How can I hash a string to an int using c++?

Question

I have to write my own hash function. If I wanted to just make the simple hash function that maps each letter in the string to a numerical value (i.e. a=1, b=2, c=3, ...), i

Answer 1

Here's a C (++) hash function that I found in Stroustrup's book:

int hash(const char *str)
{
    int h = 0;
    while (*str)
       h = h << 1 ^ *str++;
    return h;
}

If you're using it for a hash table (which Stroustrup does) then you can instead return the abs of the hash modulo a prime number. So instead

    return (h > 0 ? h : -h) % N_BUCKETS;

for the last line.

Answer 2

Another way for small strings:

int hash(const char* str) {
    int hash = 0;
    int c = 0;

    while (c < std::strlen(str)) {
        hash += (int)str[c] << (int)str[c+1];
        c++;
    }
    return hash;
}

Answer 3

You can examine each individual char from a std::string using the [] operator. However, you can look at Boost::Functional/Hash for guidance on a better hashing scheme. There is also a list of hashing functions in c located here.

Answer 4

Just posting an improvement to Arnestig's djb2 algorithm to be constexpr-friendly. I had to remove the unsigned qualifier of the argument so it can work with literal strings.

constexpr unsigned long hash(const char *str) {
    unsigned long hash = 5381;

    while (int c = *str++) {
        hash = ((hash << 5) + hash) + c; /* hash * 33 + c */
    }

    return hash;
}

Answer 5

xor the characters together, four at a time.

Answer 6

C++11 ships with a standard hashing function for strings.

https://en.cppreference.com/w/cpp/string/basic_string/hash

#include <string>
#include<functional> // hash
int main(){
    std::string s = "Hello";
    std::size_t hash = std::hash<std::string>{}(s);
}