How to convert int to apstring?

I'm trying to make a digital root calculator where the user enters a positive integer value,
and the digital root is the product of the non-zero digits of the value. The program is
supposed to find the digital root of the digital root and so on until a single digit is obtained.
I have stored the value the user enters in an apstring so that I can access the individual digits
of that value. I later convert them to digits so that I can do calculations, but at a certain point,
I need to convert from a digit back to an apstring again so that I can access the individual digits
of the next digital root. Is there a way to convert from an int to an apstring?

Here is my program thus far:


#include <iostream.h>
#include <stdlib.h>
#include "apstring.h"
#include <iomanip.h>
void printHeading(apstring title);
//Postcondition: Centers the title on the screen

apstring getInt();
//Postcondition: A valid integer entered by the user is returned to the main program
int main()
{


printHeading("Digital Product Root Calculator");

apstring integer=getInt(); //The user-entered integer value

int len=integer.length(); //The length of the entered value
int product=1; //One successive digital root
int intDigit=0; //The integer value of digit


for(int counter;counter=7;counter--)
integer=product; //<-----That's where I'm stuck at, the int value in product needs to be stored in apstring integer
for(len;len>=1;len--)
{
apstring digit=integer.substr(len-1,1);//stores a digit from the integer in digit
intDigit=atoi(digit.c_str()); //converts digit from apstring to integer
cout<<"IntDigit: "<<intDigit<<endl;
if (intDigit!=0) //multiplies an integer into the product if it is not zero
{product=intDigit*product;
cout<<"Inside Product: "<<product<<endl;
}
else
product;
}

cout<<"Digital Root: "<<product<<endl;

system("PAUSE");
return 0;
}
//Finds the length of the title, then centers the title on the screen according to its length
void printHeading(apstring title)
{
int len;
int tab;
len=title.length();
tab=len/2 +40;
cout<<setw(tab)<<title<<endl<<endl;
}
apstring getInt()
{
apstring num; //the entered integer

//Prompts user to enter a value
cout<<"Please enter a positive integer value 1-8 digits in length: ";
getline(cin,num);
cout<<endl<<endl;

//Determines whether the entered value is negative
int neg=num.find('-');

//Determines the length of the entered value
int len=num.length();

//If entered value is not between 0 and 99,999,999, prompt user to reenter the value

while (len>8||neg!=-1)
{
cout<<"Please reenter an positive integer value 1-8 digits in length: ";
getline(cin,num);
cout<<endl<<endl;

neg=num.find('-');
len=num.length();
}

return num;


}