string to float conversion?

Question

I\'m wondering what sort of algorithm could be used to take something like \"4.72\" into a float data type, equal to

float x = 4.72;

Answer 1

As you've asked for an algorithm, not a method, here is my explanation for a simple algorithm (and an implementation in C):

1. Initialize 4 integer variables, one for the value before dot, one for the after part, one for the power of the mantissa, one for the sign. Let's say, f, m, d, sign = 1.
2. First look for + or - sign at the beginning. If there are no sign characters or + sign then continue. If the first character is -, then sign = -1.
3. Then, read an integer value into f until a . or NULL character.
4. If you end up with a dot character, then start reading the mantissa part as in the previous step into m. But this time also multiply d by 10 with each digit.
5. In the end, return sign*(f + (float) m/d). The casting makes sure the division is done in floating-points and the type of the expression is float.

I guess, reading the code might be easier. So here is the code:

float atof(char *s)
{
    int f, m, sign, d=1;
    f = m = 0;

    sign = (s[0] == '-') ? -1 : 1;
    if (s[0] == '-' || s[0] == '+') s++;

    for (; *s != '.' && *s; s++) {
            f = (*s-'0') + f*10;
    }
    if (*s == '.')
            for (++s; *s; s++) {
                    m = (*s-'0') + m*10;
                    d *= 10;
            }
    return sign*(f + (float)m/d);
}