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

Okay... so i was looking into EternallyConfuzzled's website. The hash part.

I think it's written for C, but im trying to implement this stuff in C++.

How would i convert this code?

Code:

unsigned djb_hash ( void *key, int len )
 2...{
 3  unsigned char *p = key;
 4  unsigned h = 0;
 5  int i;
 6
 7  for ( i = 0; i < len; i++ )
 8    h = 33 * h + p[i];
 9
10  return h;
11}

Thanks!

Originally Posted by Junior89

How would i convert this code?

Leave it as-is?

How should i call it then? Cause i'm obviously not doing it right if i'm getting errors =P

Originally Posted by Junior89

How should i call it then? Cause i'm obviously not doing it right if i'm getting errors =P

Shall we play 20 questions, or will you post your code?

okay

Code:

#include <iostream>

using namespace std;

unsigned hash(void *key, int len);

int main()
{
	string key;
	cout<<"String: ";
	cin>>key;
	cin.ignore();
	int len = strlen(key);
	cout<<"Hashed: "<<hash(*key, len);
	cin.ignore();
	return 0;
}

And the rest of it?

[edit][Your computer doesn't read your mind any more that I do. If you have code to present to it, present it to the compler.]

Code:

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

	unsigned h=0;

	int i;

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

	return h;
}

Same as before

Code:

#include <iostream>

using namespace std;

unsigned hash(void *key, int len);

int main()
{
	char key[20];
	cout<<"String: ";
	cin.getline(key, 20);
	int len = 20;
	cout<<"Hashed: "<<hash(key, len);
	cin.ignore();
	return 0;
}

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

	unsigned h=0;

	int i;

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

	return h;
}

The whole thing so there's no confusion

Here's the error:

c:\documents and settings\doug.laptop.000\my documents\visual studio 2005\projects\hash test 1\hash test 1\main.cpp(20) : error C2440: 'initializing' : cannot convert from 'void *' to 'unsigned char *'
Conversion from 'void*' to pointer to non-'void' requires an explicit cast

Thanks! =)

> cout<<"Hashed: "<<hash(*key, len);
I presume this rather dubious way of converting a C++ String into a C-string then.

Maybe
cout<<"Hashed: "<<hash(key.c_str(), len);

While you're at it, find the correct method for doing this
int len = strlen(key);

Code:

unsigned char *p = (unsigned char *)key;

Void pointers don't look very good in C++ programs.

Just change the prototype to accept the correct pointer type:

Code:

unsigned hash(const unsigned char *key, int len)
{
	unsigned h=0;

	int i;

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

	return h;
}

And if you want to use it with a std::string, use the c_str() method.

Edit: more correctly: do the conversion to unsigned where you need the value:

Code:

unsigned hash(const char *key, int len)
{
	unsigned h=0;

	int i;

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

	return h;
}

Code:

#include <iostream>

using namespace std;

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

int main()
{
	string key;
	int len;
	cout<<"Key: ";
	cin>>key;
	cin.ignore();

	len = key.length();

	unsigned disp = hash((unsigned char *)key, len);
	
	cout<<"Hash: "<<disp;
	cin.ignore();

	return 0;
}

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

	int i;

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

Someone Please point out what i'm doing wrong here. I'm very confused. =(

Maybe try passing the address of the first element of the string as your character pointer. It worked for me... I got a compile from your code.

Code:

unsigned disp = hash(&key[0], len);