Printing an array of STL list - c++

Question

I'm having some trouble printing an array of lists (ex. list<string> hashTable[100] )
Heres what I have so far, my question is at the bottom:
hash2.h
1 #ifndef __HASH2_H
2 #define __HASH2_H
3
4 #include<string>
5 #include<list>
6 #include<fstream>
7
8 using namespace std;
9
10 class Hash
11 {
12 public:
13 void processFile(string fileName);
14 void print();
15
16 private:
17 list<string> hashTable[100];
18
19 private:
20 int hf(string ins);
21
22
23 };
24
25 #endif
hash_function2.cpp
1 #include<iostream>
2 #include<string>
3 #include<fstream>
4 #include "hash2.h"
5
6 using namespace std;
7
8 void Hash::processFile(string fileName)
9 {
10 string word;
11 ifstream myIfile(fileName.c_str());
12 while(getline(myIfile, word))
13 {
14 int index = hf(word);
15 hashTable[index].push_back(word);
16 }
17 myIfile.close();
18 }
19
20 void Hash::print()
21 {
22 for(int i = 0; i < 100; i++)
23 {
24 if(hashTable[i] != NULL)
25 {
26 list<int>::iterator i;
27 for(i = hashTable[i].begin(); i != hashTable[i].end(); ++i)
28 {
29 cout << *i << endl;
30 }
31 }
32 }
33 }
34
35 int Hash::hf(string ins)
36 {
37 return ( (int) ins[0] ) % 100;
38 }
main2.cpp
1 #include "hash2.h"
2 #include<iostream>
3 #include<iomanip>
4 #include<fstream>
5
6 using namespace std;
7
8 int main()
9 {
10 Hash hashTable;
11 hashTable.processFile("test1.txt");
12 hashTable.print();
13 }
So, what I have now is the processFile function, which takes the text file, reads each word in, performs the hash function(crappy, i know), then puts that word in the array index which the hash function returned. That is working fine I think.
What im having issues with is essentially using the STL list. So, in hash_function2.cpp, i want to print my hash table. I'm not sure how to check if the array index is empty(doesn't have any of the words I stored), and I am also not sure how to print all the strings in the list at a given array index. Basically what I would like to do is just print my hash table; get my print() function working.
If anyone could point me in the right direction that would be awesome! It would be greatly appreciated.

Answer 1

You shouldn't be checking for NULL because you have an array of std::list objects, not pointers.
You can check if the list at index i has any elements with:
if (!hashTable[i].empty())
{
// there are elements in the list
}
Or
if (hashTable[i].size())
{
// there are elements in the list
}
Also in your print() function, you're using an iterator of type std::list<int>, but it should be std::list<string>, which matches the declaration of hashTable.

Answer 2

Your problem is not with std list, it is with the array.
You could try using the std::array
#include <array>
std::array< std::list<string>, 100> hashTable;
this way you can check if it is emtpy
if (!hashTable.empty())
{
// do stuff
}
and inside you can check each list
if(!hashTable[i].empty())
{
// do even more stuff
}

Answer 3

Awesome, Thanks a lot guys! I understand how to check each array index and iterate through the list! Heres what I did:
1 #include "hash2.h"
2
3 #include<iostream>
4 #include<string>
5 #include<fstream>
6
7 using namespace std;
8
9 void Hash::processFile(string fileName)
10 {
11 string word;
12 ifstream myIfile(fileName.c_str());
13 while(getline(myIfile, word))
14 {
15 int index = hf(word);
16 hashTable[index].push_back(word);
17 }
18 myIfile.close();
19 }
20
21 void Hash::print()
22 {
23 for(int i = 0; i < 100; i++)
24 {
25 if(hashTable[i].empty() != true)
26 {
27 list<string>::iterator r;
28 for(r = hashTable[i].begin(); r != hashTable[i].end(); ++r)
29 {
30 cout << *r << endl;
31 }
32 }
33 }
34 }
35
36 int Hash::hf(string ins)
37 {
38 return ( (int) ins[0] ) % 100;
39 }