dereferencing a vector of vectors

Caudata

I am by no means an experienced c++ programmer, but I am trying to use a vector of vectors because it is convenient to store some strings while parsing a text file. I am having trouble with the nested for loop recovery of the stored data and properly dereferencing the data. Here is a bit of test code:

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 #include <iostream>

#include <string>

#include <vector>
 
using namespace std;
 
struct s {

    string strA;

    string strB;

    string strC;

};
 
void main (int argc, char *argv[]) {
 
typedef vector <s> vstype;

vstype v;
 
    s bugs;

    bugs.strA="bees";

    bugs.strB="beetles";

    bugs.strC="flys";
 
    v.push_back(bugs);
 
    s bugs2;

    bugs2.strA="chiggers";

    bugs2.strB="moths";

    bugs2.strC="praying mantii";
 
    v.push_back(bugs2);
 
    bugs.strA="mosquitos";

    bugs.strB="ants";

    bugs.strC="ticks";
 
    v.push_back(bugs);
 
    cout << bugs.strA << endl << endl;
 
    string dummy = bugs.strA;
 
    for( vstype::const_iterator p=v.begin(); p!=v.end(); ++p) {

         cout << "3 BUGS" << endl << (*p).strA << endl << (*p).strB << endl << (*p).strC << endl << endl;

    }
 
    v.clear();
 
    for( p=v.begin(); p!=v.end(); ++p) {

         cout << "3 BUGS" << endl << (*p).strA << endl << (*p).strB << endl << (*p).strC << endl << endl;

    }
 
    cout << "===============" << endl << endl;
 
vector <vstype> vx;
 
    vx.push_back(v);
 
    v[0].strA="aphids";

    v[0].strB="stink bugs";

    v[0].strC="dragonflys";
 
    vx.push_back(v);
 
    // How do you properly dereference the individual items in the following nested for loop to print all values of strings in the vector of vectors?

    // I am assuming that you do not need to use the vector[x][y] notation.
 
    for (vector<vstype>::const_iterator s=vx.begin(); s!=vx.end(); s++) {

        for (p=(*s).begin(); p!=(*s).end(); p++) {

            cout << ????? << endl;

        }

        cout << endl;

    }

}

I suppose I could try literal indexing such as vvec[x][y], but was curious whether this method would work.

weaknessforcats

Line 47 has v.clear() and I think that deletes all of the elements in your vector.

Exactly what are you trying to do??

If you need to store strings, then use vector<string>.

You say you are using vector of vectors but I don't see that. It looks just like a vector<s> where is a struct that contains three strings.

A vector of vectors would be:

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vector<vector<string> > v;

an din this case a v[i][j] will work to identify a string - provided the i and j are valid indexes into the vectors. That is, that they and do not have values that are outside the size() of the vectors.

Caudata

Thank you for your reply.

I found the vector.clear line after I posted - a mistaken leftover from a prior debug. I apologize. Please disregard this line.

Line 15 is a typedef for the vector (and vector of vectors which is declared on line 55).

The for loop on lines 68-74 is what I am interested in and more specifically, how to de-reference it effectively.

While researching this on the web, I found two methods to execute this for loop to reach the members of vectors in nested for loops.

The first is the one I listed. The example of this method I found was for a single vector - not a vector of vectors. I took the liberty of trying to expand it. I think this is using pointers (the iterator) to the actual elements of the vector. The pointer is indexed from a start (p=v.begin(); ) to the last element (p=v.end(); ) and the elements are accessed with the syntax ((*p).strA if it were a single vector for loop.)

This is effective for a single vector, but I am unsure how to implement it with a vector of vectors and reference the appropriate start, end, and each element.

A second method of doing this I found used an ordinary int as indexes in the for loop as follows:

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     for (int a=0; a<vx.size(); a++) {

        cout << "a = " << a << endl;

        for (int b=0; b<vx.at(a).size(); b++) {

            cout << "b = " << b << endl;

            cout << vx[a][b].strA << endl;

            cout << vx[a][b].strB << endl;

            cout << vx[a][b].strC << endl << endl;

        }

        cout << endl;

    }

This works, but I still wonder how to get the original single for loop to work correctly as a nested for loop for a vector of vectors.

RRick

You've got the right idea for accessing the vectors inside another vector by using an iterator. Your p variable is now a vector and you can get an iterator to p to access the struct s. Once you have struct s, you can access its variables.

Take a look at the following code. I changed the names a bit for my sanity and ease. You can access the iterator value by * or ->, your choice.

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#include <iostream>

#include <string>

#include <vector>

using namespace std;
 
struct S {

    string strA;

    string strB;

    string strC;

};
 
typedef vector <S> Vs;

typedef vector <Vs> Vvs;
 
int main (void) 

{

    S ss;  ss.strA="AAA";  ss.strB="BBB";  ss.strC="CCC"; 

    Vs  vs;  vs.push_back( ss); vs.push_back( ss); 

    Vvs vx;  vx.push_back( vs);
 
    for ( Vvs::const_iterator ivv=vx.begin(); ivv!=vx.end();  ivv++)

    {

        for ( Vs::const_iterator iv=ivv->begin(); iv!=ivv->end(); iv++)

        {

            cout << "A: " << iv->strA << endl;

            cout << "B: " << iv->strB << endl;

            cout << "C: " << iv->strC << endl << endl;

        }

    }
 
    return 0;

}

Caudata

Oh wow! That is a really clean way to do that. Thank you very much for the solution. I was struggling!

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