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, ...), is there a way I can perform this hash on a string without having to first convert it to a c-string to look at each individual char? Is there a more efficient way of hashing strings?

Answer 1

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 2

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 3

Re the first question, sure, e.g, something like:

int hash = 0;
int offset = 'a' - 1;
for(string::const_iterator it=s.begin(); it!=s.end(); ++it) {
  hash = hash << 1 | (*it - offset);
}

regarding the second, there are many better ways to hash strings. E.g., see here for a few C examples (easily translatable to C++ along the lines of the snippet above).

Answer 4

xor the characters together, four at a time.

Answer 5

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 6

You can make use of the member functions operator[] or at of the string class or iterators to access individual char of a string object without converting it to c-style char array.

To hash a string object to an integer you'll have to access each individual char of the string object which you can do as:

for (i=0; i < str.length(); i++) {
    // use str[i] or str.at(i) to access ith element.
}