my poor scala skills (and general programming) and in order to better understantig of recursion, I just want to build and return list of strings with the call stack.
For the well known factorial program:

factStringList(5) should return:

5*fact(4)
5*4*fact(3)
5*4*3*fact(2)
5*4*3*2*fact(1)
5*4*3*2*1

Here is my piece of code that does output:
5*4*3*2*fact(1)
4*3*2*fact(1)
3*2*fact(1)
2*fact(1)
fact(1)

def factString(n : Int): String = n match {
case 1 => "f(1)"
case _ =>  n.toString() + " * " + factString(n - 1)
}

def factStringList(n : Int): List[String] = n match {
case 0 => Nil
case _ => {
val s = factString( n )
s :: factStringList(n - 1)
}
}

Any help?

Thanks.