<< 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/ )
Answer from Karoly Horvath on Stack OverflowThe 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
stris not a valid integral number, or if no such sequence exists because eitherstris empty or it contains only whitespace characters, no conversion is performed and zero is returned.
<< 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
stris not a valid integral number, or if no such sequence exists because eitherstris empty or it contains only whitespace characters, no conversion is performed and zero is returned.
k = (k << 3) + (k << 1);
means
k = k * 2³ + k * 2¹ = k * 8 + k * 2 = k * 10
Does that help?
The *p - '0' term adds the value of the next digit; this works because C requires that the digit characters have consecutive values, so that '1' == '0' + 1, '2' == '0' + 2, etc.
As for your second question (atof), that should be its own question, and it's the subject for a thesis, not something simple to answer...
