CODEWITHSUNDEEP
cbit-manipulationbitwise-operatorsbitwise-and
nik
nik

Reputation: 445

Extracting rightmost N bits of a double in C

I have some natural big number in double. I need to get 30 rightmost bits of it (of integral part). If it was integer the operation was:

var & 0x3FFFFFFF

I can implement some functions for the purpose, but I need some simple solution. Is there one?

*edit:

All the answers doesn't work for me. I'll try to explain: for example I have double x = 9362446620820194.0000, 30 rightmost bits of integral part of that number is the number 957350626.

I could use uint64_t instead of double, but I need support for 32 bit systems.

p.s. I mean simple binary number representation, not the machine (memory)

Upvotes: 3

Views: 1045

Answers (2)

Fred Foo
Fred Foo

Reputation: 363767

Assuming 8-bit unsigned char:

unsigned char const *p = (unsigned char const *)&var;
long rightmost30 = (p[sizeof(double) - 4] & 0x3F) << 24
                 |  p[sizeof(double) - 3] << 16
                 |  p[sizeof(double) - 2] << 8
                 |  p[sizeof(double) - 1];

Though really, you should have a look at frexp and related functions.

Upvotes: 2

user529758
user529758

Reputation:

The casting hack:

float var = 0.2987647892;
uint32_t bit_repr = *(uint32_t *)&var;
uint32_t masked = bit_repr & 0x3FFFFFFF;

Upvotes: 0

Related Questions