Create an Array of vectors in C++

Question

I want to create a distance matrix of a big dataset, and only want to store the 'close' enough elements. The code reads like this

vector<double> * D; 
D = (vector<double> *) malloc(dim *sizeof(vector<double>) ) ;

for(i=0;i<dim;i++){
    for(j=i+1;j<dim;j++){
        dx = s[j][0] - s[i][0];
        dy = s[j][1] - s[i][1];
        d =   sqrt( dx*dx + dy*dy );
        if(d  < MAX_DISTANCE){
            D[i].push_back(d);
            D[j].push_back(d);
            }
        }

which gives me segmentation fault. I guess I have not defined the array of vector correctly. How do I get around this ?

Why make an array of vectors when you can just make a vector of vectors? — Galik, Apr 08 '16 at 10:48
Use a single `std::vector` and have its size be `width*height`. — Simple, Apr 08 '16 at 10:51
@Galik because in a vectors of vectors I would have to cycle on the full array for every element and cant start from j=i+1. Am I wrong ? — Duccio Piovani, Apr 08 '16 at 10:55
Its really no different from an array of vectors. In fact a vector is actually a dynamic array wrapped in a class. Try your example with this: `std::vector > D(dim);`. — Galik, Apr 08 '16 at 10:59
@DuccioPiovani Using `std::vector> D(dim);` solves your problem, given what you posted. Why not run it and see that it works correctly? — PaulMcKenzie, Apr 08 '16 at 11:00

Some programmer dude · Accepted Answer · 2016-04-08T10:55:15.490

4

In C++ you should never allocate object (or arrays of objects) using malloc. While malloc is good at allocating memory, that's all it does. What it doesn't do is calling constructors which means all your vector objects are uninitialized. Using them will lead to undefined behavior.

If you want to allocate an array dynamically you must use new[]. Or, better yet, use std::vector (yes, you can have a vector of vectors, std::vector<std::vector<double>> is fine).

Using the right vector constructor you can initialize a vector of a specific size:

// Create the outer vector containing `dim` elements
std::vector<std::vector<double>> D(dim);

After the above you can use the existing loops like before.

edited Apr 08 '16 at 10:55

answered Apr 08 '16 at 10:47

Some programmer dude

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Thanks Joachim. The reason I want to use the and array is for the cycle and make j start at i instead of 0. With a vector I can't do it (ora at least I wouldn't know how.) – Duccio Piovani Apr 08 '16 at 10:52
@DuccioPiovani You should read on how to [resize](http://en.cppreference.com/w/cpp/container/vector/resize) the vector to the size you want, even if you knew nothing about the constructor. – PaulMcKenzie Apr 08 '16 at 10:58
@Joachim Pileborg this was very useful. Just made me save a lot of time. I thank you very much for that. – Duccio Piovani Apr 08 '16 at 10:59

Create an Array of vectors in C++

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