Hash table creation and insertion

Hi,
I am working on implementation of a hash table.

In a given string I have to find the first word which is not repeated in the string.

Now as a hash function I am simply using atoi(str).

So for "total", I get following

t 116
o 111
t 116
a 97
l 108

I am able to do uptil this properly.
Now I want to create hash table and store this in hash table which will have 3 columns.

index........converted_int.........count

1................ 116...................... 2
2.................111.......................1
3.................97.........................1
4..................108......................1


So after this things are stored in hash table , then I can search the count values in the order of the characters in the original string and see which first character is not reapeated (seeing the count = 1)
I am almost able to do the conversion of given characters in string.

Can someone tell me how can I create hash table for this and insert these values in it? I am struggling a lot.

You don't need/want to use atoi() on a single character. You'd simply take the value of the char variable:

Code:

---

#include <stdio.h>
int main(void)
{
    char word[] = "total";
    int i;
    for (i = 0; i < sizeof word - 1; i++)
    {
        printf("%c %d\n", word[i], word[i]);
    }
    return 0;
}

---

My output:

Code:

---

t 116
o 111
t 116
a 97
l 108

---

As for your "hash table", you are not really implementing a hash table, you just need an array to store all possible character values, so something like:

Code:

---

#include <limits.h>

int count[CHAR_MAX + 1]; /* but see footnote */

---

You can then iterate through the string like my for loop above, increasing count[value] each time. Then you can iterate through the string again, finding the first character where count[value] is still 1.

Footnote:

Most systems you encouter will likely have a CHAR_MAX of 127 and a CHAR_MIN of -128, which denotes that a char on that system can be between -128 and 127. These values don't matter, but the point is that CHAR_MIN can (and often will) be negative.

Therefore you have to be careful you do not end up trying to access count[negative_number]. One simple approach is to check that it's >= 0 before assignging it to your count array and ignore characters that fall outside.

A more thorough approach would be to declare your array instead as:

Code:

---

int count[CHAR_MAX - CHAR_MIN + 1];

---

And then use an offset when you assign to make sure your result is always in range:

Code:

---

count[value - CHAR_MIN]++;

---

instead of just:

Code:

---

count[value]++;

---

Of course, another solution is just to explicitly use an unsigned char instead.