how to write data in vector>?

Question

I am trying to allocate space for vector of vector but after allocation gdb showing null in pVectorTemp_ but showing size 2

#include <iostream> 
#include<vector>
using namespace std; 
int main( )
{ 
    int index1 = 2;
    int index2 = 2;
    vector<vector<float>*>* pVectorTemp_ =  NULL;
    pVectorTemp_  = new vector<vector<float>*>();
    pVectorTemp_->resize(index1);
    for(unsigned int i=0 ;i< index1;i++)
    {
        vector<float>* pvecTemp = new vector<float>();
        pvecTemp->resize(index2);
        (*pVectorTemp_)[index1] = (pvecTemp);
    }
    return 0;
}

gdb :

(gdb) pvector  pVectorTemp_
elem[0]: $2 = (std::vector<float, std::allocator<float> > *) 0x0
elem[1]: $3 = (std::vector<float, std::allocator<float> > *) 0x0
Vector size = 2
Vector capacity = 2

So Am i doing anything wrong?

You really don't need to use pointers and dynamic allocation here. — juanchopanza, Jul 14 '14 at 09:51
`(*pVectorTemp_)[index1]` this line is wrong, should be `(*pVectorTemp_)[i]`. — Holt, Jul 14 '14 at 09:51
In other words, use `vector>` (or just `vector`) and drop all those `new`s. — juanchopanza, Jul 14 '14 at 10:02
`vector*>*` — *Cannot compute this* — Use (preferably) C++ with `std::vector` and no pointers at all or use C with pointers, don’t create Frankenstein’s monster. — bolov, Jul 14 '14 at 10:57

score 3 · Accepted Answer · edited May 23 '17 at 11:57

Inside the for loop body, you have:

(*pVectorTemp_)[index1] = (pvecTemp);

But note that the index of the for loop is i (index1 is the upper bound).

So, I think you have a typo or bug, and you may want to use i (not index1) as index inside [...].

Note also that you have a signed/unsigned mismatch, since in the loop you have unsigned int as index, and you compare that with index1, which is a signed integer.

But, anyway, your code is uselessly complicated.

You don't need to allocate all these vectors on the heap with new.
Just use automatic ("stack") allocation, e.g.:

int main()
{ 
    int index1 = 2;
    int index2 = 2;

    // Your original code is commented out:
    //
    // vector<vector<float>*>* pVectorTemp_ =  NULL;
    // pVectorTemp_  = new vector<vector<float>*>();
    vector<vector<float>> vectorTemp;

    // pVectorTemp_->resize(index1);
    // Just consider calling push_back.

    for (int i = 0; i < index1; i++)
    {
        // vector<float>* pvecTemp = new vector<float>();        
        // pvecTemp->resize(index2);
        // (*pVectorTemp_)[index1] = (pvecTemp);
        vectorTemp.push_back(vector<float>(index2));
    }

    // No need for return 0 in main().
    // return 0;
}

See how the code gets simplified!

(Code without comments follows.)

int main()
{ 
    int index1 = 2;
    int index2 = 2;

    vector<vector<float>> vectorTemp;        
    for (int i = 0; i < index1; i++)
    {
        vectorTemp.push_back(vector<float>(index2));
    }
}

As a further improvement, assuming that your C++ STL implementation provides that, you may want to use emplace_back() instead of push_back(), to build the nested vectors:

// Instead of vectorTemp.push_back(vector<float>(index2));
//
vectorTemp.emplace_back(index2);

In this case, a vector of size index2 is built directly into the vectorTemp ("outer" vector) container, without temporaries.

You may want to read also this thread on StackOverflow for further details:

push_back vs. emplace_back

@Jefffrey: Raw loops are just fine and very clear in several cases. No need to obfuscate the code with the latest feature. — Mr.C64, Jul 14 '14 at 14:21
`std::vector`'s constructor is not "*the latest feature*" by all means. — Shoe, Jul 14 '14 at 15:35
I was referring to your critics to my use of raw loops, which to me are fine and clear in several cases. — Mr.C64, Jul 14 '14 at 21:41

score 1 · Answer 2 · answered Jul 14 '14 at 10:15

These are the things I can find.

vector<vector<float>*>* pVectorTemp1 =  new vector<vector<float>*>(); // (1)(4)
pVectorTemp1->resize(index1);
for (int i=0 ; i < index1; i++) { // (2)
    vector<float>* pvecTemp2 = new vector<float>(); /(4)
    pvecTemp2->resize(index2);
    (*pVectorTemp1)[i] = (pvecTemp2); // (3)
}

Variables that differ only by trailing underscore is asking for trouble.
unsigned adds nothing, except the risk of a signed/unsigned mistake
The index should be i not index1.
It would be better to use local variables rather than heap allocation and pointers, but that would change the code too much.

score 1 · Answer 3 · answered Jul 14 '14 at 10:23

1

Change

(*pVectorTemp_)[index1] = (pvecTemp);

with

(*pVectorTemp_)[i] = (pvecTemp);

answered Jul 14 '14 at 10:23

Zorgiev

794
1
7
19

score 1 · Answer 4 · answered Jul 14 '14 at 10:44

So Am i doing anything wrong?

Yes, you are doing something wrong:

using dynamically allocation when not needed
using unnecessary raw loops

Here's a simplified version:

int main( )
{ 
    int index1 = 2;
    int index2 = 2;
    std::vector<std::vector<float>> pVectorTemp_
        ( index1
        , std::vector<float>(index2) );
}

Live demo

how to write data in vector*>*?

4 Answers4

how to write data in vector>?