Question

I really need assistance with creating a C++ hash table program for generating a hash from a string.

Create a hash function then test and evaluate the results. The set of strings are from a "words.txt" file (there are 45,402 words in the file, no repeating words. Map the words to 45,402 buckets using a hash function).

Requirements:

1. Create an array of 45,402 integers and initialize each to zero. These will be used to store how many times each index is produced from the hash function.
2. Read each line of the input file (words.txt).
3. Create a hash from each word using the hash function.
4. Modulus the results of your hash function by 45,402 to get an index, and increment the appropriate bucket (array entry).
5. After the entire file is processed, calculate and print the percentage on hashes that resulted in collisions (# of collisions / total strings hashed *100)

• The number of collisions can be obtained from the array at the end of the program. Any values >1 in the array indicate that a collision occurred on that index. For example, if theArray[102] = 3, that means two collisions occurred on index 102 because only one value should have been mapped there by a perfect hash function.

6. Write the contents of the array to a file named hashresults.csv, then import it into a spreadsheet program and graph the results.

Please use only #include<iostream>, #include<string>, and #include<fstream>.

Answer 1

Below is the program which solves your question completely. Since you haven't provided any words.txt I have taken a file which contains of 30 words and solved using that but this works for any size. To change the size, just to the program and on the top you will see a line like "#define SIZE 30". There remove 30 and keep the size whatever you want.

INPUT:

words - Notepad File Edit Format View Help this is some text some another text see the blank line in the middle

CODE:

############################

#include<iostream>
#include<fstream>
using namespace std;

#define SIZE 30

long hashArray[SIZE];

long getHash(string s)
{
long h=0, temp;
for(int i=0;i<s.size();i++)
{
temp = s[i];
h = (h*10+temp)%SIZE;
}
return h;
}

int main()
{
ifstream in_file("words.txt");
string s;
long collisions=0, h;
while(in_file>>s)
{
h = getHash(s);
hashArray[h]++;
if(hashArray[h]>1) collisions++;
}
cout<<"Total size of hash array: "<<SIZE;
cout<<"\n\nNumber of collisions: "<<collisions;
cout<<"\n\nPercentage of collisions: "<<(double)(collisions*100)/SIZE<<"\n\n";

//exporting to csv file
ofstream out_file("hashresults.csv");
for(long i=0;i<SIZE;i++)
{
out_file<<i<<','<<hashArray[i]<<'\n';
}
}

##############################

OUTPUT:

Total size of hash array: 30 Number of collisions: 14 Percentage of collisions: 46.6667 Process returned e (exe) execution ti Press any key to continue. me : 0.162 s