vector of vector initialization and assignment

Question

Hi, Refer code below:

int rows = 5;
int cols = 10;

//One Way
{
vector<vector<int>> vec(rows, vector<int>(cols, 0));
}

//Other way
{
vector<vector<int>> vec;

vector<int> temp;
temp.resize(cols);
fill(temp.begin(), temp.end(), 0);

vec.resize(rows);
fill(vec.begin(), vec.end(), temp);
}

I know first code version is efficient compared to second one and will always choose 1 over 2 as far as possible. However, assume that vec is a class member and need to be initialized in constructor based on argument of rows and cols count. Is second option the only way to do initialization or there is an effective method available for same?

Answer 1

maybe something like:

https://sololearn.com/compiler-playground/cO42x6MZC416/?ref=app

Answer 2

just a tip, vector<vector<T>> is a bad data structure, performance would suffer due to heap indirection :

the problem with nesting vectors (or any other container for that matter) is that you add more and more indirections. so an element access needs to follow more and more pointers through memory.
If you instead have a single array and linearly index into it, you only have to do a single address computation and can directly get your value.

write yourself a class Matrix that contains a single vector<T>, then you can take advantage of the C++23 multidimensional subscript operator :
https://en.cppreference.com/w/cpp/language/operators#Array_subscript_operator

C++23 Core language features:
https://en.cppreference.com/w/cpp/23

Answer 3

Thanks MO ELomari for wonderful tip

Answer 4

MO ELomari
Yes, good point and useful link. But C++23...🙄

Answer 5

Sounds good. As member initializer works, it should be ok with below price of code as well

vector<vector<int>> a;
a = vector<vector<int>>(15,vector<int>(10,-1));

I thought vector of vector is only initialized , not deep copied