Why bitwise operator produce error in calculating the maximum range possible using same number of bits as provided in an integer

Question

Why is this code not giving expected value for n = 1000000000000 Expected value = 1099511627775 While the code is giving 255

long long now = n, count = 0, len = 0;
while (now >= 1) {
  count++;
  now /= 2;
}
len = (1 << count) - 1;
cout << len;

Switch to `unsigned` types, get this working in two flicks of the wrist, then head off to the pub. — Bathsheba, Aug 30 '19 at 17:32
Your code generates C26451 warning. It's the cause of your problem — Kevin Kouketsu, Aug 30 '19 at 17:36
This [question](https://stackoverflow.com/questions/24117934/why-doesnt-left-bit-shift-shift-beyond-31-for-long-int-datatype) is a near duplicate, although the types used are different. — 1201ProgramAlarm, Aug 30 '19 at 17:36
Possible duplicate of [Why doesn't left bit shift << shift beyond 31 for long int datatype?](https://stackoverflow.com/questions/24117934/why-doesnt-left-bit-shift-shift-beyond-31-for-long-int-datatype) — Kevin Kouketsu, Aug 30 '19 at 17:37

score 7 · Accepted Answer · answered Aug 30 '19 at 17:34

7

In 1 << count the constant 1 is an int not a long long.
Then the shift overflows.
You should use 1ll << count.

answered Aug 30 '19 at 17:34

prog-fh

1 Answers1