Open hashing, add element, and find element using iterator - c++

Question

I have an open hash table using the STL.
typedef std::list<int> LIST;
typedef std::vector<LIST> HASH_TABLE;
I initialized the hash table by filling it with empty lists.
LIST mt_list;
HASH_TABLE hTable;
hTable.assign(7, mt_list);
Now if I want to add an int to my table based on:
hKey = (value*value) % 7;
and I use
hTable[hKey].push_back(value);
It should work right? I can't get it to work.
void addValue(int value){
if(val_find(value)){
std::cout << "WARNING: duplicate input: " << value << std::endl;
}
else{
calc_hash_bucket(value); //set hKey
hTable[hKey].push_back(value); //push value into list
}
}
The code above does not add the element to any of the lists within the vector.
Also, when I want to use an iterator to traverse the vector and the lists within the vector, how do I get one element at a time from a list so I can find a particular value that may or may not already be in the list?
This is what I have for finding a value within the hash table:
bool val_find(int value){
if(mt_hash()){
return false;
}
else{
for(HASH_ITER h_iter = hTable.begin(); h_iter != hTable.end(); ++h_iter){
for(LIST_ITER l_iter = h_iter->begin(); l_iter != h_iter->end(); ++l_iter){
if(*l_iter == value){
return true;
}
}
}
}
return false;
}
I'm stumped. I don't understand why it won't add the value to any of the lists.
I feel I should mention this is all in a header file and part of a class that I created. (I don't know if that matters)
Edit: The warning statement does not print. To answer questions, the mt_hash() function checks to see if the hash table is empty and I have checked it several times to make sure it outputs correctly. I fixed the hTable_1 vs hTable difference, they are the same thing. I just forgot to change it when I put it into the question.
bool mt_hash(void){ //is hash table empty?
for(unsigned int i = 0; i < hTable.size(); ++i){
if(!hTable.at(i).empty()){ //if not empty return false
return false;
}
}
return true; //else return true
}
Thanks,
Zach

Answer 1

As Pradhan points out, there is a quite a bit missing. What is the implementation of mt_hash()? Are hTable_1 and hTable the same object?
Below, I've taken your code above, and placed them in a struct with the implied functionality included. Note three changes: hTable replaces hTable_1 in val_find(); addValue() uses a local variable to store the hash key; and mt_hash() is implemented by keeping a simple element count.
#include <list>
#include <vector>
#include <iostream>
#include <iomanip>
struct open_hash {
typedef std::list<int> LIST;
typedef std::vector<LIST> HASH_TABLE;
typedef LIST::const_iterator LIST_ITER;
typedef HASH_TABLE::const_iterator HASH_ITER;
HASH_TABLE hTable;
int nbins;
int elem_count;
explicit open_hash(int nbins_): nbins(nbins_), elem_count(0) {
init_hash();
}
void init_hash() {
LIST mt_list;
hTable.assign(nbins, mt_list);
}
int hash_bucket(int value) const {
return (value*value)%nbins;
}
bool mt_hash() const {
return elem_count==0;
}
bool val_find(int value) const {
if (mt_hash()) {
return false;
}
for (HASH_ITER h_iter = hTable.begin(); h_iter != hTable.end(); ++h_iter){
for (LIST_ITER l_iter = h_iter->begin(); l_iter != h_iter->end(); ++l_iter){
if (*l_iter == value) {
return true;
}
}
}
return false;
}
void addValue(int value) {
if (val_find(value)) {
std::cout << "WARNING: duplicate input: " << value << std::endl;
}
else {
int hKey=hash_bucket(value);
hTable[hKey].push_back(value); //push value into list
++elem_count;
}
}
};
int main() {
open_hash H(7);
std::vector<int> vals={3,1,9,2,10,4,3};
for (int v: vals) {
H.addValue(v);
}
for (int i=1; i<=10; ++i) {
std::cout << "val_find(" << i << "):\t" << std::boolalpha << H.val_find(i) << "\n";
}
}
This produces expected output:
WARNING: duplicate input: 3
val_find(1): true
val_find(2): true
val_find(3): true
val_find(4): true
val_find(5): false
val_find(6): false
val_find(7): false
val_find(8): false
val_find(9): true
val_find(10): true
I suspect the original problem lies in addValue() and val_find() referring to different hash objects, or a problem in mt_hash() misreporting that the table is empty when in fact it is not.

Answer 2

The answer to the this problem is to create the class object of type HASH_TABLE in the main cpp file and then pass it by reference into the function (getCmd) that calls all of the commands and i/o.
I was calling the "getCmd" function in main() and that function (EVERY time it is called) creates a NEW instance of the HASH_TABLE class, effectively "replacing" the previous object with a new, empty object. (though I suspect it did not actually replace the previous object. I think the previous object was still taking up memory but it was not being used)
I didn't post the code for the problem area because I didn't know where the problem was.
Thanks for all your help!