我怎样才能拥有一个带有二维数组作为参数的 function，而该数组有一个我想更改的参数/维度？

Question

(I'm a student and this is my first time posting so go easy on me.) I want to create a function that takes a 2D array as an argument and in that array, I'd like to have a variable that I want to modify later in the code. This is the closest thing to an example of what I want:

int size;  //the variable I want to change later
void function(int[][size]);

int main(){
    cin >> size;
    int array[size][size];  //the array I'm using with the variable as a parameter
    function(array)
}

void function(int array[][size]){
    //Do thing....
}

The code above does give me an error (array bound is not an integer constant) so if I make the variable a constant it will compile as seen here:

const int size = 10;
void function(int[][size]);

int main(){
    int array[size][size];
    function(array)
}

void function(int array[][size]){
    //Do thing....
}

This does compile like I said, but now I can't modify the variable and need to declare its value in the code beforehand. I assume that the variable needs to be global so that I can use it in the function, and with that said, I can't get pointers to work either most likely because it's a global variable and not a local one. Here's an example of something I tried, but got an error (invalid conversion from 'const int*' to 'int*'):

const int size = 10;
void function(int[][size]);

int main(){
    int *other = &size;
    *other = 5;
}

Any help would be appreciated, thanks.

Answer 1

Plain ol' arrays aren't resizeable in C++. Even more frustrating, their size has to be a constant - you can't make the size a variable that gets set at runtime. Ever more frustrating, the size you put in an array that's a function parameter is a constraint , and it's not even enforced. It's just decor.

As it was hinted in the comments, std::vector<TYPE> is the go-to "resizeable array" in C++. You can create a vector like this:

#include <vector>
int main() {
    std::vector<int> my_int_array;
}

And you can resize it like this:

int new_size = 42;
my_int_array.resize(new_size);

And you can pass it to a function by reference (see the & ) so that changes to myint_array inside the function affect it outside the function.

void my_awesome_function(std::vector<int>& int_array);

my_awesome_function(my_int_array);

So let's say you have a 2D matrix, implemented as a vector of vectors:

std::vector<std::vector<int>> matrix = { { 1,2,3 }, { 4,5,6 } }

If you want to change the number of columns, you have to resize each row array:

int new_column_count = 10;
for (auto& row : matrix) {
    row.resize(new_column_count );
}

You can pass around matrix by reference (eg std::vector<std::vector<int>>& ) and resize it when you need to.