atoi implementation in C

Question

I can\'t understand the following atoi implementation code, specifically this line:

k = (k << 3) + (k << 1) + (*p) - \'0\';
<         


        

Answer 1

about atoi() hint code from here:

and based on the atoi(), my implementation of atof():

[have same limitation of original code, doesn't check length, etc]

double atof(const char* s)
{
  double value_h = 0;
  double value_l = 0;
  double sign = 1;

  if (*s == '+' || *s == '-')
  {
    if (*s == '-') sign = -1;
    ++s;
  }

  while (*s >= 0x30 && *s <= 0x39)
  {
    value_h *= 10;
    value_h += (double)(*s - 0x30);
    ++s;
  }

  // 0x2E == '.'
  if (*s == 0x2E)
  {
    double divider = 1;
    ++s;
    while (*s >= 0x30 && *s <= 0x39)
    {
      divider *= 10;
      value_l *= 10;
      value_l += (double)(*s - 0x30);
      ++s;
    }
    return (value_h + value_l/divider) * sign;
  }
  else
  {
    return value_h * sign;
  }
}