Vinz.R Vinz.R - 3 months ago 9
C Question

Where is the mistake in my code to calculate factorials of numbers up to 99?

I am using the old school multiplication method to calculate the factorials but still I am getting wrong answer for numbers greater than 13. I know that no data type can hold such a big value accurately so I am using an array to store each digit in the number separately.

"cas" in code denotes the number of which factorial is to be calculated.

Here is the code-

void factorial (int cas)
{
int num[158];
num[0] = 1;

for (int i=1; i<158; i++)
{
num[i] = 0;
}

int row1[158];
int row2[158];
int length = 0;
int max = 0;
int carry = 0;

for (int j=1; j<=cas; j++)
{
for (int i=0; i<158; i++)
{
row1[i] = 0;
row2[i] = 0;
}

for (length=157; length>=0; length--)
{
if (num[length] != 0)
break;
}

if (j/10 < 1)
{
int* ptr = num;
carry = 0;

for (int i=0; i<=length; i++)
{
int mult = (j * num[i]) + carry;
int units = mult % 10;
carry = mult / 10;
*ptr = units;
ptr++;
}
*ptr = carry;
}
else
{
int* ptr = num;
int* ptr1 = row1;
row2[0] = 0;
int* ptr2 = row2 + 1;
carry = 0;

for (int i=0; i<=length; i++)
{
int mult = ((j%10) * num[i]) + carry;
int units = mult % 10;
carry = mult / 10;
*ptr1 = units;
ptr1++;
}
*ptr1 = carry;

for (int i=0; i<=length; i++)
{
int mult = ((j/10) * num[i]) + carry;
int units = mult % 10;
carry = mult / 10;
*ptr2 = units;
ptr2++;
}
*ptr2 = carry;

for (max=157; max>=0; max--)
{
if (row1[max] != 0 || row2[max] != 0)
break;
}

carry = 0;

for (int i=0; i<=max; i++)
{
int add = row1[i] + row2[i] + carry;
int units = add % 10;
carry = add / 10;
*ptr = units;
ptr++;
}
*ptr = carry;
}

}
for (length=157; length>=0; length--)
{
if (num[length] != 0)
break;
}

for (;length>=0;length--)
{
printf("%i",num[length]);
}

printf("\n");
}

Answer

You forgot to reset the carry to 0 before multiplying by the tens part of number.

  }
  *ptr1 = carry;

  carry = 0;  // ADDED LINE: Reset carry before multiplying with the tens part

  for (int i=0; i<=length; i++)
  {
    int mult = ((j/10) * num[i]) + carry;