Determining values in a bitmask

Question

I have a protocol guide for a piece of hardware where I can extract 16 different kinds of data. To indicate I want all data, I would enter 65535 as the mask.

 2^0 (1) + 2^1 (2) + 2^2 (4) ... + 2^15 (32768) ============== 65535

I now need to indicate I need options 9, 10, and 13. Presumably I simply need to use the following calculation:

 2^9 (512) + 2^10 (1024) + 2^13 (8192) ============== 9728

(If I'm off-base here, or there is a programmatic way to do this, I'd be interested to know!)

What I would like to know is how I would in future extract all the numbers that were involved in the summation.

I had thought I would be able to check with (9728 & 9) == 9, (9728 & 10) == 10, and (9728 & 13) == 13, but all those return false.

You need to do the bitwise and with 2^9 instead of 9 and compare it to 2^9. — juharr
– juharr, Commented Nov 26, 2014 at 14:00

Marc Gravell · Accepted Answer · 2014-11-26 14:01:16Z

bit 9 is 256; bit 10 is 512; bit 13 is 4096.

So:

if((val & 256) != 0) { /* bit 9 is set */ } if((val & 512) != 0) { /* bit 10 is set */ } if((val & 4096) != 0) { /* bit 13 is set */ }

You could also use an enum for convenience:

[Flags] public enum MyFlags { None = 0, Foo = 1, Bar = 2, ... SomeFlag = 256, AnotherFlag = 512, ... }

then:

MyFlags flags = (MyFlags)val; if((flags & MyFlags.SomeFlag) != 0) {/* SomeFlag is set */}

And likewise:

MyFlags thingsWeWant = MyFlags.Foo | MyFlags.SomeFlag | MyFlags.AnotherFlag; int val = (int)thingsWeWant;

If you use a Flags enum, you can use the HasFlag method instead of manually "and-ing"
@SebastianNegraszus that is actually not a good idea; HasFlag takes Enum - so it involves boxing.

user2160375 · Accepted Answer · 2014-11-26 14:00:43Z

2

Did mean sth like this?

var value = 512 | 1024 | 8192; var pos = 9; var isSetNine = (value & (1 << pos)) != 0;

answered Nov 26, 2014 at 14:00

user2160375

2 Comments

Mister Epic Over a year ago

What does the pos variable signify?

user2160375 Over a year ago

@MisterEpic the position on bit (the power of two) that you want to check. It may be 9, 10 or 13 like in your sample.

Collectives™ on Stack Overflow

Determining values in a bitmask

2 Answers 2

2 Comments

2 Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Related