CODEWITHSUNDEEP
cpointers
MAA
MAA

Reputation: 1355

What's wrong with this implementation of atof from K&R?

In K&R page 71, the atof function is using array indexing. I am trying to implement the same function using pointer arithmetic(Because it makes feel like a badass :)).

double atof(char *str)
{
  int sign;
  double number = 0.0, power = 1.0;

  while( isspace(*str) )
    ++str;

  sign = (*str == '-') ? -1 : 1; // Save the sign

  if(*str == '-' || *str == '+') // Skip the sign
    str++;

  while( isdigit(*str) ) // Digits before the decimal point
  {
    number = 10.0 * number + (*str - '0');
    str++;
  }

  if(*str == '.') // Skip the decimal point
    str++;

  while( isdigit(*str) ) // Digits after the decimal point
  {
    number = 10.0 * number + (*str - '0');
    power *= 10.0;
    str++;
  }


   return sign * number / power;
}

In main

printf("%g\n", atof("  123123.123")); // Outputs 123123
printf("%g\n", atof("  1234.1222"));  // Outputs 1234.12

What's my mistake ?!

Upvotes: 2

Views: 331

Answers (1)

P.P
P.P

Reputation: 121407

The atof() implementation is fine. But printf with %g removes parts of the result.

6 is the default if you don't explicitly specify it. Since the resulting value has greater precision, it's lost. Specify the precision:

printf("%.12g\n", atof("  123123.123"));
printf("%.12g\n", atof("  1234.1222"));

Or alternatively, you can use %f (which would print trailing zeros though).

Upvotes: 4

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