atoi implementation in C

Question

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

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

Here is the code:

int my_atoi(char *p) {
    int k = 0;
    while (*p) {
        k = (k << 3) + (k << 1) + (*p) - '0';
        p++;
     }
     return k;
}

Can someone explain it to me ?

Another question: what should be the algorithm of atof implementation ?

Answer 1

<< is bit shift, (k<<3)+(k<<1) is k*10, written by someone who thought he was more clever than a compiler (well, he was wrong...)

(*p) - '0' is subtracting the value of character 0 from the character pointed by p, effectively converting the character to a number.

I hope you can figure out the rest... just remember how the decimal system works.

Here is a specification for the standard function atoi. Sorry for not quoting the standard, but this will work just as fine (from: http://www.cplusplus.com/reference/clibrary/cstdlib/atoi/ )

The function first discards as many whitespace characters (as in isspace) as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many base-10 digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed and zero is returned.