How to convert a variable to a constant?

Question

C++ / C#

I have an array
extern int vectorSync[]
After some calculations, the size of the array is calculated.
Is written to a variable.
But to declare the size of an array, I need a constant.
How to move the value of a variable into a constant, and then substitute it into the size of an array?

Answer 1

It seems to me that in any way, it is better to use a dynamic array.
int *arr = new int [ len ] ;