Why past 10^16 then you attempt minus 1 answer is 10^6 in C++

Question

So i need get answer for this 10^16-1
but then I do it it always end up being 10^16.
with -2 and more it works perfect..
Is it rounds up by default then big number needs only 1 to change into ^17???
Maybe sb had similiar problem and knows how to bypass it

Answer 1

Subtracting small numbers from large numbers loses significant precision.

Answer 2

What data type are you using to store it?

Answer 3

double because I need find sum of all numbers which has n digits 0<n<=100
I use arithmetic progression for that
a1 = pow (10, n-1)
an = pow ((10, n)-1)

Answer 4

I see, maybe you have idea how to save that precision? or manipulate to get number 10^n - 1 in other way

Answer 5

also I don't understand why then I minus 2 it works well