A code, that I do not know what it meant.

Question

1.00/5 (3 votes)

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Hello.
What does this code do?
And what is the purpose of the "for" while?

C++

int hash(const char *str) 
{
    if (str != 0)
    {
        unsigned h = str [0];
        for (int i = 1; str [i] != 0; ++i)
            h = (h << 5) - h + str [i];
        return h % HASH_SIZE; // remainder
    }
    else
    {
        return -1;
    }
}

Posted 31-Aug-12 12:57pm

smss IR

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Comments

Sandeep Mewara 31-Aug-12 23:58pm

Assignment?

Where from you picked the code that you have no idea on what is happening?

smss IR 1-Sep-12 17:17pm

I picked this code from WrightEagleBASE-3.0.0 (www.wrighteagle.org/2d) and in the ParamEngine.cpp:116
:D

Sumal.V 3-Sep-12 8:57am

Hey if you want to learn, then pick up a book and start. Learn the concept and look for examples based on that concept. like examples for threads, functions, arrays and so on. That way you can at least understand what is going on in there, rather than picking up some random code and breaking your head.

Mohibur Rashid 1-Sep-12 0:46am

If you do not understand this code then learn programming

3 solutions

Solution 1

See here[^], and preceding and succeeding pages.

Posted 31-Aug-12 21:36pm

Richard MacCutchan

Comments

smss IR 1-Sep-12 17:21pm

I know "for" and "while", but I do not know the "hash" function target's ...

cariolihome 1-Sep-12 19:45pm

Hash function is http://en.wikipedia.org/wiki/Hash_function

Solution 2

Try giving different strings as input and seeing what the output is. And debugging through the code to inspect the different values of h as the for loop progresses; and, of course, the final value of h. You need to define HASH_SIZE towards the beginning in the program.

Posted 1-Sep-12 6:34am

Amarnath S

Updated 1-Sep-12 7:38am

v3

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pasztorpisti · Accepted Answer · 2012-09-01T06:56:00

Solution 3

This seems to be the java string hashing algorithm[^]. It is a variant of the djb2[^] with different constants[^]. Here[^] you can find some explanation about the constants of djb2.

Posted 1-Sep-12 6:56am

pasztorpisti

Comments

smss IR 25-Sep-12 11:25am

Thanks my friend, but the djb2 algorithm is different from this. I wanna to know the this hash algorithm.

smss IR 25-Sep-12 11:25am

The djb2 formula is : hash = ((hash << 5) + hash) + c; /* hash * 33 + c */
but in this algorithm the code is different : h = (h << 5) - h + str [i];

pasztorpisti 25-Sep-12 11:44am

Yes. Because it uses 31 instead of 33 thats why I told you its java.
(h<<5)-h == h*31
I think the above equation clarifies some things, the original shifted version is "manual optimization" for older C compilers.

smss IR 25-Sep-12 18:44pm

Thank you my friend, but I still do not know the mean of the "hash", I think "hash" point to the count of the arguments in a string, is this true?

pasztorpisti 25-Sep-12 19:32pm

Please read these:
http://en.wikipedia.org/wiki/Hash_function
http://en.wikipedia.org/wiki/Hash_table
If you still have concenrs than search for hash-related tutorials with google, sooner or later you will get it. If you don't succeed then go on learning easier material until you reach a level to understand hashing.