Thread: Hash function in C, need to port it to C++

1. You use C++ strings but did not #include <string>.
2. As pointed out earlier, you should pass key.c_str() to hash().
3. h is uninitialised in hash().

I think that if you are going to alter your original code anyway, then it makes sense to just change it to use C++ strings, e.g.,

Code:

unsigned int hash(const std::string& key)
{
	unsigned int h = 0;
	const unsigned char* p = reinterpret_cast<const unsigned char*>(key.c_str());

	int len = key.length();
	for(int i=0; i<len; i++)
	{
		h = 33 * h + p[i];
	}
	return h;
}

This way, you can just call hash(key) in your main function.

Just curious: is one better than the other as far as using the address of the first element or using the c_str() function? My answer would be that using c_str() is easier to read.

Code:

#include <iostream>
#include <string>

using namespace std;

unsigned hash( const void* key, int len );

int main()
{
	string text;

	cout << "String: ";
	cin >> text; 
	
	cout << "Hashed: " << hash( text.c_str(), text.length() );

	cin.ignore();

	return 0;
}

unsigned hash( const void* key, int len)
{
	const unsigned char* p = (const unsigned char*)key;

	unsigned h=0;

	int i;

	for(i=0; i<len; i++)
	{
		h=33*h+p[i];
	}

	return h;
}

Those are minimal changes to your code, take a close look and find out what has changed and why. Changes are in the function declaration, it now takes a const pointer, because aside from the compiler error, it's the correct way of doing stuff.

Thanks nvoight your code works.

So c_str() is easier to read, and taking the address of the first element of the string is not guaranteed to work.

Why is it possible for c_str() to be guaranteed if passing the address of the first element is not? You say that the underlying storage of a string may not be stored contiguously, so what does c_str() really do different besides return an address? Does it make sure that the characters are arranged in order and null terminated?

Thanks Daved, that makes sense.

Could someone explain what type of data h is?

It's just declared: unsigned. What does that mean?

Thanks!

So the compiler just assumes an int then?

Hi... does this gives the same results:

Code:

#include <iostream>
#include <string>

using std::cout;
using std::cin;
using std::endl;
using std::string;

int Hash(const string& key) {
	int h = 0, i = 0;
	for(i=0; i<key.size(); i++) {
		h=33*h+key.c_str()[i];
	}
	return h;
}

int main(void) {
	string data;
	cout << "String: "; cin >> data; // Joel
	cout << "Hashed: " << Hash(data) << endl; // 2783658
	return 0;
}