Declare large array on Stack

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No there is no(we’ll say “reasonable”) way to declare this array on the stack. You can however declare the pointer on the stack, and set aside a bit of memory on the heap.

double *n = new double[4200000];

accessing n[234] of this, should be no quicker than accessing n[234] of an array that you declared like this:

double n[500];

Or even better, you could use vectors

std::vector<int> someElements(4200000);
someElements[234];//Is equally fast as our n[234] from other examples, if you optimize (-O3) and the difference on small programs is negligible if you don't(+5%)

Which if you optimize with -O3, is just as fast as an array, and much safer. As with the 

double *n = new double[4200000];

solution you will leak memory unless you do this:

delete[] n;

And with exceptions and various things, this is a very unsafe way of doing things.

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