Display a factorial sequence as a string

Question

// here is my code to find a factorial of a number:

int get_factorial(int num)
{
   auto sum = 1;
   while(num > 0)
   {
       sum = sum * num;
       num--;
   }

   return sum;
}

// this works to give me the factorial but my assignment wants me to return a string. so if my parameter is 5, instead of returning 120 like my code does currently I need it to return a string that says "1x2x3x4x5 = 120". "5x4x3x2x1 = 120" should also work.

I'm unsure where to start. I thought maybe creating a string and appending each sum as it goes through the loop but I don't know how to do that.

Answer 1

The assignment is not easy for beginners like you and me.:)

I can suggest the following solution shown in the demonstration program below.

#include <iostream>
#include <string>

std::string get_factorial( unsigned int n )
{
    std::string result;
    unsigned long long factorial = 1;

    while (n > 1)
    {
        result += std::to_string( n ) + 'x';
        factorial *= n--;
    }

    return result + std::to_string( n ) + " = " + std::to_string( factorial );
}

int main()
{
    for (unsigned int i = 0; i < 10; i++)
    {
        std::cout << get_factorial( i ) << '\n';
    }
}

The program output is

0 = 1
1 = 1
2x1 = 2
3x2x1 = 6
4x3x2x1 = 24
5x4x3x2x1 = 120
6x5x4x3x2x1 = 720
7x6x5x4x3x2x1 = 5040
8x7x6x5x4x3x2x1 = 40320
9x8x7x6x5x4x3x2x1 = 362880

Alternatively the function can look also the following way as in that demonstration program.

#include <iostream>
#include <string>

std::string get_factorial( unsigned int n )
{
    std::string result = std::to_string( n == 0 ? 0 : 1 );
    unsigned long long factorial = 1;

    for ( unsigned int i = 1; i++ < n; )
    {
        result += 'x' + std::to_string(i);
        factorial *= i;
    }

    return result  + " = " + std::to_string( factorial );
}

int main()
{
    for (unsigned int i = 0; i < 10; i++)
    {
        std::cout << get_factorial( i ) << '\n';
    }
}

The program output is

0 = 1
1 = 1
1x2 = 2
1x2x3 = 6
1x2x3x4 = 24
1x2x3x4x5 = 120
1x2x3x4x5x6 = 720
1x2x3x4x5x6x7 = 5040
1x2x3x4x5x6x7x8 = 40320
1x2x3x4x5x6x7x8x9 = 362880

Pay attention to that the type unsigned long long int can store the maximum value of factorial for n equal to 20.