checksum in C++

checksum.cpp

#include <cstdio>
#include <utility>

int get_sum(int num)
{
	num *= 2;
	int sum = 0;
	
	// if (num < 10) sum = num; else sum = num % 10 + 1;
	
	for (sum = 0; num > 0;) 
	{
		sum += (num % 10);
		num /= 10; 
	}
	
	return sum;
}

int main()
{
	int even = 0;
	int odd = 0;
	// int i = 1;
	
	FILE *fp = stdin;

	// int ch = 0;
	
	int &left = even;
	int &right = odd;
	
	while(( int ch = getc(fp)) >= '0' && ch <= '9')
	{	
		ch = ch - '0';
		
		// even += i * ch + (i^1) * get_sum(ch);
		// odd += (i^1) * ch + i * get_sum(ch);
		
		left += ch;
		right += get_sum(ch);
		
		// even %= 10;
		// odd %= 10;
		// i = i^1;
		
		left %= 10;
		right %= 10;
		
		std::swap(left,right);
	}
	
	printf("result %d\n", (10-left) % 10);
	
	// if (i) printf("%d",(10-even)%10); else printf("%d",(10-odd)%10);
	
	return (10-left)%10;
}

cheksum_book.cpp

int doubleDigitValue(int digit) 
{
  int doubledDigit = digit * 2;
  int sum;
  if (doubledDigit > 10) sum = 1 + doubledDigit % 10; else sum = doubledDigit;
  return sum; 
}


int main() 
{
  // checking if a checksum is divisible by 10
  char digit;
  int oddLengthChecksum = 0;
  int evenLengthChecksum = 0;
  int position = 1;

  cout << "Enter a number: ";
  digit = cin.get();

  // this is waiting for `Enter`
  // not taking into account other options
  while (digit != 10) {
    if (position % 2 == 0) 
    {
      // none of these sums is using % 10
      // which means it can overflow the datatype size
      oddLengthChecksum += doubleDigitValue(digit - '0'); 
      evenLengthChecksum += digit - '0';
    } else {
      oddLengthChecksum += digit - '0';
      evenLengthChecksum += doubleDigitValue(digit - '0');
    }
    digit = cin.get();

    // this is summing up the number length
    // instead of simply having a binary value
    // for odd/even numbers
    position++;
  }

  int checksum;
  if ((position - 1) % 2 == 0) checksum = evenLengthChecksum;
  else checksum = oddLengthChecksum;

  cout << "Checksum is " << checksum << ". \n";

  if (checksum % 10 == 0) 
  {
    cout << "Checksum is divisible by 10. Valid. \n"; 
  } else {
    cout << "Checksum is not divisible by 10. Invalid. \n"; 
  }
  
  return 0;
}