How can I declare an array inside a function according to the size of a parameter?

Question

Here is what I have tried:

int fun1(vector s)
{ 
    const int n = s.size();
    int arr[n]; //<----want to declare an array of length s.size()         


        

Answer 1

When you declare an array like this

int arr[n];

the compiler will allocate memory on the stack for it. In this case the C++ standard requires that n is known at compile time, i.e. it must be const.

The answer to your question is to get the memory from the heap at run time like this:

int* arr = new int[n];

In this case the memory is allocated at run time and so the value of n doesn't need to be known until run time. If you use this approach don't forget to free up the memory when you're done with:

delete [] arr;

However, as the comments suggest, using a std::vector would almost certainly be a better approach. Unless you've got a good reason not to, I'd go with that.