CODEWITHSUNDEEP
c++arraysc++11vectorinitialization
Vikas
Vikas

Reputation: 13

How to create vectors dynamically in C++

I am trying to initialize vectors depending upon user input. For example, if user provide n=4 then I have to create 4 vectors of varying sizes. As for vectors syntax is :

vector<int> v1(x);

So, similarly I want to create v2,v3 and v4 named vectors.

I am confused. How can I do this ?

Upvotes: 1

Views: 2088

Answers (1)

walnut
walnut

Reputation: 22152

The names of individual vectors v1, v2, v3, ... are defined at compile time. If you want to have a dynamic number of vectors, you need a vector of vectors, e.g.:

std::vector<std::vector<int>> vs;

or if you already know the number of vectors n that you want:

std::vector<std::vector<int>> vs(n);

Then, instead of using v1, v2, v3, you'd use vs[0], vs[1], vs[2], and your code could use dynamically a vector v[i] where i is a variable or an expression.

You can add a vector to vs with emplace_back/push_back or resize:

vs.emplace_back();

//or

vs.push_back({});

// or

vs.push_back(std::vector<int>());

// or

vs.resize(4);

In the latter case vs will contain four empty vectors (or the first 4 existing vectors if vs already had more than 4 vectors).

And you can add elements to the inner vectors as usually, e.g.:

vs[0].push_back(42);

vs[2].resize(x);

After that the first vector will have length one and the third vector will have length x.

Or you can insert a vector with size x directly after the last vector:

vs.emplace_back(x);

// or

vs.push_back({x});

// or

vs.push_back(std::vector<int>(x));

Elements can then be accessed with double indices, e.g.

vs[1][14]

reads the 15th element of the 2nd vector.

Upvotes: 6

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