CODEWITHSUNDEEP
c++dictionary
Saurabh Jain
Saurabh Jain

Reputation: 3

Why map is creating NULL in C++?

I have a 2D vector of string as

vector<vector<string>> database

it has following data

database[0] -> A C D   (database[0][0] -> A , database[0][1] -> C and so on)  
database[1] -> B C E F
database[2] -> A B C E F
database[3] -> B E
database[4] -> A C F

I am counting the occurrence of each string (in this example each character A,B etc.) and saving it in a map map_c as 

map<string,int> map_c ;

   for(i=0 ; i<database.size() ; i++)
    {
        for(j=0 ; j<database.at(i).size() ; j++)
        {
            if(map_c.find(database.at(i).at(j)) != map_c.end())
            {
                map_c[database[i][j]]++;                
            }
            else
            {
                map_c[database[i][j]] = 1;
            }
        }

    }

And printing the count of each string using the code 

for(map<string,int>::iterator it = map_c.begin() ; it != map_c.end() ; it++ )
{
    cout << it->first << " -> " << it->second << endl;
}

Output -> 

  -> 1
A -> 3
B -> 3
C -> 4
D -> 1
E -> 3
F -> 3

Why NULL key has been created with a count 1 ?

Upvotes: 0

Views: 89

Answers (3)

Saurabh Jain
Saurabh Jain

Reputation: 3

Sorry , finaly i got rid the problem. Problem was in the input file. At the end of the file , there was a space character at last line and i was thinking that my file has been over before it . That space was creating the problem. I am sorry for inconvenience and thank you so much for your replies and time .

Upvotes: 0

utnapistim
utnapistim

Reputation: 27365

The code you post should not generate an empty (or spaces) string. It is possible such a string comes with your initial data.

Either way, your code to fill the map can be made much much shorter:

map<string,int> map_c ;
for(const auto& line: database)
    for(const auto& s: line)
        ++map_c[s];

Upvotes: 4

M.M
M.M

Reputation: 141574

Replace your if...else with just ++map_c[database[i][j]]; . The default value for int in a map is 0.

The unexpected entry in your output is because your database actually had that entry in it. If you are not sure why this entry is in your database, review the code that sets up your database (and/or post that code).

Upvotes: 3

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