Substracting large integers

Asked Oct 21 '21 at 09:21

Active Oct 21 '21 at 09:45

Viewed 28 times

I am working on an assignment, but realised an error (C++). For the below code, when I input n = 62, why will the number not be subtracted ?

long long int total = pow(2,n) - 1

How should I correctly represent the value ?

I found out that if I do:

long long int total = pow(2,n);
long long int final = total - 1

It works ? Why ?

edited Oct 21 '21 at 09:45

asked Oct 21 '21 at 09:21

2

`pow(2,62)` returns a `double`, so `pow(2,62)-1` is also a `double`, which is not exactly representable, so it will be evaluated to the next representable `double`, which is `pow(2,62)`. Powers of 2 are always exactly representable in this format. – mch Oct 21 '21 at 09:23
even `2LL << 62` is likely to overflow. – 463035818_is_not_an_ai Oct 21 '21 at 09:29
If so, how should I represent the result ? What should I do if I wanna get pow(2,n) - 1 ? Thanks a lot – StrugglingCollegeStudent Oct 21 '21 at 09:33
`long long total = (1LL << n) - 1;` – chux - Reinstate Monica Oct 21 '21 at 13:25

0 Answers0