Question

Need to write a function that will convert a string to a double without calling any built-in utilities such as atof(), Double.parseDouble(), Double.valueOf(), [NSString doubleValue], [NSScanner scanDouble], std::stringstream or any similar methods.

Answer 1

const char *s = "-123.456";
int sign = 1;
int hasdp = 0;
int dplaces = 0;
float f = 0.0;
while(*s) {
    switch (*s) {
    case '-': sign = -1; break;
    case '.': hasdp = 1; break;
    default:
        f *= 10;
        f += *s - '0';
        break;
    }
    s++;
    if (hasdp) dplaces++;
}

f *= sign;
int i;
for (i = 1; i < dplaces; i++) f /= 10;
// we need to start from 1 in order to get
// the correct result, this is a property
// of this algorithm

Now f will contain the parsed value, provided its format is correct (optional – prefix, optional decimal point followed by 1 or more decimal places.