Hash function issue - adding functionality

Question

I tried adding functionality to the djb2 hash function, but it doesn't appear to like the changes. Specifically, I'm trying to include a loop that converts words (strings) to lower case. It throws the following two errors:

Incompatible integer to pointer conversion assigning to char * from int
Cannot increment value of type char *[45]

Note that in the original code *str++ appeared in the while loop. This is my first hash table, and I'm rather shaky on pointers. Any insight on where I've gone wrong would be appreciated.

// djb2 by Dan Bernstein -- slightly modified;
unsigned int hash_function(const char* str)
{
    unsigned int hash = 5381;
    int c;
    char* string[45];

    for (int i = 0; str[i] != '\0'; i++)
    {
        string[i] = (tolower(str[i]));
    }

    while (c == *string++)
        hash = ((hash << 5) + hash) + c; /* hash * 33 + c */

    return (hash % LISTS);
}

olegarch · Accepted Answer · 2016-10-24 03:12:27Z

1

More efficient version of your hash:

unsigned int hash_function(const char* str)
{
    unsigned int hash = 5381, c;
    while(c = *str++) 
      hash += (hash << 5) + (c | 040);

    return (hash % LISTS);
}

answered Oct 24, 2016 at 3:12

olegarch

3,9011 gold badge22 silver badges22 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Ryan Over a year ago

Much appreciated! I assume this is a reformulation of the original and does not convert all to lower case? Out of curiosity, what does (c | 040) mean/do, exactly. The combination of a character and a bitwise OR seems unusual. I have only recently begun doing work at the bit level, which is both cool and a total mind@#$&%.

olegarch Over a year ago

(x|040) turns "x" to lowercase, and not change numbers. However, it changes some CTRL-symbols to some printable ones. However, this is not important for hashing purposes.

unwind · Accepted Answer · 2016-10-19 20:02:50Z

1

This:

char* string[45];

means "array of 45 character pointers", you should drop the asterisk.

And you can't iterate over an array by incrementing the variable, the array variable cannot be changed. You can use a separate pointer:

const char *s = string;
while (c = *s++)

Note that assignment is spelled =, while == is comparison for equality which is not what you mean.

answered Oct 19, 2016 at 20:02

unwind

402k64 gold badges492 silver badges620 bronze badges

1 Comment

Ryan Over a year ago

Many thanks for the reply & clarification! I'll give it a shot!

Collectives™ on Stack Overflow

Hash function issue - adding functionality

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related