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Below is my code:

n = 15;
roots = Table[Sqrt[Prime[i]], {i, n}];
q = 10^125;
error = N[Abs[q*roots - Round[q* roots]], 12]

Here I have a large number q = 10^125. It is okay for other calculations (which I did not show). But because of it, I get

enter image description here

So, where did it go wrong? My OS is windows 7 and my version is 11. I am actually often confused by the operation N. When should I use it? Before any other operation or after all other operators?

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  • $\begingroup$ Try this error = Block[{$MaxExtraPrecision = 400}, N[Abs[q*roots - Round[q*roots]], 12]] $\endgroup$
    – Anjan Kumar
    Mar 14, 2017 at 9:11
  • $\begingroup$ It works. But why is not an issue in other cases. The number q is used many times in other lines. $\endgroup$
    – S. Kohn
    Mar 14, 2017 at 9:15
  • $\begingroup$ Please don't use the bugs tag when posting questions. This is a special tag that is meant to be added by someone else than the original poster after the bug has been verified by the community. $\endgroup$
    – Szabolcs
    Mar 14, 2017 at 9:27
  • 2
    $\begingroup$ BTW this is not a bug. The red box is a warning that the results have no effective precision left in them. $\endgroup$
    – Szabolcs
    Mar 14, 2017 at 9:28

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