How can I resize a 2D C++ vector?

Question

I have a 2D char vector:

vector< vector > matrix;

I will read in a matrix as an input and store it in that v

Answer 1

Given the vector is empty, you can simply resize the outer vector with preallocated inner vectors without the need of a loop:

matrix.resize(COL, vector(ROW));

Alternatively, when initializing or if you want to reset a non-empty vector, you can use the constructor overload taking a size and initial value to initialize all the inner vectors:

matrix = vector >(COL, vector(ROW));

Depending on whether your matrix is column- or row-major, you need to swap the arguments ROW and COL. The first one (the first parameter on the outer vector) is your first dimension to access the matrix, i.e. I assumed you access it with matrix[col][row].